Question about merging if-else blocks

[Hanke Zhang <hkzhang455@gmail.com>]

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?

Thanks.
Hanke Zhang.