[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #138 from pinskia at gcc dot gnu dot org 2010-09-16 17:08 --- *** Bug 45691 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||ian at macky dot net http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #137 from jvdelisle at gcc dot gnu dot org 2010-08-04 02:42 --- *** Bug 45175 has been marked as a duplicate of this bug. *** -- jvdelisle at gcc dot gnu dot org changed: What|Removed |Added CC||CycleTimeChart at yahoo dot ||com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #136 from pinskia at gcc dot gnu dot org 2010-05-19 18:05 --- *** Bug 44198 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||AlekM at hotmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #135 from jsm28 at gcc dot gnu dot org 2009-10-31 18:37 --- *** Bug 41867 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #134 from jsm28 at gcc dot gnu dot org 2009-10-29 20:26 --- *** Bug 41867 has been marked as a duplicate of this bug. *** -- jsm28 at gcc dot gnu dot org changed: What|Removed |Added CC||tydeman at tybor dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #133 from pinskia at gcc dot gnu dot org 2009-09-20 21:28 --- *** Bug 41195 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||mehta at roguewave dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #130 from nszabolcs at gmail dot com 2009-07-22 12:10 ---
(In reply to comment #129)
I am a bit wondering if this bug is also for the case (a b) (b a) ==
true. Is it?
i guess so, see:
#include stdlib.h
#include stdio.h
#define axiom_order(a,b) !(a b b a)
#define axiom_eq(a) a == a
#define third ((double)atoi(1)/atoi(3))
int main(void) {
if (axiom_order(third, third))
puts(ok);
else
puts(error);
if (axiom_eq(third))
puts(ok);
else
puts(error);
return 0;
}
here this prints
error
error
with gcc 4.2 and -S shows that this is long double vs double precision problem
Because if so, this becomes way more serious, as for example std::setdouble
is broken then (and depending on the STL implementation, it will throw
assertions then).
in C99 (+TC1,TC2,TC3) different precision is not allowed
5.1.2.3 p12:
... In particular, casts and assignments are required to perform their
specified conversion
5.2.4.2.2 p8:
Except for assignment and cast (which remove all extra range and precision),
the values 8 of operations with floating operands and values subject to the
usual arithmetic conversions and of floating constants are evaluated to a
format whose range and precision may be greater than required by the type.
i'm not sure about c++ though
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #131 from vincent at vinc17 dot org 2009-07-22 17:33 --- (In reply to comment #130) #define axiom_order(a,b) !(a b b a) #define axiom_eq(a) a == a #define third ((double)atoi(1)/atoi(3)) [...] in C99 (+TC1,TC2,TC3) different precision is not allowed It is allowed, except for... 5.1.2.3 p12: ... In particular, casts and assignments are required to perform their specified conversion But a division is not a cast, nor an assignment. 5.2.4.2.2 p8: Except for assignment and cast (which remove all extra range and precision), the values 8 of operations with floating operands and values subject to the usual arithmetic conversions and of floating constants are evaluated to a format whose range and precision may be greater than required by the type. A greater precision is OK for division, in particular. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #132 from ich at az2000 dot de 2009-07-22 20:54 ---
So that means that this C++ example could crash under certain circumstances
(depending on how far the compiler is optimising here)?
#include set
#define third ((double)atoi(1)/atoi(3))
int main() {
std::setdouble m;
m.insert(third);
m.insert(third);
return (m.find(third) != m.end()) ? 0 : 1;
}
Because this example only works if it is guaranteed that !(third third
third third), otherwise it would abort.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #128 from pinskia at gcc dot gnu dot org 2009-05-18 14:11 --- *** Bug 40186 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||ich at az2000 dot de http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #129 from ich at az2000 dot de 2009-05-18 14:24 --- I am a bit wondering if this bug is also for the case (a b) (b a) == true. Is it? Because if so, this becomes way more serious, as for example std::setdouble is broken then (and depending on the STL implementation, it will throw assertions then). If not, I guess my bug #40186 is not a duplicate of this bug. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #126 from jsm28 at gcc dot gnu dot org 2009-03-30 01:51 --- Subject: Bug 323 Author: jsm28 Date: Mon Mar 30 01:50:44 2009 New Revision: 145272 URL: http://gcc.gnu.org/viewcvs?root=gccview=revrev=145272 Log: PR rtl-optimization/323 * c-common.c (c_fully_fold, convert_and_check, c_common_truthvalue_conversion): Handle EXCESS_PRECISION_EXPR. (c_fully_fold_internal): Disallow EXCESS_PRECISION_EXPR. * c-common.def (EXCESS_PRECISION_EXPR): New. * c-cppbuiltin.c (builtin_define_float_constants): Define constants with enough digits for long double. * c-lex.c (interpret_float): Interpret constant with excess precision where appropriate. * c-opts.c (c_common_post_options): Set flag_excess_precision_cmdline. Give an error for -fexcess-precision=standard for C++ for processors where the option is significant. * c-parser.c (c_parser_conditional_expression): Handle excess precision in condition. * c-typeck.c (convert_arguments): Handle arguments with excess precision. (build_unary_op): Move excess precision outside operation. (build_conditional_expr): Likewise. (build_compound_expr): Likewise. (build_c_cast): Do cast on operand of EXCESS_PRECISION_EXPR. (build_modify_expr): Handle excess precision in RHS. (convert_for_assignment): Handle excess precision in converted value. (digest_init, output_init_element, process_init_element): Handle excess precision in initializer. (c_finish_return): Handle excess precision in return value. (build_binary_op): Handle excess precision in operands and add excess precision as needed for operation. * common.opt (-fexcess-precision=): New option. * config/i386/i386.h (X87_ENABLE_ARITH, X87_ENABLE_FLOAT): New. * config/i386/i386.md (floatSSEMODEI24:modeX87MODEF:mode2): For standard excess precision, output explicit conversion to and truncation from XFmode. (*floatSSEMODEI24:modeX87MODEF:mode2_1, *floatSSEMODEI24:modeX87MODEF:mode2_i387_with_temp, *floatSSEMODEI24:modeX87MODEF:mode2_i387, two unnamed define_splits, floatdiX87MODEF:mode2_i387_with_xmm, two unnamed define_splits, *floatunssimode2_1, two unnamed