cmd/compile: unnecessary zeroing of register on arm

[davecheney]

What steps will reproduce the problem?

func add64(a, b int) int {
        x := a + b
        return x
}

What is the expected output? What do you see instead?

Expected (from 6g)

"".add64 t=1 size=32 value=0 args=0x18 locals=0x0
        0x0000 00000 (/home/dfc/src/add64.go:3) TEXT    "".add64+0(SB),4,$0-24
        0x0000 00000 (/home/dfc/src/add64.go:3) NOP     ,
        0x0000 00000 (/home/dfc/src/add64.go:3) NOP     ,
        0x0000 00000 (/home/dfc/src/add64.go:3) FUNCDATA        $2,gclocals·f90cfd099b5ec2b453c391fece9d42bb+0(SB)
        0x0000 00000 (/home/dfc/src/add64.go:3) FUNCDATA        $3,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
        0x0000 00000 (/home/dfc/src/add64.go:4) MOVQ    "".a+8(FP),BX
        0x0005 00005 (/home/dfc/src/add64.go:4) MOVQ    "".b+16(FP),BP
        0x000a 00010 (/home/dfc/src/add64.go:4) ADDQ    BP,BX
        0x000d 00013 (/home/dfc/src/add64.go:5) MOVQ    BX,"".~r2+24(FP)
        0x0012 00018 (/home/dfc/src/add64.go:5) RET     ,

But got on 5g

"".add64 t=1 size=24 value=0 args=0xc locals=0x0 leaf
        0x0000 00000 (/home/dfc/src/add64.go:3) TEXT    "".add64+0(SB),16,$-4-12
        0x0000 00000 (/home/dfc/src/add64.go:3) FUNCDATA        $2,gclocals·f90cfd099b5ec2b453c391fece9d42bb+0(SB)
        0x0000 00000 (/home/dfc/src/add64.go:3) FUNCDATA        $3,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
        0x0000 00000 (/home/dfc/src/add64.go:3) MOVW    $0,R0
        0x0004 00004 (/home/dfc/src/add64.go:4) MOVW    "".a+0(FP),R0
        0x0008 00008 (/home/dfc/src/add64.go:4) MOVW    "".b+4(FP),R1
        0x000c 00012 (/home/dfc/src/add64.go:4) ADD     R1,R0
        0x0010 00016 (/home/dfc/src/add64.go:5) MOVW    R0,"".~r2+8(FP)
        0x0014 00020 (/home/dfc/src/add64.go:5) B       ,0(R14)

Note the unnecessary zeroing of R0, which is then overwritten by the load of 0(FP)

Please use labels and text to provide additional information.

linux/arm

% go version
go version devel +6b696a34e0af Sun Aug 03 15:14:59 2014 -0700 linux/arm

[rsc]

Comment 1:

Labels changed: added release-none, removed release-go1.4.

Status changed to Accepted.

[navytux]

As of

go version devel +3a067cc812 Fri Dec 9 06:45:08 2016 +0000 linux/amd64

this issue is no longer present:

$ GOARCH=arm go tool compile -S q.go 
"".add64 t=1 size=20 args=0xc locals=0x0 leaf
        0x0000 00000 (q.go:3)   TEXT    "".add64(SB), $-4-12
        0x0000 00000 (q.go:3)   FUNCDATA        $0, gclocals·54241e171da8af6ae173d69da0236748(SB)
        0x0000 00000 (q.go:3)   FUNCDATA        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0000 00000 (q.go:4)   MOVW    "".a(FP), R0
        0x0004 00004 (q.go:4)   MOVW    "".b+4(FP), R1
        0x0008 00008 (q.go:4)   ADD     R1, R0, R0
        0x000c 00012 (q.go:5)   MOVW    R0, "".~r2+8(FP)
        0x0010 00016 (q.go:5)   JMP     (R14)
...

on arm64 there is unnecessary (imho) more stack check but no zeroing:

$ GOARCH=arm64 go tool compile -S q.go 
"".add64 t=1 size=48 args=0x18 locals=0x0 leaf
        0x0000 00000 (q.go:3)   TEXT    "".add64(SB), $-8-24
        0x0000 00000 (q.go:3)   MOVD    16(g), R1
        0x0004 00004 (q.go:3)   MOVD    RSP, R2
        0x0008 00008 (q.go:3)   CMP     R1, R2
        0x000c 00012 (q.go:3)   BLS     36
        0x0010 00016 (q.go:3)   FUNCDATA        ZR, gclocals·54241e171da8af6ae173d69da0236748(SB)
        0x0010 00016 (q.go:3)   FUNCDATA        $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
        0x0010 00016 (q.go:4)   MOVD    "".a(FP), R0
        0x0014 00020 (q.go:4)   MOVD    "".b+8(FP), R1
        0x0018 00024 (q.go:4)   ADD     R1, R0, R0
        0x001c 00028 (q.go:5)   MOVD    R0, "".~r2+16(FP)
        0x0020 00032 (q.go:5)   RET     (R30)
        0x0024 00036 (q.go:5)   NOP
        0x0024 00036 (q.go:3)   PCDATA  $0, $-1
        0x0024 00036 (q.go:3)   MOVD    R30, R3
        0x0028 00040 (q.go:3)   CALL    runtime.morestack_noctxt(SB)
        0x002c 00044 (q.go:3)   JMP     0

[davecheney]

Thanks for confirming. Closing.