https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80631
Bug ID: 80631
Summary: Compiling with -O3 -mavx2 gives wrong code
Product: gcc
Version: 6.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: elias.rudberg at it dot uu.se
Target Milestone: ---
Created attachment 41319
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41319&action=edit
Preprocessed source generated by gcc -v -save-temps -O3 -mavx2 thecode.c
I ran into a problem with strange results when compiling with -O3 -mavx2 and
have been able to reduce it to the following small test code:
========================================
#include <stdio.h>
int main() {
const int N = 8;
int v[N];
for(int k = 0; k < N; k++)
v[k] = k;
v[0] = 77;
int found_index = -1;
for(int k = 0; k < N; k++) {
if(v[k] == 77)
found_index = k;
}
printf("found_index = %d\n", found_index);
}
========================================
If compiled correctly, running this code should give "found_index = 0".
When compiling it like this:
gcc -O3 -mavx2 thecode.c
then running the resulting a.out executable gives:
$ ./a.out
found_index = -1
which is wrong.
The output of "gcc -v -save-temps -O3 -mavx2 thecode.c" looks as follows:
========================================
$ gcc -v -save-temps -O3 -mavx2 thecode.c
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-linker-hash-style=gnu --enable-plugin --enable-initfini-array
--disable-libgcj --with-isl --enable-libmpx --enable-gnu-indirect-function
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 6.3.1 20161221 (Red Hat 6.3.1-1) (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx2' '-mtune=generic'
'-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/cc1 -E -quiet -v thecode.c -mavx2
-mtune=generic -march=x86-64 -O3 -fpch-preprocess -o thecode.i
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/6.3.1/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/6.3.1/include
/usr/local/include
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx2' '-mtune=generic'
'-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/cc1 -fpreprocessed thecode.i -quiet
-dumpbase thecode.c -mavx2 -mtune=generic -march=x86-64 -auxbase thecode -O3
-version -o thecode.s
GNU C11 (GCC) version 6.3.1 20161221 (Red Hat 6.3.1-1) (x86_64-redhat-linux)
compiled by GNU C version 6.3.1 20161221 (Red Hat 6.3.1-1), GMP version
6.1.1, MPFR version 3.1.5, MPC version 1.0.2, isl version 0.14 or 0.13
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C11 (GCC) version 6.3.1 20161221 (Red Hat 6.3.1-1) (x86_64-redhat-linux)
compiled by GNU C version 6.3.1 20161221 (Red Hat 6.3.1-1), GMP version
6.1.1, MPFR version 3.1.5, MPC version 1.0.2, isl version 0.14 or 0.13
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 67626b9d441eed376539391e660a9413
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx2' '-mtune=generic'
'-march=x86-64'
as -v --64 -o thecode.o thecode.s
GNU assembler version 2.26.1 (x86_64-redhat-linux) using BFD version version
2.26.1-1.fc25
COMPILER_PATH=/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/:/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/:/usr/libexec/gcc/x86_64-redhat-linux/:/usr/lib/gcc/x86_64-redhat-linux/6.3.1/:/usr/lib/gcc/x86_64-redhat-linux/
LIBRARY_PATH=/usr/lib/gcc/x86_64-redhat-linux/6.3.1/:/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx2' '-mtune=generic'
'-march=x86-64'
/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/collect2 -plugin
/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/liblto_plugin.so
-plugin-opt=/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/lto-wrapper
-plugin-opt=-fresolution=thecode.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 --build-id
--no-add-needed --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../../lib64/crt1.o
/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-redhat-linux/6.3.1/crtbegin.o
-L/usr/lib/gcc/x86_64-redhat-linux/6.3.1
-L/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../../lib64 -L/lib/../lib64
-L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../.. thecode.o
-lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s
--no-as-needed /usr/lib/gcc/x86_64-redhat-linux/6.3.1/crtend.o
/usr/lib/gcc/x86_64-redhat-linux/6.3.1/../../../../lib64/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx2' '-mtune=generic'
'-march=x86-64'
========================================
I have tested this with a few different gcc versions:
gcc 4.8.3 --> OK
gcc 4.9.4 --> OK
gcc 5.3.0 --> OK
gcc 5.4.0 --> OK
gcc 6.1.0 --> WRONG
gcc 6.2.0 --> WRONG
gcc 6.3.1 --> WRONG
gcc 7.1.0 --> WRONG
I don't know what goes wrong but it seems somehow related to the beginning of
the list v in the code; if I change v[0]=77 to e.g. v[3]=77 then that gives
found_index=3 as it should, it is only v[0] that somehow is missed.