define_splits, floatunssimode2, addmode3, submode3, mulmode3, divdf3, divsf3, *fop_mode_comm_i387, *fop_mode_1_i387, *fop_MODEF:mode_2_i387, *fop_MODEF:mode_3_i387, *fop_df_4_i387, *fop_df_5_i387, *fop_df_6_i387, two unnamed define_splits, sqrtmode2): Disable where appropriate for standard excess precision. * convert.c (convert_to_real): Do not shorten arithmetic to type for which excess precision would be used. * defaults.h (TARGET_FLT_EVAL_METHOD_NON_DEFAULT): Define. * doc/invoke.texi (-fexcess-precision=): Document option. (-mfpmath=): Correct index entry. * flags.h (enum excess_precision, flag_excess_precision_cmdline, flag_excess_precision): New. * langhooks.c (lhd_post_options): Set flag_excess_precision_cmdline. * opts.c (common_handle_option): Handle -fexcess-precision=. * toplev.c (flag_excess_precision_cmdline, flag_excess_precision, init_excess_precision): New. (lang_dependent_init_target): Call init_excess_precision. * tree.c (excess_precision_type): New. * tree.h (excess_precision_type): Declare. ada: * gcc-interface/misc.c (gnat_post_options): Set flag_excess_precision_cmdline. Give an error for -fexcess-precision=standard for processors where the option is significant. fortran: * options.c (gfc_post_options): Set flag_excess_precision_cmdline. Give an error for -fexcess-precision=standard for processors where the option is significant. java: * lang.c (java_post_options): Set flag_excess_precision_cmdline. Give an error for -fexcess-precision=standard for processors where the option is significant. testsuite: * gcc.target/i386/excess-precision-1.c, gcc.target/i386/excess-precision-2.c, gcc.target/i386/excess-precision-3.c, gcc.target/i386/excess-precision-4.c, gcc.target/i386/excess-precision-5.c, gcc.target/i386/excess-precision-6.c: New tests. Added: trunk/gcc/testsuite/gcc.target/i386/excess-precision-1.c trunk/gcc/testsuite/gcc.target/i386/excess-precision-2.c trunk/gcc/testsuite/gcc.target/i386/excess-precision-3.c trunk/gcc/testsuite/gcc.target/i386/excess-precision-4.c trunk/gcc/testsuite/gcc.target/i386/excess-precision-5.c trunk/gcc/testsuite/gcc.target/i386/excess-precision-6.c Modified: trunk/gcc/ChangeLog trunk/gcc/ada/ChangeLog trunk/gcc/ada/gcc-interface/misc.c
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #127 from jsm28 at gcc dot gnu dot org 2009-03-30 01:57 --- Fixed for C (and ObjC) for 4.5 with the new -fexcess-precision=standard support. The issue remains for other languages (and maybe for some m68k processors); I quote from my original message http://gcc.gnu.org/ml/gcc-patches/2008-11/msg00105.html: It would be possible to implement the option for non-C languages, to provide whatever predictable semantics are appropriate for those languages (whether or not described in their standards). Note that bug 323 was originally reported with a C++ testcase. If implemented for all languages, the option might supersede -ffloat-store. Right now, -ffloat-store checks are scattered about the optimizers and it seems unlikely that -ffloat-store really implements any form of predictable semantics now; such semantic effect as it was intended to have could be better represented as an alias for a -fexcess-precision=standard option supported for all languages. It would probably be most appropriate not to close bug 323 without having some form of predictable semantics available for each language. and: I have not changed the m68k back end in this patch. Thus the option may not be fully effective for the affected m68k processors (classic m68k with 68881, before 68040, only, not ColdFire, not 68040 or later). If anyone wishes to make it fully effective for such processors they should copy the testcases to gcc.target/m68k/ and go through m68k insn patterns appropriately adjusting them. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #125 from vincent at vinc17 dot org 2008-11-11 10:13 --- (In reply to comment #124) It seems like the C99 standard prohibits double rounding, only if Annex F is claimed to be supported (note: Annex F is not just IEEE 754, it also contains specific bindings). IEEE 754 doesn't prohibit double rounding either (this depends on the bindings), but with C99 + Annex F, double rounding is prohibited. Now, bug 323 is not about double rounding specifically. There are two potential problems: 1. A double variable (or result of a cast) contains a long double value (not exactly representable in a double). This is prohibited by C99 (5.1.2.3#12, 6.3.1.5#2 and 6.3.1.8#2[52]). This problem seems to be fixed by Joseph Myers' patch mentioned in comment #123 (but I haven't tried). 2. Computations on double expressions are carried out in extended precision. This is allowed by C99 (except for casts and assignments), e.g. when FLT_EVAL_METHOD=2. But if the implementation (i.e. here compiler + library + ...) claims to support Annex F, then this is prohibited. This point is rather tricky because the compiler (GCC) and library (e.g. GNU libc) settings must be consistent, so their developers need to talk with each other. FYI, I reported the following bug concerning glibc: http://sourceware.org/bugzilla/show_bug.cgi?id=6981 because it sets __STDC_IEC_559__ to 1 unconditionally. The short answer is that no compiler, be it gcc, will be modified so that complex sequences of operations are used for floating-point operations in lieu of directly using x87 instructions! At least for two reasons: * x87 is now fading away (its use is deprecated on x86-64, it's not used by default on Intel Macintosh...) * Most people don't want to pay the performance hit. That's why in Joseph's patch, it's just an option (disabled by default, but enabled by -std=c99 because one should assume that if a user wants C99, then he really wants it, and if he is able to add an option, then he is also able to add another one if he wants to disable this fix in case he knows it is useless for his application -- this is also true for -ffast-math). GCC already supports SSE, but this patch is for processors that don't. Also the performance hit depends very much on the application. Performance hit is reduced in applications that do not use intensive FP or mostly interactive applications. In addition, I think there are more urgent things to fix in gcc's floating-point system, such as support for #pragma STDC FENV ACCESS FYI, this is bug 34678. And I submitted bug 37845 concerning the FP_CONTRACT pragma. * It is possible to force the x87 to use reduced precision for the mantissa (with inline asm or even now with gcc options). Unfortunately, this means that long double wouldn't behave as expected, and the behavior is not controllable enough (e.g. due to libraries, plugins...). Such a change should have been system-wide. Now, this is needed in software where double rounding is prohibited (e.g. XSLT processor). