[gcc r13-1944] Improved pre-reload split of double word comparison against -1 on x86.

[Roger Sayle <sayle@gcc.gnu.org>]

https://gcc.gnu.org/g:e6b011bcfd52c245978ccd540e3f929571c59471

commit r13-1944-ge6b011bcfd52c245978ccd540e3f929571c59471
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Aug 3 09:03:17 2022 +0100

    Improved pre-reload split of double word comparison against -1 on x86.
    
    This patch adds an extra optimization to *cmp<dwi>_doubleword to improve
    the code generated for comparisons against -1.  Hypothetically, if a
    comparison against -1 reached this splitter we'd currently generate code
    that looks like:
    
            notq    %rdx            ; 3 bytes
            notq    %rax            ; 3 bytes
            orq     %rdx, %rax      ; 3 bytes
            setne   %al
    
    With this patch we would instead generate the superior:
    
            andq    %rdx, %rax      ; 3 bytes
            cmpq    $-1, %rax       ; 4 bytes
            setne   %al
    
    which is both faster and smaller, and also what's currently generated
    thanks to the middle-end splitting double word comparisons against
    zero and minus one during RTL expansion.  Should that change, this would
    become a missed-optimization regression, but this patch also (potentially)
    helps suitable comparisons created by CSE and combine.
    
    2022-08-03  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            * config/i386/i386.md (*cmp<dwi>_doubleword): Add a special case
            to split comparisons against -1 using AND and CMP -1 instructions.

Diff:
---
 gcc/config/i386/i386.md | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index f1158e1356b..e8f3851be01 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -1526,6 +1526,15 @@
       operands[i] = force_reg (<MODE>mode, operands[i]);
 
   operands[4] = gen_reg_rtx (<MODE>mode);
+
+  /* Special case comparisons against -1.  */
+  if (operands[1] == constm1_rtx && operands[3] == constm1_rtx)
+    {
+      emit_insn (gen_and<mode>3 (operands[4], operands[0], operands[2]));
+      emit_insn (gen_cmp_1 (<MODE>mode, operands[4], constm1_rtx));
+      DONE;
+    }
+
   if (operands[1] == const0_rtx)
     emit_move_insn (operands[4], operands[0]);
   else if (operands[0] == const0_rtx)