This patch is a follow-up to my previous PR target/110551 patch, this
time to address the additional move after mulx, seen on TARGET_BMI2
architectures (such as -march=haswell).  The complication here is
that the flexible multiple-set mulx instruction is introduced into
RTL after reload, by split2, and therefore can't benefit from register
preferencing.  This results in RTL like the following:

(insn 32 31 17 2 (parallel [
            (set (reg:DI 4 si [orig:101 r ] [101])
                (mult:DI (reg:DI 1 dx [109])
                    (reg:DI 5 di [109])))
            (set (reg:DI 5 di [ r+8 ])
                (umul_highpart:DI (reg:DI 1 dx [109])
                    (reg:DI 5 di [109])))
        ]) "pr110551-2.c":8:17 -1
     (nil))

(insn 17 32 9 2 (set (reg:DI 0 ax [107])
        (reg:DI 5 di [ r+8 ])) "pr110551-2.c":9:40 90 {*movdi_internal}
     (expr_list:REG_DEAD (reg:DI 5 di [ r+8 ])
        (nil)))

Here insn 32, the mulx instruction, places its results in si and di,
and then immediately after decides to move di to ax, with di now dead.
This can be trivially cleaned up by a peephole2.  I've added an
additional constraint that the two SET_DESTs can't be the same
register to avoid confusing the middle-end, but this has well-defined
behaviour on x86_64/BMI2, encoding a umul_highpart.

For the new test case, compiled on x86_64 with -O2 -march=haswell:

Before:
mulx64: movabsq $-7046029254386353131, %rdx
        mulx    %rdi, %rsi, %rdi
        movq    %rdi, %rax
        xorq    %rsi, %rax
        ret

After:
mulx64: movabsq $-7046029254386353131, %rdx
        mulx    %rdi, %rsi, %rax
        xorq    %rsi, %rax
        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?

2023-10-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR target/110551
        * config/i386/i386.md (*bmi2_umul<mode><dwi>3_1): Tidy condition
        as operands[2] with predicate register_operand must be !MEM_P.
        (peephole2): Optimize a mulx followed by a register-to-register
        move, to place result in the correct destination if possible.

gcc/testsuite/ChangeLog
        PR target/110551
        * gcc.target/i386/pr110551-2.c: New test case.


Thanks in advance,
Roger
--