i386: Optimize carry flag comparisons a bit

In ix86_int_compare, opportunistically swap operands of GTU and LEU comparisons
to emit carry flag comparison, with the expectation that the comparison will
combine to *add<mode>3_carry_0 or *sub<mode>3_carry_0 insn pattern.

Do not use ix86_expand_carry_flag_compare because this function prefers
carry flag comparisons too much - it forces the constants into registers
and/or emits additional arithmetic instructions to convert simple
comparisons into carry flag comparisons - but simply swap operands to
convert GTU and LEU comparisons into GEU and LTU ones.

Also, change the insn predicates of *add<mode>3_carry_0 and
*sub<mode>3_carry_0 insn patterns to allow more combine opportunities
with memory operands.

2021-04-29  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
	* config/i386/i386-expand.c (ix86_expand_int_compare):
	Swap operands of GTU and LEU comparison to emit carry flag comparison.
	* config/i386/i386.md (*add<mode>3_carry_0): Change insn
	predicate to allow more combine opportunities with memory operands.
	(*sub<mode>3_carry_0): Ditto.

1 files changed, 8 insertions, 0 deletions

diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
index 76990628150..fee4d07b7fd 100644
--- a/gcc/config/i386/i386-expand.c
+++ b/gcc/config/i386/i386-expand.c
@@ -2658,6 +2658,14 @@ ix86_expand_int_compare (enum rtx_code code, rtx op0, rtx op1)
   machine_mode cmpmode;
   rtx tmp, flags;
 
+  /* Swap operands to emit carry flag comparison.  */
+  if ((code == GTU || code == LEU)
+      && nonimmediate_operand (op1, VOIDmode))
+    {
+      std::swap (op0, op1);
+      code = swap_condition (code);
+    }
+
   cmpmode = SELECT_CC_MODE (code, op0, op1);
   flags = gen_rtx_REG (cmpmode, FLAGS_REG);