[PATCH] Prefer small negative constants in InstCombine

[Eli Friedman]

On Wed, Jun 26, 2013 at 6:56 AM, Tim Northover <tnorthover at apple.com> wrote:

> Hi,
>
> The attached patch adds a heuristic to ShrinkDemandedConstant so that
> small negative constants are preferred over large ones even if as an
> unsigned value the transformation would be an improvement.
>
> The motivating case is added as a test:
> define i32 @foo(i64 %in) {
>   %sub = sub i64 %in, 4
>   %shift = lshr i64 %sub, 2
>   %res = trunc i64 %shift to i32
>
>   ret i32 %res
> }
>
> On x86-64 this currently generates:
>
> _foo:
>         movabsq $17179869180, %rax      ## imm = 0x3FFFFFFFC
>         addq    %rdi, %rax
>         shrq    $2, %rax
>         ret
>
> with an unnecessary and large movabsq. With this change the fact that
> we’re really subtracting 4 survives to CodeGen and we just emit
>
> _foo:
>         leaq -4(%rdi), %rax
>         shrq $2, %rax
>         ret
>
> Does anyone have any objections to this change? Is it likely to cause more
> damage than good for later passes?
>
>
This seems like a big change that could affect all sorts of optimizations;
I'd like to see performance numbers from the nightly test suite.

Also, I'm not sure this code is actually performing the check you want it
to; it appears that, for example, it would prefer -3 over -4.

-Eli
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