https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89008
Bug ID: 89008
Summary: O2 and O1 results differ for simple test
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: konstantin.vladimirov at gmail dot com
Target Milestone: ---
Simple test:
----
#include <stdlib.h>
unsigned long a, c;
unsigned b;
int d, e;
long f() {
unsigned long g = 0;
for (d = 0; d < 5; d += 2)
for (e = 0; e < 5; e += 3) {
c = 4 + b;
g = -b - b;
b = 5 * (b << 24);
}
a = g;
return 0;
}
int main() {
f();
if (a)
abort();
}
---
Obviously, b initially 0 and 5*(b<<24) is 0 too on every iteration. But compile
and run it with gcc 8.1
> gcc -O1 min-05.c
> a.out
> gcc -O2 min-05.c
> a.out
Aborted
Compiler information:
> gcc --verbose
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)
