[Bug tree-optimization/90055] New: Incorrect result with ffast-math + tree-vectorize

wence at gmx dot li gcc-bugzilla@gcc.gnu.org
Fri Apr 12 10:35:00 GMT 2019 

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90055

            Bug ID: 90055
           Summary: Incorrect result with ffast-math + tree-vectorize
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wence at gmx dot li
  Target Milestone: ---

Created attachment 46147
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46147&action=edit
Bug-provoking file

The attached code should produce the answer 0:

$ gcc-7 -O0 bug.c -o bug
$ ./bug
sum: 0.0000

Conversely:

$ gcc-7 -O3 -ffast-math -march=haswell -mtune=generic bug.c -o bug
$ ./bug
sum: -0.3750

And also

$ gcc-7 -O3 -ffast-math -march=haswell -mtune=haswell bug.c -o bug
$ ./bug
sum: -0.0937

If I turn off -ffast-math or add -fno-tree-loop-vectorize, the problem
disappears.

Output of gcc-7 -v -save-temps -O3 -ffast-math -march=haswell -mtune=generic
bug.c -o bug:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
7.3.0-27ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-ffast-math' '-march=haswell'
'-mtune=generic' '-o' 'bug'
 /usr/lib/gcc/x86_64-linux-gnu/7/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu
bug.c -march=haswell -mtune=generic -ffast-math -O3 -fpch-preprocess
-fstack-protector-strong -Wformat -Wformat-security -o bug.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/7/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-ffast-math' '-march=haswell'
'-mtune=generic' '-o' 'bug'
 /usr/lib/gcc/x86_64-linux-gnu/7/cc1 -fpreprocessed bug.i -quiet -dumpbase
bug.c -march=haswell -mtune=generic -auxbase bug -O3 -version -ffast-math
-fstack-protector-strong -Wformat -Wformat-security -o bug.s
GNU C11 (Ubuntu 7.3.0-27ubuntu1~18.04) version 7.3.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.3.0, GMP version 6.1.2, MPFR version 4.0.1,
MPC version 1.1.0, isl version isl-0.19-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C11 (Ubuntu 7.3.0-27ubuntu1~18.04) version 7.3.0 (x86_64-linux-gnu)
        compiled by GNU C version 7.3.0, GMP version 6.1.2, MPFR version 4.0.1,
MPC version 1.1.0, isl version isl-0.19-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: c8081a99abb72bbfd9129549110a350c
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-ffast-math' '-march=haswell'
'-mtune=generic' '-o' 'bug'
 as -v --64 -o bug.o bug.s
GNU assembler version 2.30 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.30
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/7/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/7/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-ffast-math' '-march=haswell'
'-mtune=generic' '-o' 'bug'
 /usr/lib/gcc/x86_64-linux-gnu/7/collect2 -plugin
/usr/lib/gcc/x86_64-linux-gnu/7/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
-plugin-opt=-fresolution=bug.res -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --sysroot=/
--build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -z now -z relro -o bug
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/7/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/7
-L/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/7/../../../../lib -L/lib/x86_64-linux-gnu
-L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib
-L/usr/lib/gcc/x86_64-linux-gnu/7/../../.. bug.o -lgcc --push-state --as-needed
-lgcc_s --pop-state -lc -lgcc --push-state --as-needed -lgcc_s --pop-state
/usr/lib/gcc/x86_64-linux-gnu/7/crtfastmath.o
/usr/lib/gcc/x86_64-linux-gnu/7/crtendS.o
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-ffast-math' '-march=haswell'
'-mtune=generic' '-o' 'bug'

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