From c59b1cd18a0a346a75f6324e0bcf9623b96569e6 Mon Sep 17 00:00:00 2001
From: Uros Bizjak <ubizjak@gmail.com>
Date: Thu, 1 Feb 2024 21:27:19 +0100
Subject: [PATCH] i386: Improve *cmp<dwi>_doubleword splitter [PR113701]

The fix for PR70321 introduced a splitter that split a doubleword
comparison into a pair of XORs followed by an IOR to set the (zero)
flags register.  To help the reload, splitter forced SUBREG pieces of
double-word input values to a pseudo, but this regressed
gcc.target/i386/pr82580.c

int f0 (U x, U y) { return x == y; }

from:
    xorq    %rdx, %rdi
    xorq    %rcx, %rsi
    xorl    %eax, %eax
    orq     %rsi, %rdi
    sete    %al
    ret

to:
    xchgq   %rdi, %rsi
    movq    %rdx, %r8
    movq    %rcx, %rax
    movq    %rsi, %rdx
    movq    %rdi, %rcx
    xorq    %rax, %rcx
    xorq    %r8, %rdx
    xorl    %eax, %eax
    orq     %rcx, %rdx
    sete    %al
    ret

To mitigate the regression, remove this legacy heuristic (workaround?).
There have been many incremental changes and improvements to x86 TImode
and register allocation, so this legacy workaround is not only no longer
useful, but it actually hurts register allocation.  The patched compiler
now produces:

        xchgq   %rdi, %rsi
        xorl    %eax, %eax
        xorq    %rsi, %rdx
        xorq    %rdi, %rcx
        orq     %rcx, %rdx
        sete    %al
        ret

    PR target/113701

gcc/ChangeLog:

    * config/i386/i386.md (*cmp<dwi>_doubleword):
    Do not force SUBREG pieces to pseudos.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Uros.
---
 gcc/config/i386/i386.md | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index e6ebc461e524..d6bd637e8105 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -1617,10 +1617,6 @@
 	      (set (match_dup 4) (ior:DWIH (match_dup 4) (match_dup 5)))])]
 {
   split_double_mode (<DWI>mode, &operands[0], 2, &operands[0], &operands[2]);
-  /* Placing the SUBREG pieces in pseudos helps reload.  */
-  for (int i = 0; i < 4; i++)
-    if (SUBREG_P (operands[i]))
-      operands[i] = force_reg (<MODE>mode, operands[i]);
 
   operands[4] = gen_reg_rtx (<MODE>mode);