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cmd/compile: stores to unused variables on the stack not eliminated #19765

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mundaym opened this issue Mar 29, 2017 · 2 comments
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cmd/compile: stores to unused variables on the stack not eliminated #19765

mundaym opened this issue Mar 29, 2017 · 2 comments
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@mundaym
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mundaym commented Mar 29, 2017 •
edited

I am trying to replace math.Float64bits and friends with register to register moves and noticed that stores to variables on the stack aren't eliminated when their all their uses have been optimized away. Is there an existing issue for this?

For example, on tip (6e9ec14):

package main

var Sink int

func main() {
	x := 1
	Sink = *(&x)
}

Compiles to:

   	00000 (/tmp/main.go:5)	TEXT	"".main(SB)
   	00001 (/tmp/main.go:5)	FUNCDATA	$0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
   	00002 (/tmp/main.go:5)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
v7	00003 (/tmp/main.go:6)	MOVQ	$1, "".x-8(SP)
v13	00004 (/tmp/main.go:7)	MOVQ	$1, "".Sink(SB)
b1	00005 (/tmp/main.go:8)	RET
   	00006 (<unknown line number>)	END

The MOVQ $1, "".x-8(SP) could be eliminated by the compiler but isn't.
@randall77
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Kind of a dup of #13095

@gopherbot
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CL https://golang.org/cl/38746 mentions this issue.

@golang golang locked and limited conversation to collaborators Aug 24, 2018
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@randall77 @gopherbot @mundaym