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i386: Use SHR to compare with large power-of-two constants [PR94650]

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Convert unsigned compares where

	m >= LARGE_POWER_OF_TWO

and LARGE_POWER_OF_TWO represent an immediate where bit 33+ is set to use
a SHR instruction and compare the result to 0.  This avoids loading a
large immediate with MOVABS insn.

        movabsq $1099511627775, %rax
        cmpq    %rax, %rdi
        ja      .L5

gets converted to:

	shrq	$40, %rdi
	jne	.L5

	PR target/94650
	* config/i386/predicates.md (shr_comparison_operator): New predicate.
	* config/i386/i386.md (compare->shr splitter): New splitters.

testsuite/ChangeLog:

	PR target/94650
	* gcc.targeti/i386/pr94650.c: New test.
This commit is contained in:
Uros Bizjak 2020-05-04 13:49:14 +02:00
parent 6b5c7ee0df
commit 8ea03e9016
5 changed files with 110 additions and 0 deletions
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6
gcc/ChangeLog
View File

@ -1,3 +1,9 @@
2020-05-04 Uroš Bizjak <ubizjak@gmail.com>
PR target/94650
* config/i386/predicates.md (shr_comparison_operator): New predicate.
* config/i386/i386.md (compare->shr splitter): New splitters.
2020-05-04 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/94718

66
gcc/config/i386/i386.md
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@ -12310,6 +12310,36 @@
;; Store-flag instructions.
(define_split
[(set (match_operand:QI 0 "nonimmediate_operand")
(match_operator:QI 1 "shr_comparison_operator"
[(match_operand:DI 2 "register_operand")
(match_operand 3 "const_int_operand")]))]
"TARGET_64BIT
&& IN_RANGE (exact_log2 (UINTVAL (operands[3]) + 1), 32, 63)"
[(parallel
[(set (reg:CCZ FLAGS_REG)
(compare:CCZ
(lshiftrt:DI (match_dup 2) (match_dup 4))
(const_int 0)))
(clobber (scratch:DI))])
(set (match_dup 0)
(match_op_dup 1 [(reg:CCZ FLAGS_REG) (const_int 0)]))]
{
enum rtx_code new_code;
operands[1] = shallow_copy_rtx (operands[1]);
switch (GET_CODE (operands[1]))
{
case GTU: new_code = NE; break;
case LEU: new_code = EQ; break;
default: gcc_unreachable ();
}
PUT_CODE (operands[1], new_code);
operands[4] = GEN_INT (exact_log2 (UINTVAL (operands[3]) + 1));
})
;; For all sCOND expanders, also expand the compare or test insn that
;; generates cc0. Generate an equality comparison if `seq' or `sne'.
@ -12473,6 +12503,42 @@
(set_attr "mode" "<MODE>")])
;; Basic conditional jump instructions.
(define_split
[(set (pc)
(if_then_else
(match_operator 1 "shr_comparison_operator"
[(match_operand:DI 2 "register_operand")
(match_operand 3 "const_int_operand")])
(label_ref (match_operand 0))
(pc)))]
"TARGET_64BIT
&& IN_RANGE (exact_log2 (UINTVAL (operands[3]) + 1), 32, 63)"
[(parallel
[(set (reg:CCZ FLAGS_REG)
(compare:CCZ
(lshiftrt:DI (match_dup 2) (match_dup 4))
(const_int 0)))
(clobber (scratch:DI))])
(set (pc)
(if_then_else (match_op_dup 1 [(reg:CCZ FLAGS_REG) (const_int 0)])
(label_ref (match_operand 0))
(pc)))]
{
enum rtx_code new_code;
operands[1] = shallow_copy_rtx (operands[1]);
switch (GET_CODE (operands[1]))
{
case GTU: new_code = NE; break;
case LEU: new_code = EQ; break;
default: gcc_unreachable ();
}
PUT_CODE (operands[1], new_code);
operands[4] = GEN_INT (exact_log2 (UINTVAL (operands[3]) + 1));
})
;; We ignore the overflow flag for signed branch instructions.
(define_insn "*jcc"

3
gcc/config/i386/predicates.md
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@ -1290,6 +1290,9 @@
(define_predicate "bt_comparison_operator"
(match_code "ne,eq"))
(define_predicate "shr_comparison_operator"
(match_code "gtu,leu"))
;; Return true if OP is a valid comparison operator in valid mode.
(define_predicate "ix86_comparison_operator"
(match_operand 0 "comparison_operator")

5
gcc/testsuite/ChangeLog
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@ -1,3 +1,8 @@
2020-05-04 Uroš Bizjak <ubizjak@gmail.com>
PR target/94650
* gcc.targeti/i386/pr94650.c: New test.
2020-05-04 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/94718

30
gcc/testsuite/gcc.target/i386/pr94650.c Normal file
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@ -0,0 +1,30 @@
/* PR target/94650 */
/* { dg-do compile { target { ! ia32 } } } */
/* { dg-options "-O2" } */
#define LARGE_POWER_OF_TWO (1ULL << 40)
int
check (unsigned long long m)
{
return m >= LARGE_POWER_OF_TWO;
}
void g (int);
void
test0 (unsigned long long m)
{
if (m >= LARGE_POWER_OF_TWO)
g (0);
}
void
test1 (unsigned long long m)
{
if (m >= LARGE_POWER_OF_TWO)
g (m);
}
/* { dg-final { scan-assembler-not "movabs" } } */
/* { dg-final { scan-assembler-times "shr" 3 } } */