[Bug rtl-optimization/94026] combine missed opportunity to simplify comparisons with zero

[wdijkstr at arm dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94026

Wilco <wdijkstr at arm dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |wdijkstr at arm dot com

--- Comment #5 from Wilco <wdijkstr at arm dot com> ---
(In reply to Fei Yang from comment #4)
> (In reply to Fei Yang from comment #0)
> > Created attachment 47966 [details]
> > proposed patch to fix this issue
> > 
> > Simple test case:
> > int
> > foo (int c, int d)
> > {
> >   int a = (c >> d) & 7;
> > 
> >   if (a >= 2) {
> >     return 1;
> >   }
> > 
> >   return 0;
> > }
> > 
> > Compile option: gcc -S -O2 test.c
> > 
> > 
> > On aarch64, GCC trunk emits 4 instrunctions:
> >         asr     w0, w0, 8
> >         tst     w0, 6
> >         cset    w0, ne
> >         ret
> > 
> > which can be further simplified into:
> >         tst     x0, 1536
> >         cset    w0, ne
> >         ret
> > 
> > We see the same issue on other targets such as i386 and x86-64.
> > 
> > Attached please find proposed patch for this issue.
> 
> The previously posted test case is not correct.
> Test case should be:
> int fifth (int c)
> {
>     int a = (c >> 8) & 7;
> 
>     if (a >= 2) {
>         return 1;
>     } else {
>         return 0;
>     }
> }

Simpler cases are:

int f1(int x) { return ((x >> 8) & 6) != 0; }
int f2(int x) { return ((x << 2) & 24) != 0; }
int f3(unsigned x) { return ((x << 2) & 15) != 0; }
int f4(unsigned x) { return ((x >> 2) & 14) != 0; }