[Bug c++/50364] New: Volatile miscompilation
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50364 Bug #: 50364 Summary: Volatile miscompilation Classification: Unclassified Product: gcc Version: 4.6.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: abramobagn...@tin.it $ cat p.cc volatile int a, b; void f() { b = 1 + (a = 0); } $ g++ -O2 -S p.cc $ cat p.s .file "p.cc" .text .p2align 4,,15 .globl _Z1fv .type _Z1fv, @function _Z1fv: .LFB0: .cfi_startproc movl$0, a movl$1, b ret .cfi_endproc ... As the typescript above shows, the conversion between lvalue (a = 0) and relative rvalue does not generate a volatile read memory access unlike what I think that standard mandates.
[Bug target/37581] IEEE inexact-flag not working on the Alpha
--- Comment #6 from abramobagnara at tin dot it 2009-01-24 15:41 --- (In reply to comment #5) > Because it looks to me that this is libc problem with fetestexcept. As far as > the compiler is concerned, GCC correctly decorates trapping instructions with > /sui and puts trap barrier to expected place: > > ... > jsr $26,($27),__nldbl_printf!lituse_jsr!5 > ldah $29,0($26) !gpdisp!6 > divs/sui $f3,$f2,$f0 > trapb > ... I've reported a bug according to the valuable info you provide. http://sourceware.org/bugzilla/show_bug.cgi?id=9783 -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581
[Bug middle-end/38960] Wrong floating point reorder
--- Comment #1 from abramobagnara at tin dot it 2009-01-24 15:14 --- Created an attachment (id=17176) --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17176&action=view) Assembler generated by gcc -S -O2 bug.c -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38960
[Bug c/38960] New: Wrong floating point reorder
The program below show that gcc reorder floating point instructions in such a way to make inexact checking fruitless. Reading generated assembler I see two problems: 1) the cast to float in x assignment is executed *after* fetestexcept and not before as it's written (and needed to get the correct result). This infringes C99 standard sequence point rules. 2) the second division is not recomputed (because CSE), then inexact flag is not changed after feclearexcept I guess that the latter is due to missing #pragma STDC FENV_ACCESS implementation, but the former undermine the whole fetestexcept usability. $ cat bug.c #include #include double vf = 0x0fff; double vg = 0x1000; /* vf/vg is exactly representable as IEC559 64 bit floating point, while it's not representable exactly as a 32 bit one */ int main() { double a = vf; double b = vg; feclearexcept(FE_INEXACT); float x; x = a / b; printf("%i %.1000g\n", fetestexcept(FE_INEXACT), x); feclearexcept(FE_INEXACT); double y; y = a / b; printf("%i %.1000g\n", fetestexcept(FE_INEXACT), y); return 0; } $ gcc -O2 bug.c -lm $ ./a.out 0 1 0 0.62747097015380859375 $ -- Summary: Wrong floating point reorder Product: gcc Version: 4.3.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: abramobagnara at tin dot it GCC target triplet: i486-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38960
[Bug target/37581] IEEE inexact-flag not working on the Alpha
--- Comment #3 from abramobagnara at tin dot it 2009-01-24 05:43 --- The test case you have used it's different from the original that showed the bug. Nevertheless it's useful to understand the possible nature of the bug. It seems that if feenableexcept(FE_INEXACT) is not called fetestexcept(FE_INEXACT) doesn't work as expected (and as C99 standard provides). Please note that according to documentation GNU extension feenableexcept/fedisablexcept does not enable/disable inexact detection, but does enable/disable inexact *trapping* via SIGFPE signal. Here below there is a new test case that shows the wrong behaviour. $ cat sf2.c #define _GNU_SOURCE #include #include #include static void foo (int sig) { printf("inexact\n"); } float __attribute__((noinline)) test (float x) { printf("%f / %f\n", 2.0f, x); return 2.0f / x; } void t() { volatile float x; feclearexcept (FE_ALL_EXCEPT); x = test (3.0f); printf("fetestexcept(FE_INEXACT) = %d\n", fetestexcept(FE_INEXACT)); feclearexcept (FE_ALL_EXCEPT); x = test (2.0f); printf("fetestexcept(FE_INEXACT) = %d\n", fetestexcept(FE_INEXACT)); } int main() { printf("\nWith FE_INEXACT SIGFPE disabled\n"); t(); printf("\nWith FE_INEXACT SIGFPE enabled\n"); signal (SIGFPE, foo); feenableexcept (FE_INEXACT); t(); } $ gcc -O2 -lm -mieee-with-inexact sf2.c $ ./a.out With FE_INEXACT SIGFPE disabled 2.00 / 3.00 fetestexcept(FE_INEXACT) = 0 2.00 / 2.00 fetestexcept(FE_INEXACT) = 0 With FE_INEXACT SIGFPE enabled 2.00 / 3.00 inexact fetestexcept(FE_INEXACT) = 2097152 2.000000 / 2.00 fetestexcept(FE_INEXACT) = 0 $ -- abramobagnara at tin dot it changed: What|Removed |Added ---------------- CC||abramobagnara at tin dot it http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37581