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cmd/compile: unnecessary nil checks on address of return values #7994

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randall77 opened this issue May 14, 2014 · 5 comments
Closed

cmd/compile: unnecessary nil checks on address of return values #7994

randall77 opened this issue May 14, 2014 · 5 comments
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FrozenDueToAge Performance
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Unplanned

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@randall77
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package main
func foo(x int) (y int) {
     p := &x
     q := &y
     *q = *p
     return
}

compile with "go tool 6g -S foo.go"

The assembly is 

0005  MOVQ    $0,y+8(FP)
0006  LEAQ    x+0(FP),BX
0007  MOVQ    BX,CX
0008  LEAQ    y+8(FP),BX
0009  CMPQ    BX,$0
0010  JNE     $1,12
0011  MOVL    AX,(BX)
0012  NOP     ,
0013  MOVQ    (CX),R8
0014  MOVQ    R8,(BX)
0015  RET     ,

There is a nil check (CMPQ/JNE/MOVL/NOP) on the address of the return value, a value
that certainly non-nil.

Note that there is no nil check on the address of the argument.
@ianlancetaylor
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Comment 1:

Labels changed: added repo-main.

@rsc
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rsc commented Sep 15, 2014

Comment 2:

Labels changed: added release-go1.5, removed release-go1.4.

Status changed to Accepted.

@bradfitz bradfitz modified the milestone: Go1.5 Dec 16, 2014
@rsc rsc removed accepted labels Apr 14, 2015
@rsc
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rsc commented Jun 8, 2015

Too late for Go 1.5. SSA may replace this code.

@rsc rsc modified the milestones: Unplanned, Go1.5 Jun 8, 2015
@rsc rsc changed the title cmd/gc: unnecessary nil checks on address of return values cmd/compile: unnecessary nil checks on address of return values Jun 8, 2015
@tzneal
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tzneal commented Aug 21, 2015

dev.ssa currently generates:

00000  MOVQ    $0, AX
00002  MOVQ    AX, 16(SP) 
00007  MOVQ    8(SP), AX
00012  MOVQ    AX, 16(SP)
00017  RET

deadstore elimination doesn't work across blocks, or it would be even better.

@randall77
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Fixed with SSA backend.

@golang golang locked and limited conversation to collaborators Apr 24, 2017
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@bradfitz @tzneal @rsc @ianlancetaylor @randall77 @gopherbot