[Mesa-dev] [PATCH 3/3] nir: Simplify ine/ieq(ineg(a), a) -> ine/ieq(a, 0)

[Matt Turner]

(Don't top quote)

On Thu, Aug 6, 2015 at 7:22 AM, Boyan Ding <boyan.j.ding at gmail.com> wrote:
> But, isn't -INT_MIN == INT_MIN?

In C (-INT_MIN) is undefined since it relies on behavior of signed
integer overflow.

In GLSL though, I think you're right. The spec says that addition
subtraction, and shift operations (weird to list shift operations)
will wrap. Multiplication and division however do not -- when they
overflow their results are undefined (per a clarification/change in
the 4.30 spec).

So yeah, I don't think this is safe.

By the way, a good sanity check is to write a C function and compile
with gcc and see if it performs the optimization you're implementing.

bool int_cmp(int x) {
    return -x == x;
}

0000000000000000 <int_cmp>:
   0: 89 f8                 mov    %edi,%eax
   2: 01 f8                 add    %edi,%eax
   4: 0f 94 c0             sete   %al
   7: c3                   retq

I'm not actually sure what it's doing, but the function does return
true for INT_MIN.