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: unassig...@gcc.gnu.org ReportedBy: mi...@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.