This patch addresses PR rtl-optimization/96692 on x86_64, by providing
a define_split for combine to convert the three operation ((A|B)^C)^D
into a two operation sequence using andn when either A or B is the same
register as C or D.  This is essentially a reassociation problem that's
only a win if the target supports an and-not instruction (as with -mbmi).

Hence for the new test case:

int f(int a, int b, int c)
{
    return (a ^ b) ^ (a | c);
}

GCC on x86_64-pc-linux-gnu wth -O2 -mbmi would previously generate:

        xorl    %edi, %esi
        orl     %edx, %edi
        movl    %esi, %eax
        xorl    %edi, %eax
        ret

but with this patch now generates:

        andn    %edx, %edi, %eax
        xorl    %esi, %eax
        ret

I'll investigate whether this optimization can also be implemented
more generically in simplify_rtx when the backend provides
accurate rtx_costs for "(and (not ..." (as there's no optab for
andn).

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?


2022-06-26  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/96692
	* config/i386/i386.md (define_split): Split ((A | B) ^ C) ^ D
	as (X & ~Y) ^ Z on target BMI when either C or D is A or B.

gcc/testsuite/ChangeLog
	PR rtl-optimization/96692
	* gcc.target/i386/bmi-andn-4.c: New test case.


Thanks in advance,
Roger
--