Re: Question about merging if-else blocks

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Sep 27, 2023 at 7:21 AM Hanke Zhang via Gcc <gcc@gcc.gnu.org> wrote:
>
> Thanks! I understand what you mean, then can I think that if the
> function here is not an external function, but a function visible to
> the compiler and the function doesn't modify `a`, then these two
> blocks can be merged?

Yes.  The key transform you'd see before any of the merging is
CSE of the loads from 'a', then the rest is equivalent to the local
variable case.

Richard.

> Marc Glisse <marc.glisse@inria.fr> 于2023年9月27日周三 12:51写道：
> >
> > On Wed, 27 Sep 2023, Hanke Zhang via Gcc wrote:
> >
> > > Hi, I have recently been working on merging if-else statement blocks,
> > > and I found a rather bizarre phenomenon that I would like to ask
> > > about.
> > > A rough explanation is that for two consecutive if-else blocks, if
> > > their if statements are exactly the same, they should be merged, like
> > > the following program:
> > >
> > > int a = atoi(argv[1]);
> > > if (a) {
> > >  printf("if 1");
> > > } else {
> > >  printf("else 1");
> > > }
> > > if (a) {
> > >  printf("if 2");
> > > } else {
> > >  printf("else 2");
> > > }
> > >
> > > After using the -O3 -flto optimization option, it can be optimized as follows:
> > >
> > > int a = atoi(argv[1]);
> > > if (a) {
> > >  printf("if 1");
> > >  printf("if 2");
> > > } else {
> > >  printf("else 1");
> > >  printf("else 2");
> > > }
> > >
> > > But `a` here is a local variable. If I declare a as a global variable,
> > > it cannot be optimized as above. I would like to ask why this is? And
> > > is there any solution?
> >
> > If 'a' is a global variable, how do you know 'printf' doesn't modify its
> > value? (you could know it for printf, but it really depends on the
> > function that is called)
> >
> > --
> > Marc Glisse