[Bug middle-end/44081] Incorrect nonnull assumed in code generation
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081 Martin Sebor changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED CC||msebor at gcc dot gnu.org Resolution|--- |FIXED --- Comment #15 from Martin Sebor --- I'm unable to reproduce any unexpected/erroneous behavior with the test cases submitted in comment #11 and later. Recent versions of GCC (5.x and beyond) all print the same output (marked <- Ok in the comments). Thus resolving as fixed.
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081 --- Comment #14 from Hugo van der Sanden --- (In reply to Manuel López-Ibáñez from comment #13) > Should we close this? With what status? I think it should at least be updated to CONFIRMED, based on the comments from valeriy. I should note also that I was discouraged by the quality of response, and in particular that I never got a reply to #9.
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081 Manuel López-Ibáñez changed: What|Removed |Added CC||manu at gcc dot gnu.org --- Comment #13 from Manuel López-Ibáñez --- Should we close this?
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
--- Comment #12 from Valeriy 2011-01-20 18:23:29 UTC ---
/*attr_nn.c*/
#include
static void attr_nn(int i1, int *i2, int *i3) __attribute__((__nonnull__(2)));
static void attr_nn(int i1, int *i2, int *i3) {
*i2 = i1;
if (i3 == 0 || i3 == (int *) 8) printf("%p if (0 or 8)\n", i3);
else printf("%p else \n", i3);
}
int i2a;
int main(int argc, char *argv[]) {
attr_nn(1, &i2a, (int *) 0);
attr_nn(2, &i2a, (int *) 4);
attr_nn(3, &i2a, (int *) 8);
return 0;
}
/*
-
Ubuntu GCC-4.4.5 - Error
$ gcc -O0 attr_nn.c && ./a.out
$ gcc -O1 attr_nn.c && ./a.out
$ gcc -O3 attr_nn.c && ./a.out
(nil) if (0 or 8) <- Ok
0x4 else
0x8 if (0 or 8)
$ gcc -O2 attr_nn.c && ./a.out
$ gcc -Os attr_nn.c && ./a.out
(nil) else <- Error
0x4 else
0x8 if (0 or 8)
$ gcc -v
Using built-in specs.
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.4.4-14ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc
--enable-targets=all --disable-werror --with-arch-32=i686 --with-tune=generic
--enable-checking=release --build=i686-linux-gnu --host=i686-linux-gnu
--target=i686-linux-gnu
Thread model: posix
gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5)
-
Cygwin GCC-4.3.4 - Ok
$ gcc -O0 attr_nn.c && ./a.exe
$ gcc -O1 attr_nn.c && ./a.exe
$ gcc -O2 attr_nn.c && ./a.exe
$ gcc -O3 attr_nn.c && ./a.exe
$ gcc -Os attr_nn.c && ./a.exe
0x0 if (0 or 8) <- Ok
0x4 else
0x8 if (0 or 8)
-
Cygwin GCC-4.5.0 - Error
$ gcc -O0 attr_nn.c && ./a.exe
$ gcc -O1 attr_nn.c && ./a.exe
$ gcc -O3 attr_nn.c && ./a.exe
0x0 if (0 or 8) <- Ok
0x4 else
0x8 if (0 or 8)
$ gcc -O2 attr_nn.c && ./a.exe
$ gcc -Os attr_nn.c && ./a.exe
0x0 else <- Error
0x4 else
0x8 if (0 or 8)
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-cygwin/4.5.0/lto-wrapper.exe
Target: i686-pc-cygwin
Configured with:
/gnu/gcc/releases/respins/4.5.0-1/gcc4-4.5.0-1/src/gcc-4.5.0/configure
--srcdir=/gnu/gcc/releases/respins/4.5.0-1/gcc4-4.5.0-1/src/gcc-4.5.0
--prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin
--libexecdir=/usr/lib --datadir=/usr/share --localstatedir=/var
--sysconfdir=/etc --datarootdir=/usr/share --docdir=/usr/share/doc/gcc4
--datadir=/usr/share --infodir=/usr/share/info --mandir=/usr/share/man -v
--with-gmp=/usr --with-mpfr=/usr --enable-bootstrap
--enable-version-specific-runtime-libs --libexecdir=/usr/lib --enable-static
--enable-shared --enable-shared-libgcc --disable-__cxa_atexit --with-gnu-ld
--with-gnu-as --with-dwarf2 --disable-sjlj-exceptions
--enable-languages=ada,c,c++,fortran,java,lto,objc,obj-c++ --enable-graphite
--enable-lto --enable-java-awt=gtk --disable-symvers --enable-libjava
--program-suffix=-4 --enable-libgomp --enable-libssp --enable-libada
--enable-threads=posix --with-arch=i686 --with-tune=generic
--enable-libgcj-sublibs CC=gcc-4 CXX=g++-4 CC_FOR_TARGET=gcc-4
CXX_FOR_TARGET=g++-4 GNATMAKE_FOR_TARGET=gnatmake GNATBIND_FOR_TARGET=gnatbind
--with-ecj-jar=/usr/share/java/ecj.jar
Thread model: posix
gcc version 4.5.0 (GCC)
-
MinGW GCC-4.4.3 (Strawberry Perl) - Error
>gcc -O0 attr_nn.c && a.exe
>gcc -O1 attr_nn.c && a.exe
>gcc -O3 attr_nn.c && a.exe
if (0 or 8)<- Ok
0004 else
0008 if (0 or 8)
>gcc -O2 attr_nn.c && a.exe
>gcc -Os attr_nn.c && a.exe
else <- Error
0004 else
0008 if (0 or 8)
>gcc -v
Using built-in specs.
