https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81633
Bug ID: 81633
Summary: Incorrect floating point result with tree vectoriser
Product: gcc
Version: 7.1.0
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 41873
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41873&action=edit
Preprocessed source.
The attached source (reduced from a real kernel) multiplies a 4x4 identity
matrix by its first column, It should produce the output:
1 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
With GCC 7.1.0, (and a recent 8.0.0 snapshot) it produces instead:
1 1 1 1
0 0 0 0
0 0 0 0
0 0 0 0
The bug disappears if I say either -fno-tree-vectorize or
-fno-tree-loop-vectorize
$ gcc -o prog.exe -Wall -Wextra -std=c99 -pedantic -O3 -v prog.c
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-7.1.0/bin/gcc
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-7.1.0/configure --prefix=/opt/wandbox/gcc-7.1.0
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-7.1.0/lib,-rpath,/opt/wandbox/gcc-7.1.0/lib64,-rpath,/opt/wandbox/gcc-7.1.0/lib32
--enable-lto
Thread model: posix
gcc version 7.1.0 (GCC)
COLLECT_GCC_OPTIONS='-o' 'prog.exe' '-Wpedantic' '-std=c99' '-Wall' '-Wextra'
'-O3' '-v' '-mtune=generic' '-march=x86-64'
/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/cc1 -quiet -v
-imultiarch x86_64-linux-gnu prog.c -quiet -dumpbase prog.c -mtune=generic
-march=x86-64 -auxbase prog -O3 -Wpedantic -Wall -Wextra -std=c99 -version -o
/tmp/ccorXUPc.s
GNU C99 (GCC) version 7.1.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.1.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory
"/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/include
/opt/wandbox/gcc-7.1.0/include
/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
GNU C99 (GCC) version 7.1.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.1.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 5122689c716743eb07fee255d0dfe7c4
COLLECT_GCC_OPTIONS='-o' 'prog.exe' '-Wpedantic' '-std=c99' '-Wall' '-Wextra'
'-O3' '-v' '-mtune=generic' '-march=x86-64'
as -v --64 -o /tmp/cc0c1aeA.o /tmp/ccorXUPc.s
GNU assembler version 2.26.1 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.26.1
COMPILER_PATH=/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/:/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/:/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/:/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/:/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/:/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../lib64/:/lib/x86_64-linux-gnu/:/lib/../lib64/:/usr/lib/x86_64-linux-gnu/:/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-o' 'prog.exe' '-Wpedantic' '-std=c99' '-Wall' '-Wextra'
'-O3' '-v' '-mtune=generic' '-march=x86-64'
/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/collect2 -plugin
/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/liblto_plugin.so
-plugin-opt=/opt/wandbox/gcc-7.1.0/libexec/gcc/x86_64-pc-linux-gnu/7.1.0/lto-wrapper
-plugin-opt=-fresolution=/tmp/ccfS7LEX.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
--eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o
prog.exe /usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o
/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/crtbegin.o
-L/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0
-L/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../../../lib64
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/../../.. -rpath
/opt/wandbox/gcc-7.1.0/lib64 /tmp/cc0c1aeA.o -lgcc --as-needed -lgcc_s
--no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/opt/wandbox/gcc-7.1.0/lib/gcc/x86_64-pc-linux-gnu/7.1.0/crtend.o
/usr/lib/x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-o' 'prog.exe' '-Wpedantic' '-std=c99' '-Wall' '-Wextra'
'-O3' '-v' '-mtune=generic' '-march=x86-64'
$ ./prog.exe
1 1 1 1
0 0 0 0
0 0 0 0
0 0 0 0
