On Thu, Jun 01, 2017 at 11:54:33PM +0000, Michael Collison wrote:
> This patch improves code generation for relational compares against zero that
> are not merged into a subtract instruction. This patch improves the >= and <
> cases.
>
> An example of the '<' case:
>
> int lt (int x, int y)
> {
> if ((x - y) < 0)
> return 10;
>
> return 0;
> }
>
> Trunk generates:
>
> lt:
> sub w1, w0, w1
> mov w0, 10
> cmp w1, 0
> csel w0, w0, wzr, lt
> ret
>
> With the patch we can eliminate the redundant subtract and now generate:
>
> lt:
> cmp w0, w1
> mov w0, 10
> csel w0, w0, wzr, mi
> ret
I'm not up to speed on the way we use CC register modes in the AArch64
Backend. On the one hand looking at patterns like *sub<mode>3_compare0, this
patch looks correct. Those too generate subs instructions, and only set the
CC_NZ CC register. But on the other hand cmp<mode> sets the full CC
register. As cmp is an alias for subs, should they not set the same
CC register mode? Certainly the instruction sets more than just N and Z.
As I say, I don't understand this area, and I'm not sure if my objection
is reasonable. Hopefully someone who knows CC modes better could help me
understand why this is correct.
Thanks,
James
>
> Bootstrapped and tested on aarch64-linux-gnu. Okay for trunk?
>
> 2017-06-01 Michael Collison <[email protected]>
>
> * config/aarch64/aarch64-simd.md(aarch64_sub<mode>_compare0):
> New pattern.
> * testsuite/gcc.target/aarch64/cmp-2.c: New testcase.
