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cmd/compile: more boolean optimizations #17905

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randall77 opened this issue Nov 13, 2016 · 4 comments
Closed

cmd/compile: more boolean optimizations #17905

randall77 opened this issue Nov 13, 2016 · 4 comments
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FrozenDueToAge Performance Suggested Issues that may be good for new contributors looking for work to do.
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Go1.9

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@randall77
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func f(a *int, b *int) bool {
	if (a == nil) != (b == nil) {
		return false
	}
	return true
}

Compiles to:

	0x0000 00000 (tmp1.go:4)	MOVQ	"".a+8(FP), AX
	0x0005 00005 (tmp1.go:4)	TESTQ	AX, AX
	0x0008 00008 (tmp1.go:4)	SETNE	AL
	0x000b 00011 (tmp1.go:4)	XORL	$1, AX
	0x000e 00014 (tmp1.go:4)	MOVQ	"".b+16(FP), CX
	0x0013 00019 (tmp1.go:4)	TESTQ	CX, CX
	0x0016 00022 (tmp1.go:4)	SETNE	CL
	0x0019 00025 (tmp1.go:4)	XORL	$1, CX
	0x001c 00028 (tmp1.go:4)	CMPB	AL, CL
	0x001e 00030 (tmp1.go:4)	JEQ	38

SETNE/XORL can be replaced with SETEQ.
It might also be worthwhile to rewrite NeqB to XOR instead of SETNE/CMPB. Not sure it would help here, but it may in other places.
@randall77 randall77 added this to the Go1.9 milestone Nov 13, 2016
@mvdan
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mvdan commented Nov 13, 2016 •
edited

Is there a reason why the code above is in the form if x { return false; }; return true? You could just write return (a == nil) == (b == nil). Or was that just to prove the point?

@randall77
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No reason, it was just simplified from an example where I saw the suboptimal code.

@josharian josharian added the Suggested Issues that may be good for new contributors looking for work to do. label Feb 28, 2017
@josharian
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It might also be worthwhile to rewrite NeqB to XOR instead of SETNE/CMPB.

In this code, that causes a regression, because the result of the XOR then gets TESTed, whereas the SETNE flags can be re-used.

The rest is coming in a CL shortly.

@gopherbot
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CL https://golang.org/cl/42491 mentions this issue.

@golang golang locked and limited conversation to collaborators May 9, 2018
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