This patch is the next instalment in a set of backend patches around
improvements to ptest/vptest.  A previous patch optimized the sequence
t=pand(x,y); ptestz(t,t) into the equivalent ptestz(x,y), using the
property that ZF is set to (X&Y) == 0.  This patch performs a similar
transformation, converting t=pandn(x,y); ptestz(t,t) into the (almost)
equivalent ptestc(y,x), using the property that the CF flags is set to
(~X&Y) == 0.  The tricky bit is that this sets the CF flag instead of
the ZF flag, so we can only perform this transformation when we can
also convert the flags' consumer, as well as the producer.

For the test case:

int foo (__m128i x, __m128i y)
{
  __m128i a = x & ~y;
  return __builtin_ia32_ptestz128 (a, a);
}

With -O2 -msse4.1 we previously generated:

foo:    pandn   %xmm0, %xmm1
        xorl    %eax, %eax
        ptest   %xmm1, %xmm1
        sete    %al
        ret

with this patch we now generate:

foo:    xorl    %eax, %eax
        ptest   %xmm0, %xmm1
        setc    %al
        ret

At the same time, this patch also provides alternative fixes for
PR target/109973 and PR target/110118, by recognizing that ptestc(x,x)
always sets the carry flag (X&~X is always zero).  This is achieved
both by recognizing the special case in ix86_expand_sse_ptest and with
a splitter to convert an eligible ptest into an stc.

The next piece is, of course, STV of "if (x & ~y)..."

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?

2023-06-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * config/i386/i386-expand.cc (ix86_expand_sse_ptest): Recognize
        expansion of ptestc with equal operands as returning const1_rtx.
        * config/i386/i386.cc (ix86_rtx_costs): Provide accurate cost
        estimates of UNSPEC_PTEST, where the ptest performs the PAND
        or PAND of its operands.
        * config/i386/sse.md (define_split): Transform CCCmode UNSPEC_PTEST
        of reg_equal_p operands into an x86_stc instruction.
        (define_split): Split pandn/ptestz/setne into ptestc/setnc.
        (define_split): Split pandn/ptestz/sete into ptestc/setc.
        (define_split): Split pandn/ptestz/je into ptestc/jc.
        (define_split): Split pandn/ptestz/jne into ptestc/jnc.

gcc/testsuite/ChangeLog
        * gcc.target/i386/avx-vptest-4.c: New test case.
        * gcc.target/i386/avx-vptest-5.c: Likewise.
        * gcc.target/i386/avx-vptest-6.c: Likewise.
        * gcc.target/i386/pr109973-1.c: Update test case.
        * gcc.target/i386/pr109973-2.c: Likewise.
        * gcc.target/i386/sse4_1-ptest-4.c: New test case.
        * gcc.target/i386/sse4_1-ptest-5.c: Likewise.
        * gcc.target/i386/sse4_1-ptest-6.c: Likewise.


Thanks in advance,
Roger
--