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From: Hanke Zhang <hkzhang455@gmail.com>
Date: Wed, 27 Sep 2023 11:47:16 +0800
Message-ID: <CAM_DAs8=kgTU3UbH+KtGdYHQzPc4SM=_owW-mHqVvcrzx47rvg@mail.gmail.com>
Subject: Question about merging if-else blocks
To: gcc@gcc.gnu.org
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List-Id: <gcc.gcc.gnu.org>

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.