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.

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index bac0a6ade676285a60ea85a19c6b087099030aa1..a82f2e456feb1cf555ca765dd3a44c0e85c5d45b 100644 (file)
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -1632,10 +1632,6 @@
              (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);