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randall77 opened this issue Jul 6, 2020 · 1 comment
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cmd/compile: teach prove about arguments of & expressions #40083

randall77 opened this issue Jul 6, 2020 · 1 comment
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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
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Unplanned

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@randall77
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package main

import "math/bits"

func f(x, y uint64) int {
	if x&y != 0 {
		return bits.TrailingZeros64(x)
	}
	return 0
}

The compiler could infer from x&y != 0 that x != 0 (and y != 0). It can use that here, for example, to get rid of the code that deals with the x==0 special case on x86.

This currently generates

	0x0000 00000 (/Users/khr/gowork/tmp1.go:6)	MOVQ	"".x+8(SP), AX
	0x0005 00005 (/Users/khr/gowork/tmp1.go:6)	MOVQ	"".y+16(SP), CX
	0x000a 00010 (/Users/khr/gowork/tmp1.go:6)	TESTQ	AX, CX
	0x000d 00013 (/Users/khr/gowork/tmp1.go:6)	JEQ	34
	0x000f 00015 (/Users/khr/gowork/tmp1.go:7)	BSFQ	AX, AX
	0x0013 00019 (/Users/khr/gowork/tmp1.go:7)	MOVL	$64, CX
	0x0018 00024 (/Users/khr/gowork/tmp1.go:7)	CMOVQEQ	CX, AX
	0x001c 00028 (/Users/khr/gowork/tmp1.go:7)	MOVQ	AX, "".~r2+24(SP)
	0x0021 00033 (/Users/khr/gowork/tmp1.go:7)	RET
	0x0022 00034 (/Users/khr/gowork/tmp1.go:9)	MOVQ	$0, "".~r2+24(SP)
	0x002b 00043 (/Users/khr/gowork/tmp1.go:9)	RET

We could get rid of the MOVL $64/CMOVQEQ pair.

@zdjones @rasky @josharian
@randall77 randall77 added this to the Unplanned milestone Jul 6, 2020
@dmitshur dmitshur added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jul 7, 2020
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jul 13, 2022
@randall77
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randall77 commented Oct 29, 2024 •
edited by gabyhelp
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This is fixed, probably by CL 599096.

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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
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