Optimize double word negation of zero extended values on x86.

It's not uncommon for GCC to convert between a (zero or one) Boolean
value and a (zero or all ones) mask value, possibly of a wider type,
using negation.

Currently on x86_64, the following simple test case:
__int128 foo(unsigned long x) { return -(__int128)x; }

compiles with -O2 to:

        movq    %rdi, %rax
        xorl    %edx, %edx
        negq    %rax
        adcq    $0, %rdx
        negq    %rdx
        ret

with this patch, which adds an additional peephole2 to i386.md,
we instead generate the improved:

        movq    %rdi, %rax
        negq    %rax
        sbbq    %rdx, %rdx
        ret

[and likewise for the (DImode) long long version using -m32.]
A peephole2 is appropriate as the double word negation and the
operation providing the xor are typically only split after combine.

In fact, the new peephole2 sequence:
;; Convert:
;;   xorl %edx, %edx
;;   negl %eax
;;   adcl $0, %edx
;;   negl %edx
;; to:
;;   negl %eax
;;   sbbl %edx, %edx    // *x86_mov<mode>cc_0_m1

is nearly identical to (and placed immediately after) the existing:
;; Convert:
;;   mov %esi, %edx
;;   negl %eax
;;   adcl $0, %edx
;;   negl %edx
;; to:
;;   xorl %edx, %edx
;;   negl %eax
;;   sbbl %esi, %edx

One potential objection/concern is that "sbb? %reg,%reg" may possibly be
incorrectly perceived as a false register dependency on older hardware,
much like "xor? %reg,%reg" may be perceived as a false dependency on
really old hardware.  This doesn't currently appear to be a concern
for the i386 backend's *x86_move<mode>cc_0_m1 as shown by the following
test code:

int bar(unsigned int x, unsigned int y) {
  return x > y ? -1 : 0;
}

which currently generates a "naked" sbb:
        cmp     esi, edi
        sbb     eax, eax
        ret

If anyone does potentially encounter a stall, it would easy to add
a splitter or peephole2 controlled by a tuning flag to insert an additional
xor to break the false dependency chain (when not optimizing for size),
but I don't believe this is required on recent microarchitectures.

2022-05-24 Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (peephole2): Convert xor;neg;adc;neg,
	i.e. a double word negation of a zero extended operand, to
	neg;sbb.

gcc/testsuite/ChangeLog
	* gcc.target/i386/neg-zext-1.c: New test case for -m32.
	* gcc.target/i386/neg-zext-2.c: New test case for -m64.

0 files changed, 0 insertions, 0 deletions