Optimize combination of comparisons to dec+compare

[Richard Biener]

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.
>