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cmd/compile: inefficient compilation of (64 - x) #20677

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josharian opened this issue Jun 14, 2017 · 1 comment

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commented Jun 14, 2017

package p

var sink uint8

func BenchmarkSub(b *testing.B) {
	for i := 0; i < b.N; i++ {
		sink = 64 - sink
	}
}

Currently, the core of the inner loop compiles to:

	0x0009 00009 (/Users/josh/src/mask_test.go:29)	MOVBLZX	"".sink(SB), DX
	0x0010 00016 (/Users/josh/src/mask_test.go:29)	ADDL	$-64, DX
	0x0013 00019 (/Users/josh/src/mask_test.go:29)	NEGL	DX
	0x0015 00021 (/Users/josh/src/mask_test.go:29)	MOVB	DL, "".sink(SB)

Disabling a few optimizations allows this to revert back to:

	0x0009 00009 (/Users/josh/src/mask_test.go:29)	MOVBLZX	"".sink(SB), DX
	0x0010 00016 (/Users/josh/src/mask_test.go:29)	MOVL	$64, BX
	0x0015 00021 (/Users/josh/src/mask_test.go:29)	SUBL	DX, BX
	0x0017 00023 (/Users/josh/src/mask_test.go:29)	MOVB	BL, "".sink(SB)

It is two bytes longer, but executes faster: 1.60 ns vs 2.06 ns. (Both measurements are extremely consistent.)

Perhaps we should remove the optimizations, or perhaps we should make them contingent on something (what?).

cc @randall77

@josharian josharian added this to the Go1.10 milestone Jun 14, 2017

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commented Jun 14, 2017

I guess the existing code is not just shorter, it only requires a single register. I'll find a different attack on this code.

@josharian josharian closed this Jun 14, 2017

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