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #124 from David dot Monniaux at imag dot fr 2008-11-11 07:46 --- Vincent Lefèvre is right: the issue is quite subtle. (I should mention that Vincent is an expert in computer arithmetics, which I'm not.) As he rightly points, conformance to IEEE-754 should be evaluated for a whole software/hardware system - it is possible to have a IEEE-754 system entirely implemented in software. It seems like the C99 standard prohibits double rounding, and prohibits having values depend on the vagaries of register spilling. Except that this prohibition is explicit only in non-normative sections. Language lawyers have sent me justifications that this prohibition is implied by various normative prescriptions of the standard. I think that in any case we should not spend too much energy trying to assign blame (who conforms to the standard) but rather try to find solutions. The short answer is that no compiler, be it gcc, will be modified so that complex sequences of operations are used for floating-point operations in lieu of directly using x87 instructions! At least for two reasons: * x87 is now fading away (its use is deprecated on x86-64, it's not used by default on Intel Macintosh...) * Most people don't want to pay the performance hit. In addition, I think there are more urgent things to fix in gcc's floating-point system, such as support for #pragma STDC FENV ACCESS Now for some additional facts: * It is possible to force the x87 to use reduced precision for the mantissa (with inline asm or even now with gcc options). * This setting does not affect the range of exponents. so you can still have surprises if a very tiny nonzero value is kept in a register then is rounded to zero when spilled to memory. * In some rare cases, you can have double rounding on underflow: you get a different result by computing on SSE in double precision mode on the one hand, and by computing on x87 in double precision then writing to a double variable in memory. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #123 from jsm28 at gcc dot gnu dot org 2008-11-04 13:25 --- Subject: Bug 323 Author: jsm28 Date: Tue Nov 4 13:24:30 2008 New Revision: 141578 URL: http://gcc.gnu.org/viewcvs?root=gccview=revrev=141578 Log: PR rtl-optimization/323 * c-common.c (convert_and_check, c_common_truthvalue_conversion): Handle EXCESS_PRECISION_EXPR. * c-common.def (EXCESS_PRECISION_EXPR): New. * c-cppbuiltin.c (builtin_define_float_constants): Define constants with enough digits for long double. * c-lex.c (interpret_float): Interpret constant with excess precision where appropriate. * c-opts.c (c_common_post_options): Set flag_excess_precision_cmdline. * c-parser.c (c_parser_conditional_expression): Handle excess precision in condition. * c-typeck.c (c_fully_fold): Handle EXCESS_PRECISION_EXPR. (c_fully_fold_internal): Disallow EXCESS_PRECISION_EXPR. (convert_arguments): Handle arguments with excess precision. (build_unary_op): Move excess precision outside operation. (build_conditional_expr): Likewise. (build_compound_expr): Likewise. (build_c_cast): Do cast on operand of EXCESS_PRECISION_EXPR. (build_modify_expr): Handle excess precision in RHS. (convert_for_assignment): Handle excess precision in converted value. (digest_init, output_init_element, process_init_element): Handle excess precision in initializer. (c_finish_return): Handle excess precision in return value. (build_binary_op): Handle excess precision in operands and add excess precision as needed for operation. * common.opt (-fexcess-precision=): New option. * config/i386/i386.h (X87_ENABLE_ARITH, X87_ENABLE_FLOAT): New. * config/i386/i386.md (floatSSEMODEI24:modeX87MODEF:mode2): For standard excess precision, output explicit conversion to and truncation from XFmode. (*floatSSEMODEI24:modeX87MODEF:mode2_1, *floatSSEMODEI24:modeX87MODEF:mode2_i387_with_temp, *floatSSEMODEI24:modeX87MODEF:mode2_i387, two unnamed define_splits, floatdiX87MODEF:mode2_i387_with_xmm, two unnamed define_splits, *floatunssimode2_1, two unnamed define_splits, floatunssimode2, addmode3, submode3, mulmode3, divdf3, divsf3, *fop_mode_comm_i387, *fop_mode_1_i387, *fop_MODEF:mode_2_i387, *fop_MODEF:mode_3_i387, *fop_df_4_i387, *fop_df_5_i387, *fop_df_6_i387, two unnamed define_splits, sqrtmode2): Disable where appropriate for standard excess precision. * convert.c (convert_to_real): Do not shorten arithmetic to type for which excess precision would be used. * doc/invoke.texi (-fexcess-precision=): Document option. (-mfpmath=): Correct index entry. * flags.h (enum excess_precision, flag_excess_precision_cmdline, flag_excess_precision): New. * langhooks.c (lhd_post_options): Set flag_excess_precision_cmdline. * opts.c (common_handle_option): Handle -fexcess-precision=. * toplev.c (flag_excess_precision_cmdline, flag_excess_precision, init_excess_precision): New. (lang_dependent_init_target): Call init_excess_precision. * tree.c (excess_precision_type): New. * tree.h (excess_precision_type): Declare. ada: * gcc-interface/misc.c (gnat_post_options): Set flag_excess_precision_cmdline. fortran: * options.c (gfc_post_options): Set flag_excess_precision_cmdline. java: * lang.c (java_post_options): Set flag_excess_precision_cmdline. testsuite: * gcc.target/i386/excess-precision-1.c, gcc.target/i386/excess-precision-2.c, gcc.target/i386/excess-precision-3.c, gcc.target/i386/excess-precision-4.c, gcc.target/i386/excess-precision-5.c, gcc.target/i386/excess-precision-6.c: New tests. Added: branches/c-4_5-branch/gcc/ada/ChangeLog.c45 branches/c-4_5-branch/gcc/fortran/ChangeLog.c45 branches/c-4_5-branch/gcc/java/ChangeLog.c45 branches/c-4_5-branch/gcc/testsuite/gcc.target/i386/excess-precision-1.c branches/c-4_5-branch/gcc/testsuite/gcc.target/i386/excess-precision-2.c branches/c-4_5-branch/gcc/testsuite/gcc.target/i386/excess-precision-3.c branches/c-4_5-branch/gcc/testsuite/gcc.target/i386/excess-precision-4.c branches/c-4_5-branch/gcc/testsuite/gcc.target/i386/excess-precision-5.c branches/c-4_5-branch/gcc/testsuite/gcc.target/i386/excess-precision-6.c Modified: branches/c-4_5-branch/gcc/ChangeLog.c45 branches/c-4_5-branch/gcc/ada/gcc-interface/misc.c branches/c-4_5-branch/gcc/c-common.c branches/c-4_5-branch/gcc/c-common.def branches/c-4_5-branch/gcc/c-cppbuiltin.c branches/c-4_5-branch/gcc/c-lex.c branches/c-4_5-branch/gcc/c-opts.c branches/c-4_5-branch/gcc/c-parser.c
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #122 from pinskia at gcc dot gnu dot org 2008-09-06 18:01 --- *** Bug 37390 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||doko at ubuntu dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #119 from mbrudka at filbico dot pl 2008-07-17 10:45 ---
Another example related with fp on x87?