Target: i686-w64-mingw32
Configured with: ../gcc44-svn/configure --target=i686-w64-mingw32
--host=i686-w64-mingw32 --disable-multilib --disable-nls
--disable-win32-registry --prefix=/mingw32 --with-gmp=/mingw32
-with-mpfr=/mingw32 --enable-languages=c,c++
Thread model: win32
gcc version 4.4.3 (GCC)
-
MinGW GCC-4.5.0 (20101030) - Error
>gcc -O0 attr_nn.c && a.exe
>gcc -O1 attr_nn.c && a.exe
>gcc -O3 attr_nn.c && a.exe
if (0 or 8)<- Ok
0004 else
0008 if (0 or 8)
>gcc -O2 attr_nn.c && a.exe
>gcc -Os attr_nn.c && a.exe
else <- Error
0004 else
0008 if (0 or 8)
gcc -v
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
Valeriy changed:
What|Removed |Added
CC||nordq at ya dot ru
--- Comment #11 from Valeriy 2010-12-26 22:57:48 UTC ---
/*
nonnull_2.c
gcc -O0 nonnull_2.c
gcc -O1 nonnull_2.c
gcc -O3 nonnull_2.c
if (0 or 8) <- Ok
0004 else
0008 if (0 or 8)
gcc -O2 nonnull_2.c
gcc -Os nonnull_2.c
else <- Error
0004 else
0008 if (0 or 8)
*/
#include
static void attribute_nonnull_2(int i1, int *i2, int *i3)
__attribute__((__nonnull__(2)));
static void attribute_nonnull_2(int i1, int *i2, int *i3) {
*i2 = i1;
if (i3 == 0 || i3 == (int *) 8)
printf("%p if (0 or 8)\n", i3);
else
printf("%p else \n", i3);
}
int i2a;
int main(int argc, char *argv[]) {
attribute_nonnull_2(1, &i2a, (int *) 0);
attribute_nonnull_2(2, &i2a, (int *) 4);
attribute_nonnull_2(3, &i2a, (int *) 8);
return 0;
}
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
--- Comment #10 from joseph at codesourcery dot com 2010-05-12 11:52 --- Subject: Re: Incorrect nonnull assumed in code generation On Wed, 12 May 2010, pmoulder at mail dot csse dot monash dot edu dot au wrote: > Coming back to memcpy etc.: As this is such an edge case, I'll give more > detail > for my (revised) reasoning (partly for the benefit of certain others that I > intend to refer to this discussion): It's a case about which the standard is completely explicit. 7.21.1#2. "Where an argument declared as size_t n specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4." -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
--- Comment #9 from hv at crypt dot org 2010-05-12 10:54 --- The direction of discussion has centred so far on the documentation, but as far as I can tell the only point at which the documentation confused someone was the triage at #3. Should there not be a separate bug opened for problems with the documentation? Can somone with access to a stock 4.5.0 confirm that the originally reported testcase does indeed show a bug? In #7, Jakub says: > 4.5.0 (rather than current 4.5 snapshots) had a bug with nonnull attribute > if a function is IPA optimized and some arguments are optimized out, but I > don't think that's the case here. Do you have a reference for this bug? What makes you think this isn't the same? Note that in the process of cutting down the original code to a testcase, one constant that I found had to be preserved was that the test() function be static, so that gcc had freedom to fiddle with the calling conventions; another was that the first parameter be preserved even though unused; another was that the second parameter needed its nonnull attribute. It sounds very similar to me. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
--- Comment #8 from pmoulder at mail dot csse dot monash dot edu dot au 2010-05-12 09:25 --- OK, on careful reading, I agree re memcpy etc. (see below). Consequently, I amend my suggested change to the documentation to: - Add a sentence about implicit `this' argument (copied & pasted from the format_arg entry). - Although largely taken care of by the above, I'd still be inclined to rename the attribute parameter from âarg-indexâ to âarg-numberâ; and similarly to rename âstring-indexâ to âstring-arg-numberâ elsewhere in the texinfo node (in format and format_arg items). - Reword âargument index listâ to âargument number listâ (in the nonnull item). - Despite retracting the âwrongly used in glibcâ grounds, I think the concerns raised on the page I linked to are sufficient to merit giving more emphasis to the "The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null" sentence: that it be put it in a paragraph by itself, and adding a warning that a consequence would be that any checks for NULL in the function may be optimized away. Coming back to memcpy etc.: As this is such an edge case, I'll give more detail for my (revised) reasoning (partly for the benefit of certain others that I intend to refer to this discussion): - C99 §7.21.2.1 [#2]: âThe memcpy function copies n [i.e. 0] characters from the object pointed to by s2 into the object pointed to by s1.