Optimize combination of comparisons to dec+compare

[Eugene Rozenfeld]

Re-sending my question and re-attaching the patch.

Richard, can you please clarify your feedback?

Thanks,

Eugene

-----Original Message-----
From: Gcc-patches <gcc-patches-bounces@gcc.gnu.org> On Behalf Of Eugene Rozenfeld via Gcc-patches
Sent: Tuesday, December 15, 2020 2:06 PM
To: Richard Biener <richard.guenther@gmail.com>
Cc: gcc-patches@gcc.gnu.org
Subject: [EXTERNAL] Re: Optimize combination of comparisons to dec+compare

Richard,

> Do we already handle x < y || x <= CST to x <= y - CST?

That is an invalid transformation: e.g., consider x=3, y=4, CST=2.
Can you please clarify?

Thanks,

Eugene

-----Original Message-----
From: Richard Biener <richard.guenther@gmail.com> 
Sent: Thursday, December 10, 2020 12:21 AM
To: Eugene Rozenfeld <Eugene.Rozenfeld@microsoft.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: Optimize combination of comparisons to dec+compare

On Thu, Dec 10, 2020 at 1:52 AM Eugene Rozenfeld via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>
> This patch adds a pattern for optimizing x < y || x == XXX_MIN to x <= 
> y-1 if y is an integer with TYPE_OVERFLOW_WRAPS.

Do we already handle x < y || x <= CST to x <= y - CST?
That is, the XXX_MIN case is just a special-case of generic anti-range testing?  For anti-range testing with signed types we pun to unsigned when possible.

> This fixes pr96674.
>
> Tested on x86_64-pc-linux-gnu.
>
> For this function
>
> bool f(unsigned a, unsigned b)
> {
>     return (b == 0) | (a < b);
> }
>
> the code without the patch is
>
> test   esi,esi
> sete   al
> cmp    esi,edi
> seta   dl
> or     eax,edx
> ret
>
> the code with the patch is
>
> sub    esi,0x1
> cmp    esi,edi
> setae  al
> ret
>
> Eugene
>
> gcc/
> PR tree-optimization/96674
> * match.pd: New pattern x < y || x == XXX_MIN --> x <= y - 1
>
> gcc/testsuite
> * gcc.dg/pr96674.c: New test.
>
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