PR tree-optimization/40210: Fold (bswap(X)>>C1)&C2 to (X>>C3)&C2 in match.pd

All of the optimizations/transformations mentioned in bugzilla for
PR tree-optimization/40210 are already implemented in mainline GCC,
with one exception.  In comment #5, there's a suggestion that
(bswap64(x)>>56)&0xff can be implemented without the bswap as
(unsigned char)x, or equivalently x&0xff.

This patch implements the above optimization, and closely related
variants.  For any single bit, (bswap(X)>>C1)&1 can be simplified
to (X>>C2)&1, where bit position C2 is the appropriate permutation
of C1.  Similarly, the bswap can eliminated if the desired set of
bits all lie within the same byte, hence (bswap(x)>>8)&255 can
always be optimized, as can (bswap(x)>>8)&123.

Previously,
int foo(long long x) {
  return (__builtin_bswap64(x) >> 56) & 0xff;
}

compiled with -O2 to
foo:	movq    %rdi, %rax
        bswap   %rax
        shrq    $56, %rax
        ret

with this patch, it now compiles to
foo:	movzbl  %dil, %eax
        ret

2021-07-08  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR tree-optimization/40210
	* match.pd (bswap optimizations): Simplify (bswap(x)>>C1)&C2 as
	(x>>C3)&C2 when possible.  Simplify bswap(x)>>C1 as ((T)x)>>C2
	when possible.  Simplify bswap(x)&C1 as (x>>C2)&C1 when 0<=C1<=255.

gcc/testsuite/ChangeLog
	PR tree-optimization/40210
	* gcc.dg/builtin-bswap-13.c: New test.
	* gcc.dg/builtin-bswap-14.c: New test.

0 files changed, 0 insertions, 0 deletions