https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88732
Bug ID: 88732
Summary: different results on -O0 and -O1, -O2, -O3, -Os
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: amocywang at gmail dot com
Target Milestone: ---
> $ gcc -v
>
>
Using built-in specs.
COLLECT_GCC=/home/wgc/installs/gcc_trunks/trunk_r267598/bin/gcc
COLLECT_LTO_WRAPPER=/home/wgc/installs/gcc_trunks/trunk_r267598/libexec/gcc/x86_64-pc-linux-gnu/9.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ./configure LDFLAGS=-Wl,--no-as-needed
--prefix=/home/wgc/installs/gcc_trunks/trunk_r267598
--with-gmp=/home/wgc/installs/gmp-6.1.2
--with-mpfr=/home/wgc/installs/mpfr-4.0.1
--with-mpc=/home/wgc/installs/mpc-1.1.0 --with-isl= --with-cloog=
--enable-languages=c,c++ --disable-multilib --disable-bootstrap
Thread model: posix
gcc version 9.0.0 20190105 (experimental) (GCC)
$ gcc -O0 program.c
$ ./a.out
63
$ gcc -O1 program.c
$ ./a.out
64
$ gcc -O2 program.c
$ ./a.out
64
$ gcc -O3 program.c
$ ./a.out
64
$ gcc -Os program.c
$ ./a.out
64
$ cat program.c
static int a, b;
unsigned short int c() {
b = __builtin_clzl(a);
return a;
}
int main() {
c();
printf("%d\n", b);
return 0;
}
I have validated this difference with the command line option
"-fno-strict-aliasing -fwrapv" and the results are different on different
optimizing options, same with the results above. It seems the test program is
correct.
