This patch is the latest revision of my patch to add support for the
STC (set carry flag), CLC (clear carry flag) and CMC (complement
carry flag) instructions to the i386 backend, incorporating Uros'
previous feedback.  The significant changes are (i) the inclusion
of CMC, (ii) the use of UNSPEC for pattern, (iii) Use of a new
X86_TUNE_SLOW_STC tuning flag to use alternate implementations on
pentium4 (which has a notoriously slow STC) when not optimizing
for size.

An example of the use of the stc instruction is:
unsigned int foo (unsigned int a, unsigned int b, unsigned int *c) {
  return __builtin_ia32_addcarryx_u32 (1, a, b, c);
}

which previously generated:
        movl    $1, %eax
        addb    $-1, %al
        adcl    %esi, %edi
        setc    %al
        movl    %edi, (%rdx)
        movzbl  %al, %eax
        ret

with this patch now generates:
        stc
        adcl    %esi, %edi
        setc    %al
        movl    %edi, (%rdx)
        movzbl  %al, %eax
        ret

An example of the use of the cmc instruction (where the carry from
a first adc is inverted/complemented as input to a second adc) is:
unsigned int bar (unsigned int a, unsigned int b,
                  unsigned int c, unsigned int d)
{
  unsigned int c1 = __builtin_ia32_addcarryx_u32 (1, a, b, &o1);
  return __builtin_ia32_addcarryx_u32 (c1 ^ 1, c, d, &o2);
}

which previously generated:
        movl    $1, %eax
        addb    $-1, %al
        adcl    %esi, %edi
        setnc   %al
        movl    %edi, o1(%rip)
        addb    $-1, %al
        adcl    %ecx, %edx
        setc    %al
        movl    %edx, o2(%rip)
        movzbl  %al, %eax
        ret

and now generates:
        stc
        adcl    %esi, %edi
        cmc
        movl    %edi, o1(%rip)
        adcl    %ecx, %edx
        setc    %al
        movl    %edx, o2(%rip)
        movzbl  %al, %eax
        ret


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?


2022-06-03  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * config/i386/i386-expand.cc (ix86_expand_builtin) <handlecarry>:
        Use new x86_stc or negqi_ccc_1 instructions to set the carry flag.
        * config/i386/i386.h (TARGET_SLOW_STC): New define.
        * config/i386/i386.md (UNSPEC_CLC): New UNSPEC for clc.
        (UNSPEC_STC): New UNSPEC for stc.
        (UNSPEC_CMC): New UNSPEC for cmc.
        (*x86_clc): New define_insn.
        (*x86_clc_xor): New define_insn for pentium4 without -Os.
        (x86_stc): New define_insn.
        (define_split): Convert x86_stc into alternate implementation
        on pentium4.
        (x86_cmc): New define_insn.
        (*x86_cmc_1): New define_insn_and_split to recognize cmc pattern.
        (*setcc_qi_negqi_ccc_1_<mode>): New define_insn_and_split to
        recognize (and eliminate) the carry flag being copied to itself.
        (*setcc_qi_negqi_ccc_2_<mode>): Likewise.
        (neg<mode>_ccc_1): Renamed from *neg<mode>_ccc_1 for gen function.
        * config/i386/x86-tune.def (X86_TUNE_SLOW_STC): New tuning flag.

gcc/testsuite/ChangeLog
        * gcc.target/i386/cmc-1.c: New test case.
        * gcc.target/i386/stc-1.c: Likewise.


Thanks,
Roger
--