Re: Question about merging if-else blocks

[Marc Glisse <marc.glisse@inria.fr>]

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