Re: Question about merging if-else blocks

[Hanke Zhang <hkzhang455@gmail.com>]

Richard Biener <richard.guenther@gmail.com> 于2023年9月27日周三 15:30写道：
>
> 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.

Hi, Richard

I'm still a little confused about this.

I want to change the default behavior of gcc. We know that printf
won't change the value of 'a'. I'd like to let the compiler to get
this information as well. How can I do that? Or which pass should I
focus on?

By disassembling the exe file generated by icc, I found that icc will
merge these two blocks with the example code below. So I think there
maybe some ways to make it.

Thanks.
Hanke Zhang.

>
> > 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