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cmd/compile: optimize bitset tests

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The assembly output for x & c == c, where c is power of 2:

	MOVQ	"".set+8(SP), AX
	ANDQ	$8, AX
	CMPQ	AX, $8
	SETEQ	"".~r2+24(SP)

With optimization using bitset:

	MOVQ	"".set+8(SP), AX
	BTL	$3, AX
	SETCS	"".~r2+24(SP)

output less than 1 instruction.

However, there is no speed improvement:

name         old time/op  new time/op  delta
AllBitSet-8  0.35ns ± 0%  0.35ns ± 0%   ~     (all equal)

Fixes #31904

Change-Id: I5dca4e410bf45716ed2145e3473979ec997e35d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/175957
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
LE Manh Cuong 2019-05-08 17:02:23 +07:00 committed by Daniel Martí
parent ae68a91272
commit c5f142fa9f
3 changed files with 1468 additions and 5 deletions
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6
src/cmd/compile/internal/ssa/gen/generic.rules
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@ -1733,6 +1733,12 @@
(Eq(8|16|32|64) s:(Sub(8|16|32|64) x y) (Const(8|16|32|64) [0])) && s.Uses == 1 -> (Eq(8|16|32|64) x y)
(Neq(8|16|32|64) s:(Sub(8|16|32|64) x y) (Const(8|16|32|64) [0])) && s.Uses == 1 -> (Neq(8|16|32|64) x y)
// Optimize bitsets
(Eq(8|16|32|64) (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [y])) (Const(8|16|32|64) <t> [y])) && isPowerOfTwo(y)
-> (Neq(8|16|32|64) (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [y])) (Const(8|16|32|64) <t> [0]))
(Neq(8|16|32|64) (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [y])) (Const(8|16|32|64) <t> [y])) && isPowerOfTwo(y)
-> (Eq(8|16|32|64) (And(8|16|32|64) <t> x (Const(8|16|32|64) <t> [y])) (Const(8|16|32|64) <t> [0]))
// Reassociate expressions involving
// constants such that constants come first,
// exposing obvious constant-folding opportunities.

1455
src/cmd/compile/internal/ssa/rewritegeneric.go
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File diff suppressed because it is too large Load Diff

12
test/codegen/bits.go
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@ -314,3 +314,15 @@ func op_orn(x, y uint32) uint32 {
// arm64:`ORN\t`,-`ORR`
return x | ^y
}
// check bitsets
func bitSetPowerOf2Test(x int) bool {
// amd64:"BTL\t[$]3"
return x&8 == 8
}
func bitSetTest(x int) bool {
// amd64:"ANDQ\t[$]9, AX"
// amd64:"CMPQ\tAX, [$]9"
return x&9 == 9
}