EXPECTED RESULT:
0 (with EPS accuracy)
0 (with EPS accuracy)
0 (with EPS accuracy)
0 (with EPS accuracy)
REAL RESULT:
5.313991e+33
5.313991e+33
0.00e+00
0.00e+00
CODE
#include stdio.h
int main( void )
{
/* register */ double d1 = 1e50;
/* register */ double d2 = -2.7438011834107752e+51;
/* register */ double s = 0.036445789368634796;
/* register */ double d3 = -d1/s;
/* register */ double d4 = s*d2;
/* register */ double d5 = s*d3;
printf( %e\n, d1 + s*d2);
printf( %e\n, d1 + s*d3);
printf( %e\n, d1 + d4);
printf( %e\n, d1 + d5);
return 0;
}
--
mbrudka at filbico dot pl changed:
What|Removed |Added
CC||mbrudka at filbico dot pl
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #120 from vincent at vinc17 dot org 2008-07-17 12:41 --- (In reply to comment #119) REAL RESULT: 5.313991e+33 5.313991e+33 0.00e+00 0.00e+00 Only without optimizations. But since the ISO C standard allows expressions to be evaluated in a higher precision, there's no bug here (unless you show a contradiction with the value of FLT_EVAL_METHOD, but the FP_CONTRACT pragma should also be set to OFF -- though this currently has no effect on gcc). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #121 from mbrudka at filbico dot pl 2008-07-17 12:51 --- Thank you Vincent. I fact after commenting I realized that this is a plain numerical error on the last digit of double in multiplication. I think that my comment was rather irrelevant and I am the more ashamed the more I cannot remove it from bugzilla :) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #117 from pepalogik at seznam dot cz 2008-06-24 20:12 --- (In reply to comment #116) Yes, but this requires quite a complicated workaround (solution (4) in my comment #109). The problem is on the compiler side, which could store every result of a cast or an assignment to memory (this is inefficient, but that's what you get with the x87, and the ISO C language could be blamed too for *requiring* something like that instead of being more flexible). So you could say that the IEEE754 double precision type is available even on a processor without any FPU because this can be emulated using integers. Yes, but a conforming implementation would be the processor + a library, not just the processor with its instruction set. Moreover, if we assess things pedantically, the workaround (4) still doesn't fully obey the IEEE single/double precision type(s), because there remains the problem of double rounding of denormals. As I said, in this particular case (underflow/overflow), double rounding is allowed by the IEEE standard. It may not be allowed by some languages (e.g. XPath, and Java in some mode) for good or bad reasons, but this is another problem. OK, thanks for explanation. I think now it's clear. I quote, too: Applies To Microsoft#174; Visual C++#174; Now I assume that it follows the MS-Windows API (though nothing is certain with Microsoft). And the other compilers under MS-Windows could (or should) do the same thing. By a lucky hit, I have found this in the GCC documentation: -mpc32 -mpc64 -mpc80 Set 80387 floating-point precision to 32, 64 or 80 bits. When '-mpc32' is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); '-mpc64' rounds the the significands of results of floatingpoint operations to 53 bits (double precision) and '-mpc80' rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. [...] So GCC sets extended precision by default. And it's easy to change it. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #118 from vincent at vinc17 dot org 2008-06-24 20:45 --- (In reply to comment #117) By a lucky hit, I have found this in the GCC documentation: -mpc32 -mpc64 -mpc80 OK, this is new in gcc 4.3. I haven't tried, but if gcc just changes the precision without changing the values of float.h macros to make them correct, this is just a workaround (better than nothing, though). Also, this is a problem for library code if it requires to have double precision instead of extended precision, as these options won't probably be taken into account at that point. (Unfortunately it's probably too late to have a clean ABI.) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #114 from pepalogik at seznam dot cz 2008-06-22 16:59 --- (In reply to comment #113) It is available when storing a result to memory. Yes, but this requires quite a complicated workaround (solution (4) in my comment #109). So you could say that the IEEE754 double precision type is available even on a processor without any FPU because this can be emulated using integers. Moreover, if we assess things pedantically, the workaround (4) still doesn't fully obey the IEEE single/double precision type(s), because there remains the problem of double rounding of denormals. The IEEE754-1985 allows this. Section 4.3: Normally, a result is rounded to the precision of its destination. However, some systems deliver results only to double or extended destinations. On such a system the user, which may be a high-level language compiler, shall be able to specify that a result be rounded instead to single precision, though it may be stored in the double or extended format with its wider exponent range. [...] [...] AFAIK, the IEEE754-1985 standard was designed from the x87 implementation, so it would have been very surprising that x87 didn't conform to IEEE754-1985. So it seems I was wrong but the IEEE754-1985 standard is also quite wrong. Do you mean that on Windows, long double has (by default) no more precision than double? I don't think so (it's confirmed by my experience). I don't remember my original reference, but here's a new one: http://msdn.microsoft.com/en-us/library/aa289157(vs.71).aspx In fact, this depends on the architecture. I quote: x86. Intermediate expressions are computed at the default 53-bit precision with an extended range [...] I quote, too: Applies To Microsoft#174; Visual C++#174; -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #115 from pepalogik at seznam dot cz 2008-06-22 17:28 --- That #174; should be (R). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #116 from vincent at vinc17 dot org 2008-06-22 21:14 --- (In reply to comment #114) Yes, but this requires quite a complicated workaround (solution (4) in my comment #109). The problem is on the compiler side, which could store every result of a cast or an assignment to memory (this is inefficient, but that's what you get with the x87, and the ISO C language could be blamed too for *requiring* something like that instead of being more flexible). So you could say that the IEEE754 double precision type is available even on a processor without any FPU because this can be emulated using integers. Yes, but a conforming implementation would be the processor + a library, not just the processor with its instruction set. Moreover, if we assess things pedantically, the workaround (4) still doesn't fully obey the IEEE single/double precision type(s), because there remains the problem of double rounding of denormals. As I said, in this particular case (underflow/overflow), double rounding is allowed by the IEEE standard. It may not be allowed by some languages (e.g. XPath, and Java in some mode) for good or bad reasons, but this is another problem. I quote, too: Applies To Microsoft#174; Visual C++#174; Now I assume that it follows the MS-Windows API (though nothing is certain with Microsoft). And the other compilers under MS-Windows could (or should) do the same thing. