https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87074
Bug ID: 87074
Summary: Vectorization bug: O3 result differ from O2
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: konstantin.vladimirov at gmail dot com
Target Milestone: ---
Reproduction:
long b;
unsigned c[5];
unsigned long long d = 3;
int e, f, g;
void h() {
for (; f < 11; f++) {
b = g;
for (e = 0; e < 5; e++) {
c[e] = e - b - (c[e] >> 5);
g = c[e];
}
}
if (c[0])
d = 0;
}
int main() {
h();
return d;
}
Compiler:
> gcc -v
Reading specs from
/apps/gcc/8.1.0/.bin/../lib64/gcc/x86_64-suse-linux/8.1.0/specs
COLLECT_GCC=/apps/gcc/8.1.0/.bin/gcc
COLLECT_LTO_WRAPPER=/apps/gcc/8.1.0/.bin/../libexec/gcc/x86_64-suse-linux/8.1.0/lto-wrapper
Target: x86_64-suse-linux
Configured with: ./configure --prefix=/apps/gcc/8.1.0
--libdir=/apps/gcc/8.1.0/lib64 --libexecdir=/apps/gcc/8.1.0/libexec
--bindir=/apps/gcc/8.1.0/bin --with-isl=/apps/gcc/8.1.0
--with-libelf=/apps/gcc/8.1.0 --with-mpfr=/apps/gcc/8.1.0
--with-gmp=/apps/gcc/8.1.0 --with-mpc=/apps/gcc/8.1.0
--disable-gnu-unique-object --enable-gold=yes --enable-lto
--enable-languages=c,c++,objc,fortran --build=x86_64-suse-linux
--host=x86_64-suse-linux --target=x86_64-suse-linux --enable-libotm
--disable-multilib --disable-bootstrap --disable-libstdcxx-pch
Thread model: posix
gcc version 8.1.0 (GCC)
Compile and run with:
> gcc -O2 minimize.c
> ./a.out
> echo $?
0
> gcc -O3 minimize.c
> ./a.out
> echo $?
3
0 is correct, 3 is incorrect
I also tried with sanitizer, just in case I missed some UB here
> gcc -O2 -fsanitize=undefined minimize.c
> ./a.out
Sanitizer catchs nothing. So if there is really UB (not sure) it is sanitizer
bug.
Clang 6 returns 0 in both cases. GCC 7.3.0 also returns 0 both times.
