On Tue, Oct 3, 2023 at 6:30 PM Hanke Zhang via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi, I'm a little confused about the behavior of gcc when the function
> is not inlined.
>
> Here is an example code:
>
> int __attribute__((noinline)) foo() {
> return 1;
> }
>
> int main() {
> if (foo()) {
> printf("foo() returned 1\n");
> } else {
> printf("foo() returned 0\n");
> }
> return 0;
> }
>
> After compiling this via `-O3 -flto`, the else block isn't been
> optimized and still exists.
>
> Even it's so obvious that the function will return '1', can't the
> compiler see that? Does gcc only get this information by inlining the
> function? Or is that what the gcc does?
>
> If so, how to make a change to let gcc get this information then?
I think IPA-CP would be doing this but the issue is that historically
'noinline' also disabled other IPA transforms and we've kept that
for backward compatibility even when we introduced the separate 'noipa'
attribute.
Richard.
>
> Thanks
> Hanke Zhang