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #112 from pepalogik at seznam dot cz 2008-06-21 22:38 --- (In reply to comment #111) Concerning the standards: The x87 FPU does obey the IEEE754-1985 standard, which *allows* extended precision, and double precision is *available*. It's true that double *precision* is available on x87. But not the *IEEE-754 double precision type*. Beside the precision of mantissa, this includes also the range of exponent. On the x87, it is possible to set the precision of mantissa but not the range of exponent. That's why I believe it doesn't obey the IEEE. (I haven't ever seen the IEEE-754 standard but I base on the work of David Monniaux.) Note: the solution chosen by some OS'es (*BSD, MS-Windows...) is to configure the processor to the IEEE double precision by default (thus long double is also in double precision, but this is OK as far as the C language is concerned, there's still a problem with float, but in practice, nobody cares AFAIK). Do you mean that on Windows, long double has (by default) no more precision than double? I don't think so (it's confirmed by my experience). According to the paper of David Monniaux, only FreeBSD 4 sets double precision by default (but I know almost nothing about BSD). (1) A very simple solution: Use long double everywhere. This avoids the bug, but this is not possible for software that requires double precision exactly, e.g. XML tools that use XPath. Yes, of course. I don't say this can be used everywhere. (But be careful when transfering binary data in long double format between computers because this format is not standardized and so the concrete bit representations vary between different CPU architectures.) Well, this is not specific to long double anyway: there exist 3 possible endianess for the double format (x86, PowerPC, ARM). OK but David Monniaux mentions portability issues just in the case of long double, so the differences are probably more frequent in this case (maybe even within the x86 architecture). Yes, but note that this is not the only problem with compilers. See e.g. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578 Thanks for info. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #113 from vincent at vinc17 dot org 2008-06-22 00:52 --- (In reply to comment #112) It's true that double *precision* is available on x87. But not the *IEEE-754 double precision type*. It is available when storing a result to memory. Beside the precision of mantissa, this includes also the range of exponent. On the x87, it is possible to set the precision of mantissa but not the range of exponent. The IEEE754-1985 allows this. Section 4.3: Normally, a result is rounded to the precision of its destination. However, some systems deliver results only to double or extended destinations. On such a system the user, which may be a high-level language compiler, shall be able to specify that a result be rounded instead to single precision, though it may be stored in the double or extended format with its wider exponent range. [...] That's why I believe it doesn't obey the IEEE. (I haven't ever seen the IEEE-754 standard but I base on the work of David Monniaux.) See above. Also beware of subtilities in the wording used by David Monniaux. FYI, the IEEE754-1985 standard (with minor corrections) is available from the following page: http://www.validlab.com/754R/ (look at the end). AFAIK, the IEEE754-1985 standard was designed from the x87 implementation, so it would have been very surprising that x87 didn't conform to IEEE754-1985. Do you mean that on Windows, long double has (by default) no more precision than double? I don't think so (it's confirmed by my experience). I don't remember my original reference, but here's a new one: http://msdn.microsoft.com/en-us/library/aa289157(vs.71).aspx In fact, this depends on the architecture. I quote: x86. Intermediate expressions are computed at the default 53-bit precision with an extended range provided by a 16-bit exponent. When these 53:16 values are spilled to memory (as can happen during a function call), the extended exponent range will be narrowed to 11-bits. That is, spilled values are cast to the standard double precision format with only an 11-bit exponent. A user may switch to extended 64-bit precision for intermediate rounding by altering the floating-point control word using _controlfp and by enabling FPU environment access (see The fpenv_access Pragma). However, when extended precision register-values are spilled to memory, the intermediate results will still be rounded to double precision. This particular semantic is subject to change. Note that the behavior has changed in some version of Windows (it was using the extended precision, then it switched to double precision for x86). Now, this may also depend on the compiler. According to the paper of David Monniaux, only FreeBSD 4 sets double precision by default (but I know almost nothing about BSD). I've noted that amongst the BSD's, NetBSD does this too (I don't remember if I've tried or got it from some document, and this might also depend on the NetBSD version and/or the processor). