public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug tree-optimization/53501] New: incorrect loop optimization with -O2
@ 2012-05-27 23:11 misof at gcc dot ksp.sk
  2012-05-29 16:15 ` [Bug tree-optimization/53501] [4.5/4.6/4.7/4.8 Regression] scev introduces signed overflow amonakov at gcc dot gnu.org
                   ` (10 more replies)
  0 siblings, 11 replies; 12+ messages in thread
From: misof at gcc dot ksp.sk @ 2012-05-27 23:11 UTC (permalink / raw)
  To: gcc-bugs

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53501

             Bug #: 53501
           Summary: incorrect loop optimization with -O2
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: misof@gcc.ksp.sk


Created attachment 27509
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27509
preprocessed file for which the bug manifests

Sorry if this is a duplicate or if I miscategorized it (my first bug report
here, I did search for this and found nothing similar enough).

This bug appears both in gcc and g++, my best guess is that it has to do with
incorrect optimization of a loop. Bug present at amd64, gcc versions 4.6.3
(Ubuntu) and 4.7.0 (compiled), details follow:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) 

$ ./gcc -v # my build of 4.7.0
Using built-in specs.
COLLECT_GCC=./gcc
COLLECT_LTO_WRAPPER=/home/misof/Desktop/newgcc/libexec/gcc/x86_64-unknown-linux-gnu/4.7.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --prefix=/home/misof/Desktop/newgcc/
--disable-multilib
Thread model: posix
gcc version 4.7.0 (GCC) 

Here is the offending source code:
$ cat bug.c
#include <stdio.h>

int e[100], n;

void foo(void) {
    int i, k = 0;
    for (i = 0; i < n; ++i) { e[k] = 10; ++k; e[k] = 10; ++k; }
    for (i = 0; i < k; ++i) printf("%d\n",e[i]);
    printf("k=%d\n",k);
}

int main(void) {
    n = 10;
    foo();
    return 0;
}

Up to my best knowledge this should be valid C code. When compiled without
optimizations or with -O1, everything works as expected:

$ gcc -Wall -Wextra bug.c && ./a.out
10
10
10
10
k=4

However, once -O2 or more is used, the second for-cycle seems to be incorrectly
optimized away:
$ gcc -Wall -Wextra -O2 bug.c && ./a.out
k=4

Interestingly, -fwrapv does make a difference, even though there seem to be no
integer overflows in the code:
$ gcc -Wall -Wextra -O2 -fwrapv bug.c && ./a.out 
10
10
10
10
k=4

Below is the complete stdout of
$ gcc -v -save-temps -Wall -Wextra -O2 bug.c 
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-mtune=generic'
'-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1 -E -quiet -v -imultilib . -imultiarch
x86_64-linux-gnu bug.c -mtune=generic -march=x86-64 -Wall -Wextra -O2
-fpch-preprocess -fstack-protector -o bug.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../x86_64-linux-gnu/include"
ignoring duplicate directory "/usr/include/x86_64-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/x86_64-linux-gnu
 /usr/lib/gcc/x86_64-linux-gnu/4.6/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-mtune=generic'
'-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1 -fpreprocessed bug.i -quiet -dumpbase
bug.c -mtune=generic -march=x86-64 -auxbase bug -O2 -Wall -Wextra -version
-fstack-protector -o bug.s
GNU C (Ubuntu/Linaro 4.6.3-1ubuntu5) version 4.6.3 (x86_64-linux-gnu)
        compiled by GNU C version 4.6.3, GMP version 5.0.2, MPFR version
3.1.0-p3, MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C (Ubuntu/Linaro 4.6.3-1ubuntu5) version 4.6.3 (x86_64-linux-gnu)
        compiled by GNU C version 4.6.3, GMP version 5.0.2, MPFR version
3.1.0-p3, MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 75e879ed14f91af504f4150eadeaa0e6
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-mtune=generic'
'-march=x86-64'
 as --64 -o bug.o bug.s
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-mtune=generic'
'-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/collect2 --sysroot=/ --build-id
--no-add-needed --as-needed --eh-frame-hdr -m elf_x86_64 --hash-style=gnu
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o
-L/usr/lib/gcc/x86_64-linux-gnu/4.6
-L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib -L/lib/x86_64-linux-gnu
-L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib
-L/usr/lib/x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../.. bug.o
-lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s
--no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.6/crtend.o
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crtn.o

Attached is the resulting bug.i file.


^ permalink raw reply	[flat|nested] 12+ messages in thread
end of thread, other threads:[~2013-04-12 16:25 UTC | newest]

Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2012-05-27 23:11 [Bug tree-optimization/53501] New: incorrect loop optimization with -O2 misof at gcc dot ksp.sk
2012-05-29 16:15 ` [Bug tree-optimization/53501] [4.5/4.6/4.7/4.8 Regression] scev introduces signed overflow amonakov at gcc dot gnu.org
2012-05-30  9:58 ` rguenth at gcc dot gnu.org
2012-05-30 10:55 ` rguenth at gcc dot gnu.org
2012-05-30 12:31 ` rguenth at gcc dot gnu.org
2012-05-30 12:34 ` rguenth at gcc dot gnu.org
2012-05-30 12:34 ` [Bug tree-optimization/53501] [4.5/4.6 " rguenth at gcc dot gnu.org
2012-06-01 17:01 ` ebotcazou at gcc dot gnu.org
2012-06-01 17:03 ` ebotcazou at gcc dot gnu.org
2012-06-20 13:23 ` rguenth at gcc dot gnu.org
2012-07-02 10:57 ` rguenth at gcc dot gnu.org
2013-04-12 16:25 ` [Bug tree-optimization/53501] [4.6 " jakub at gcc dot gnu.org

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).