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i386: Optimize carry flag comparisons a bit

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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.
This commit is contained in:
Uros Bizjak 2021-04-29 22:02:00 +02:00
parent b24d8acbff
commit 86403f4e6e
2 changed files with 10 additions and 2 deletions
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8
gcc/config/i386/i386-expand.c
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@ -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);

4
gcc/config/i386/i386.md
View file

@ -6777,7 +6777,7 @@
[(match_operand 2 "flags_reg_operand") (const_int 0)])
(match_operand:SWI 1 "nonimmediate_operand" "0")))
(clobber (reg:CC FLAGS_REG))]
"ix86_unary_operator_ok (PLUS, <MODE>mode, operands)"
"!MEM_P (operands[0]) || rtx_equal_p (operands[0], operands[1])"
"adc{<imodesuffix>}\t{$0, %0|%0, 0}"
[(set_attr "type" "alu")
(set_attr "use_carry" "1")
@ -6919,7 +6919,7 @@
(match_operator:SWI 3 "ix86_carry_flag_operator"
[(match_operand 2 "flags_reg_operand") (const_int 0)])))
(clobber (reg:CC FLAGS_REG))]
"ix86_unary_operator_ok (MINUS, <MODE>mode, operands)"
"!MEM_P (operands[0]) || rtx_equal_p (operands[0], operands[1])"
"sbb{<imodesuffix>}\t{$0, %0|%0, 0}"
[(set_attr "type" "alu")
(set_attr "use_carry" "1")