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #111 from vincent at vinc17 dot org 2008-06-20 16:09 --- (In reply to comment #109) WHERE'S THE BUG This is really not a GCC bug. The bug is actually in the x87 FPU because it doesn't obey the IEEE standard. Concerning the standards: The x87 FPU does obey the IEEE754-1985 standard, which *allows* extended precision, and double precision is *available*. In fact, one could say that GCC even obeys the IEEE standard (which doesn't define bindings: the definition of destination page 4 of the IEEE754-1985 standard is rather vague and lets the language to define it exactly), but it doesn't obey the ISO C99 standard on some point. Concerning the x87 FPU: One can say however that the x87 is a badly designed because it is not possible to statically specify the precision. Nevertheless the OS/language implementations should take care of this problem. Note: the solution chosen by some OS'es (*BSD, MS-Windows...) is to configure the processor to the IEEE double precision by default (thus long double is also in double precision, but this is OK as far as the C language is concerned, there's still a problem with float, but in practice, nobody cares AFAIK). If you wish to compile for processors which don't have SSE, you have a few possibilities: (1) A very simple solution: Use long double everywhere. This avoids the bug, but this is not possible for software that requires double precision exactly, e.g. XML tools that use XPath. See other examples here: http://www.vinc17.org/research/extended.en.html Also this makes maintenance of software more difficult because long double can be much slower on some platforms, which support this type in software to provide more precision (e.g. PowerPC Linux and Mac OS X implement a double-double arithmetic, Solaris and HPUX implement quadruple precision). (But be careful when transfering binary data in long double format between computers because this format is not standardized and so the concrete bit representations vary between different CPU architectures.) Well, this is not specific to long double anyway: there exist 3 possible endianess for the double format (x86, PowerPC, ARM). (2) A partial but simple solution: Do comparisons on volatile variables only. Yes (but this is also a problem concerning the maintenance of portable programs). (4) A complex solution: [...] Yes, this is the workaround I use in practice. RECOMMENDATIONS I think this problem is really serious and general. Therefore, programmers should be warned soon enough. Yes, but note that this is not the only problem with compilers. See e.g. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36578 for a bug related to casts to long double on x86_64 and ia64. This one is now tested by: http://www.vinc17.org/software/tst-ieee754.c (which has also tested bug 323 for a long time). -- vincent at vinc17 dot org changed: What|Removed |Added CC||vincent at vinc17 dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #110 from pepalogik at seznam dot cz 2008-06-12 14:14 --- I used an old version of GCC documentation so I omitted some new processors with SSE: core2, k8-sse3, opteron-sse3, athlon64-sse3, amdfam10 and barcelona. I think you can use -march=pentium3 for all Intel's CPUs (of course, starting with P3). I'm unsure about AMD. (Maybe you know it better.) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #109 from pepalogik at seznam dot cz 2008-05-20 16:59 ---
I also encountered such problems and was going to report it as a bug in GCC...
But in the GCC bug (not) reporting guide, there is fortunately a link to this
page and here (comment #96) is a link to David Monniaux's paper about
floating-point computations. This explains it closely but it is maybe too long.
I have almost read it and hope I have understood it properly. So I'll give a
brief explanation (for those who don't know it yet) of the reasons of such a
strange behaviour. Then I'll assess where the bug actually is (in GCC or CPU).
Then I'll write the solution (!) and finally a few recommendations to the GCC
team.
EXPLANATION
The x87 FPU was originally designed in (or before) 1980. I think that's why it
is quite simple: it has only one unit for all FP data types. Of course, the
precision must be of the widest type, which is the 80-bit long double.
Consider you have a program, where all the FP variables are of the type double.
You perform some FP operations and one of them is e.g. 1e-300/1e300, which
results in 1e-600. Despite this value cannot be held by a double, it is
stored in an 80-bit FPU register as the result. Consider you use the variable
x to hold that result. If the program has been compiled with optimization,
the value need not be stored in RAM. So, say, it is still in the register.
Consider you need x to be nonzero, so you perform the test x != 0. Since 1e-600
is not zero, the test yields true. While you perform some other computations,
the value is moved to RAM and converted to 0 because x is of type double. Now
you want to use your certainly nonzero x... Hard luck :-(
Note that if the result doesn't have its corresponding variable and you perform
the test directly on an expression, the problem can come to light even without
optimization.
It could seem that performing all FP operations in extended precision can bring
benefits only. But it introduces a serious pitfall: moving a value may change
the value!!!
WHERE'S THE BUG
This is really not a GCC bug. The bug is actually in the x87 FPU because it
doesn't obey the IEEE standard.
SOLUTION
The x87 FPU is still present in contemporary processors (including AMD) due to
compatibility. I think most of PC software still uses it. But new processors
have also another FPU, called SSE, and this do obey the IEEE. GCC in 32-bit
mode compiles for x87 by default but it is able to compile for the SSE, too. So
the solution is to add these options to the compilation command:
-march=* -msse -mfpmath=sse
Yes, this definitely resolves the problem - but not for all processors. The *
can be one of the following: pentium3, pentium3m, pentium-m, pentium4,
pentium4m, prescott, nocona, athlon-4, athlon-xp, athlon-mp, k8, opteron,
athlon64, athlon-fx and c3-2 (I'm unsure about athlon and athlon-tbird). Beside
-msse, you can also add some of -mmmx, -msse2, -msse3 and -m3dnow, if the CPU
supports them (see GCC doc or CPU doc).
If you wish to compile for processors which don't have SSE, you have a few
possibilities:
(1) A very simple solution: Use long double everywhere. (But be careful when
transfering binary data in long double format between computers because this
format is not standardized and so the concrete bit representations vary between
different CPU architectures.)
(2) A partial but simple solution: Do comparisons on volatile variables only.
(3) A similar solution: Try to implement a discard_extended_precision
function suggested by Egon in comment #88.
(4) A complex solution: Before doing any mathematical operation or comparison,
put the operands into variables and put also the result to a variable (i.e.
don't use complex expressions). For example, instead of { c = 2*(a+b); } ,
write { double s = a+b; c = 2*s; } . I'm unsure about arrays but I think they
should be OK. When you have modified your code in this manner, then compile it
either without optimization or, when using optimization, use -ffloat-store. In
order to avoid double rounding (i.e. rounding twice), it is also good to
decrease the FPU precision by changing its control word in the beginning of
your program (see comment #60). Then you should also apply -frounding-math.
(5) A radical solution: Find a job/hobby where computers are not used at all.
RECOMMENDATIONS
I think this problem is really serious and general. Therefore, programmers
should be warned soon enough.