â Text is similar for memcmp, memmove, memset, and their w- (wmemcmp etc.) counterparts, in each case referring to âthe object pointed to by s/s1/s2â; while text for qsort says âsorts an array of nmemb objects, the initial element of which is pointed to by baseâ. - C99 §6.3.2.3 [#3]: âSuch a pointer [as NULL], called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.â Thus, if s/s1/s2 is a null pointer, then âthe object pointed to by s/s1/s2â isn't defined, so the sentence doesn't apply (despite copying/comparing only 0 bytes/characters from or to this undefined thing); and in each case there's no other text giving the behaviour when s/s1/s2 is a null pointer; so I conclude that the behaviour of these functions is indeed undefined when s/s1/s2 is a null pointer, so the âundefined behaviourâ provisions of §3.18 apply, and optimizing out the test for NULL is conformant behaviour as an instance of âignoring the situation [of s/s1/s2 == NULL] completely with unpredictable resultsâ (§3.18 [#2]). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
--- Comment #7 from jakub at gcc dot gnu dot org 2010-05-12 06:25 --- I can't reproduce the problem either, neither in 4.6 nor in 4.4. 4.5.0 (rather than current 4.5 snapshots) had a bug with nonnull attribute if a function is IPA optimized and some arguments are optimized out, but I don't think that's the case here. The nonnull arguments are 1 based, sorry for not reading the testcase carefully when replying in #c3. memcpy etc. with NULL argument is invalid, the standard say that those functions copy from object pointed by the argument (or to object pointed by the argument). NULL doesn't point to any object (the standard say that it compares unequal to pointer to any object). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
--- Comment #6 from pmoulder at mail dot csse dot monash dot edu dot au 2010-05-12 05:29 --- Part of the problem is a documentation bug: the nonnull attribute's parameter is named "arg-index", suggesting that the first parameter is 0, while the example strongly suggests that the first parameter is 1. If parameters are numbered from 1, then please change the name to "arg-number"; if they are numbered from 0, then please change the example to match. On the subject of nonnull's documentation (and only weakly related to the current bug report): To judge from the fact that this attribute is being incorrectly used in glibc on functions like memcmp,memcpy,memmove,qsort etc. where NULL is a valid value (so long as n==0), and also from comments such as at http://chtekk.longitekk.com/index.php?/archives/43-GCCs-__attribute__-nonnull-...-not-helpful-at-all.html, I suggest that the "The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null" sentence be given more emphasis: I suggest putting it in a paragraph by itself, and adding a warning that a consequence would be that any checks for NULL in the function will be âoptimized awayâ, and also warning that the attribute is inappropriate for functions such as memcmp,memcpy,memmove,qsort where NULL is only unusual (but valid when n==0) rather than unconditionally invalid. -- pmoulder at mail dot csse dot monash dot edu dot au changed: What|Removed |Added CC||pmoulder at mail dot csse ||dot monash dot edu dot au http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
[Bug middle-end/44081] Incorrect nonnull assumed in code generation
--- Comment #5 from pinskia at gcc dot gnu dot org 2010-05-11 22:59 --- Works for me with: .ident "GCC: (GNU) 4.6.0 20100427 (experimental) [trunk revision 158795]" on x86_64 with -m32 -O2 -fno-inline. -- pinskia at gcc dot gnu dot org changed: What|Removed |Added Component|c |middle-end http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44081
