i386: Optimize ptestz(x,-1) as ptestz(x,x) on x86

i386: Optimize ptestz(x,-1) as ptestz(x,x) on x86

This patch, inspired by PR target/90483 and libstdc++/118416, implements
some RTL expansion-time simplifications of ptest. A common idiom for
testing a vector against zero is to use ptestz(mask,-1).  Alas the code
generated for this is suboptimal, requiring materialization of an all_ones
vector.  Given that ptestz(x,y) is defined as (x & y) == 0, an equivalent
form is ptestz(mask,mask), saving an instruction (if ~0 isn't available).

Consider the function:

typedef long long v2di __attribute__ ((__vector_size__ (16)));

int foo (v2di x)
{
  return __builtin_ia32_ptestz128(x,~(v2di){0,0});
}

with -O2 -mavx2, GCC currently generates:

foo:	vpcmpeqd        %xmm1, %xmm1, %xmm1
        xorl    %eax, %eax
        vptest  %xmm1, %xmm0
        sete    %al
        ret

with this patch, it now generates:

foo:	xorl    %eax, %eax
        vptest  %xmm0, %xmm0
        sete    %al
        ret

2026-05-19  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/90483
	PR libstdc++/118416
	* config/i386/i386-expand.cc (ix86_expand_sse_ptest):  Refactor
	with optimizations for PTESTZ*, PTESTC* and PTESTNZC*, including
	transforming ptestz(x,-1) into ptestz(x,x).

gcc/testsuite/ChangeLog
	PR target/90483
	PR libstdc++/118416
	* gcc.target/i386/sse4_1-ptest-8.c: New test case.
	* gcc.target/i386/sse4_1-ptest-9.c: Likewise.