This recommendation should be addressed especially to authors of programming
coursebooks. But I think there could also be a paragraph about it in the GCC
documentation (I haven't read it wholly but it doesn't seem there's any warning
against x87). And, of course, there should be a warning in the bug reporting
guide (http://gcc.gnu.org/bugs.html). It's fine there's a link to this page
(Bug 323) but the example with (int)(a/b) is insufficient. It only demonstrates
that real numbers are often not represented exactly in the computer. It doesn't
demonstrate the
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #108 from rguenth at gcc dot gnu dot org 2008-03-14 17:30 --- *** Bug 35585 has been marked as a duplicate of this bug. *** -- rguenth at gcc dot gnu dot org changed: What|Removed |Added CC||nicos at maunakeatech dot ||com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #105 from pinskia at gcc dot gnu dot org 2008-03-06 22:44 --- *** Bug 35488 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||purnnam1 at naver dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #106 from rguenth at gcc dot gnu dot org 2008-03-06 23:14 --- *** Bug 35488 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #107 from pinskia at gcc dot gnu dot org 2008-03-06 23:58 --- *** Bug 35489 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #103 from pinskia at gcc dot gnu dot org 2008-02-27 20:38 --- *** Bug 35389 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||ywei at qualcomm dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #104 from ywei at qualcomm dot com 2008-02-27 21:41 --- Subject: RE: optimized code gives strange floating point results Not sure this is the same issues as 323. All three numbers, 8, 1 and 65, should be able to represented in IEEE 754 floating-point format exactly without any rounding or approximation. That is 8 = 1* 2^3 1 = 1* 2^0 65 = (1 + 1/64) * 2^6 -Original Message- From: pinskia at gcc dot gnu dot org [mailto:[EMAIL PROTECTED] Sent: Wednesday, February 27, 2008 12:39 PM To: Wei, Yongbin Subject: [Bug rtl-optimization/323] optimized code gives strange floating point results --- Comment #103 from pinskia at gcc dot gnu dot org 2008-02-27 20:38 --- *** Bug 35389 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||ywei at qualcomm dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323 --- You are receiving this mail because: --- You are on the CC list for the bug, or are watching someone who is. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #102 from pinskia at gcc dot gnu dot org 2008-01-27 23:17 --- *** Bug 34992 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||roebel at ircam dot fr http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #101 from pinskia at gcc dot gnu dot org 2008-01-24 04:51 --- *** Bug 34951 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||tterribe at xiph dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #99 from macracan at yahoo dot com 2007-10-01 17:43 --- *** Bug 33611 has been marked as a duplicate of this bug. *** -- macracan at yahoo dot com changed: What|Removed |Added CC||macracan at yahoo dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #98 from sliwa at cft dot edu dot pl 2007-08-03 12:09 --- *** Bug 32976 has been marked as a duplicate of this bug. *** -- sliwa at cft dot edu dot pl changed: What|Removed |Added CC||sliwa at cft dot edu dot pl http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #97 from pinskia at gcc dot gnu dot org 2007-06-19 08:11 --- *** Bug 32391 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||sunjoong at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #96 from David dot Monniaux at ens dot fr 2007-04-20 21:19 --- The following paper explains how this kind of behaviour occurs, why it is correct, why it is difficult to fix but how it can be partly fixed, and how this breaks many testing and proving techniques: http://hal.archives-ouvertes.fr/hal-00128124 -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #95 from guillaume dot melquiond at ens-lyon dot fr 2007-04-03 17:51 --- I think that Uros' patch to add a -mpc switch for precision control would fix this. The real fix would be to automatically insert fldcw instructions before float/double operations, in order to limit the precision of the operations. But I think that it would kill speed even more than -ffloat-store. Unfortunately, it is not that simple. The -mpc switch and the fldcw instructions control the size of the significant, but they don't control the range of the exponent. So it will solve the issue with the first testcase of this bug-report, but you could still build examples where two execution paths that perform the same floating-point computations produce completely different results. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #94 from bonzini at gnu dot org 2007-04-02 16:20 --- I think that Uros' patch to add a -mpc switch for precision control would fix this. The real fix would be to automatically insert fldcw instructions before float/double operations, in order to limit the precision of the operations. But I think that it would kill speed even more than -ffloat-store. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #91 from pinskia at gcc dot gnu dot org 2007-03-09 20:11 --- *** Bug 31114 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||terry at chem dot gu dot se http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #92 from whaley at cs dot utsa dot edu 2007-03-09 20:22 --- I'd like to welcome the newest members of the bug 323 community, where all x87 floating point errors in gcc come to die! All floating point errors that use the x87 are welcome, despite the fact that many of them are easily fixable, and many are not! We're all one happy family, making the egregious mistake of wanting accuracy out of the most accurate general purpose FPU on the market! Cheers, Clint -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #93 from pinskia at gcc dot gnu dot org 2007-03-09 22:45 --- *** Bug 31114 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #90 from rguenth at gcc dot gnu dot org 2007-03-01 16:43 --- *** Bug 31008 has been marked as a duplicate of this bug. *** -- rguenth at gcc dot gnu dot org changed: What|Removed |Added CC||gcczilla at achurch dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #89 from brooks at gcc dot gnu dot org 2007-02-10 00:55 --- *** Bug 30752 has been marked as a duplicate of this bug. *** -- brooks at gcc dot gnu dot org changed: What|Removed |Added CC||brooks at gcc dot gnu dot ||org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #87 from bruno at clisp dot org 2006-12-21 15:08 ---
The option -ffloat-store, recommended by Richard Henderson, has the effect of
decreasing the performance of floating-point operations for the entire
compilation unit. If you want a minimal fix that does not affect other
functions in the same compilation unit, you can use 'volatile double'
instead of 'double'. It's like a one-shot -ffloat-store. Example:
#include stdio.h
void test(double x, double y)
{
const volatile double y2 = x + 1.0;
if (y != y2) printf(error\n);
}
void main()
{
const double x = .012;
const double y = x + 1.0;
test(x, y);
