This patch addresses PR rtl-optimization/107991, which is a P2 regression
where GCC currently requires more "mov" instructions than GCC 7.

The x86's two address ISA creates some interesting challenges for reload.
For example, the tricky "x = y - x" usually needs to be implemented on x86
as

        tmp = x
        x = y
        x -= tmp

where a scratch register and two mov's are required to work around
the lack of a subf (subtract from) or rsub (reverse subtract) insn.

Not uncommonly, if y is dead after this subtraction, register allocation
can be improved by clobbering y.

        y -= x
        x = y

For the testcase in PR 107991, things are slightly more complicated,
where y is not itself dead, but is assigned from (i.e. equivalent to)
a value that is dead.  Hence we have something like:

        y = z
        x = y - x

so, GCC's reload currently generates the expected shuffle (as y is live):

        y = z
        tmp = x
        x = y
        x -= tmp

but we can use a peephole2 that understands that y and z are equivalent,
and that z is dead, to produce the shorter sequence:

        y = z
        z -= x
        x = z

In practice, for the new testcase from PR 107991, which before produced:

foo:    movl    %edx, %ecx
        movl    %esi, %edx
        movl    %esi, %eax
        subl    %ecx, %edx
        testb   %dil, %dil
        cmovne  %edx, %eax
        ret

with this patch/peephole2 we now produce the much improved:

foo:    movl    %esi, %eax
        subl    %edx, %esi
        testb   %dil, %dil
        cmovne  %esi, %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?


2023-01-09  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR rtl-optimization/107991
        * config/i386/i386.md (peephole2): New peephole2 to avoid register
        shuffling before a subtraction, after a register-to-register move.

gcc/testsuite/ChangeLog
        PR rtl-optimization/107991
        * gcc.target/i386/pr107991.c: New test case.


Thanks in advance,
Roger
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