}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #85 from pinskia at gcc dot gnu dot org 2006-12-18 20:16 --- *** Bug 30255 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||whaley at cs dot utsa dot ||edu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #86 from pinskia at gcc dot gnu dot org 2006-12-18 22:04 --- *** Bug 30255 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #84 from pinskia at gcc dot gnu dot org 2006-10-25 22:05 --- *** Bug 29597 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||brad_atcheson at yahoo dot ||ca http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #83 from aph at gcc dot gnu dot org 2006-08-02 10:56 --- *** Bug 16825 has been marked as a duplicate of this bug. *** -- aph at gcc dot gnu dot org changed: What|Removed |Added CC||mark at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #82 from pinskia at gcc dot gnu dot org 2006-06-28 18:49 --- *** Bug 28191 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||rozenman at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #81 from pinskia at gcc dot gnu dot org 2006-01-28 03:56 --- *** Bug 26000 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||pr2345 at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #80 from pinskia at gcc dot gnu dot org 2005-12-08 01:47 --- *** Bug 25303 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||hjl at lucon dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #79 from pinskia at gcc dot gnu dot org 2005-11-10 03:41 --- *** Bug 7935 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||sk2alexa at math dot ||uwaterloo dot ca http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #78 from pinskia at gcc dot gnu dot org 2005-10-21 20:24 --- *** Bug 24479 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||dyang at mathworks dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Comment #77 from pinskia at gcc dot gnu dot org 2005-10-15 18:34 --- *** Bug 24387 has been marked as a duplicate of this bug. *** -- pinskia at gcc dot gnu dot org changed: What|Removed |Added CC||neff dot kevin at mayo dot ||edu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-09-22 15:48 --- *** Bug 24014 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||u dot strempel at gmx dot de http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-08-16 13:22 --- *** Bug 23318 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-08-15 21:23 --- *** Bug 23407 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||dir at lanl dot gov http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-08-09 15:59 --- *** Bug 23298 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||dsell at agleader dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-07-10 18:09 --- *** Bug 22394 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||glenn at aoi-industries dot ||com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From steven at gcc dot gnu dot org 2005-06-19 11:05 --- Reopening... -- What|Removed |Added Status|RESOLVED|REOPENED Resolution|INVALID | http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From steven at gcc dot gnu dot org 2005-06-19 11:09 --- ...to end this pointless discussion. Some people call this a bug in the x87 series. Other call it a bug in gcc. See these mails at least for the reason why this could be considered a bug in gcc: http://gcc.gnu.org/ml/gcc/2003-08/msg01195.html http://gcc.gnu.org/ml/gcc/2003-08/msg01234.html http://gcc.gnu.org/ml/gcc/2003-08/msg01257.html Regardless of where one wishes to put the blame, this problem will _not_ be fixed. Period. -- What|Removed |Added Status|REOPENED|SUSPENDED http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-06-19 12:18 --- *** Bug 1098 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||green at rhythm dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 16:24 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||themis_hv at yahoo dot co ||dot uk http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 18:27 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 19:06 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 19:24 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 19:35 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 19:43 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 19:47 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-05-29 19:56 --- *** Bug 21809 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From mario dot tragni at planetek dot it 2005-05-20 08:40 --- (In reply to comment #2) State-Changed-From-To: open-closed State-Changed-Why: See any faq on numerical analysis that mentions the x86. You are seeing the results of excess precision in the FPU. Either change the rounding precision in the FPCR, or work around the problem with -ffloat-store. I had this bug on x86 architecture, with no optimization of the code (no -OX) and with float-store on. My workaround was to store the return of the double function in a auxliar double variable before comparison. Have you an other suggestion ? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From cognot at earthdecision dot com 2005-05-20
10:03 ---
(In reply to comment #59)
I had this bug on x86 architecture, with no optimization of the code (no -OX)
and with float-store on. My workaround was to store the return of the double
function in a auxliar double variable before comparison.
Have you an other suggestion ?
The way I've fixed (more like avoided) this problem is to have:
#include fpu_control.h
void set_math_double_precision() {
fpu_control_t fpu_control = 0x027f ;
_FPU_SETCW(fpu_control);
}
and make sure this function is called before doing any FP operations. It only
needs to be called once.
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-04-20 03:00 --- *** Bug 7719 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||petr dot savicky at cuni dot ||cz http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-03-29 14:17 --- *** Bug 20674 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-03-28 21:53 --- *** Bug 20674 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||piaget at us dot ibm dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-02-17 13:37 --- *** Bug 20026 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||denis dot nagorny at intel ||dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-02-09 06:35 --- *** Bug 19837 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||wirawan0 at softhome dot net http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-01-28 18:47 --- *** Bug 19675 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||cognot at earthdecision dot ||com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-01-28 19:07 --- *** Bug 19675 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-01-17 15:40 --- *** Bug 19469 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||debian-gcc at lists dot ||debian dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-15 14:05 --- *** Bug 19011 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||konstantin at mysql dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-15 15:10 --- *** Bug 19011 has been marked as a duplicate of this bug. *** -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-02 14:22 --- *** Bug 18784 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||Graham dot Murphy at jhuapl ||dot edu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2004-11-19 15:33 --- *** Bug 18567 has been marked as a duplicate of this bug. *** --- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-01 15:12 --- *** Bug 18756 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||eda-qa at disemia dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From eda-qa at disemia dot com 2004-12-01 16:24 --- To summarize, this defect effectively states that: assert( (x/y) == (x/y) ) may cause an assertion if compiled with optimization. While I understand why it happens, that doesn't mean it isn't a defect. This makes it impossible to turn on the optimizer with any code using floating point and still expect to get a correct result. Perhaps in some situations this is okay, but in general this is not. This would also mean the following are also invalid code -- which I'm fairly certain the C/C++ standards would state otherwise: a = (x/y); assert( a == x/y ) //may Abort if( a == x/y ) assert( a == x/y ) //may Abort -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
[Bug rtl-optimization/323] optimized code gives strange floating point results
--- Additional Comments From pinskia at gcc dot gnu dot org 2004-11-19 15:32 --- *** Bug 18567 has been marked as a duplicate of this bug. *** -- What|Removed |Added CC||benoit dot sibaud at rd dot ||francetelecom dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
