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cmd/gc: recognize small TPTR64 values as small integer constants.

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Given the following Go program:

func sum(s []int) int {
        ret := 0
        for _, x := range s {
                ret += x
        }
        return ret
}

6g would previously generate:

--- prog list "sum" ---
0000 (main.go:3) TEXT    sum+0(SB),$0-24
0001 (main.go:5) MOVQ    s+0(FP),CX
0002 (main.go:5) MOVL    s+8(FP),DI
0003 (main.go:5) MOVL    s+12(FP),BX
0004 (main.go:4) MOVL    $0,SI
0005 (main.go:5) MOVL    $0,AX
0006 (main.go:5) JMP     ,8
0007 (main.go:5) INCL    ,AX
0008 (main.go:5) CMPL    AX,DI
0009 (main.go:5) JGE     $0,16
0010 (main.go:5) MOVL    (CX),DX
0011 (main.go:5) MOVQ    $4,BX
0012 (main.go:5) ADDQ    CX,BX
0013 (main.go:5) MOVQ    BX,CX
0014 (main.go:6) ADDL    DX,SI
0015 (main.go:5) JMP     ,7
0016 (main.go:8) MOVL    SI,.noname+16(FP)
0017 (main.go:8) RET     ,

and now generates:

--- prog list "sum" ---
0000 (main.go:3) TEXT    sum+0(SB),$0-24
0001 (main.go:5) MOVQ    s+0(FP),CX
0002 (main.go:5) MOVL    s+8(FP),DI
0003 (main.go:5) MOVL    s+12(FP),BX
0004 (main.go:4) MOVL    $0,SI
0005 (main.go:5) MOVL    $0,AX
0006 (main.go:5) JMP     ,8
0007 (main.go:5) INCL    ,AX
0008 (main.go:5) CMPL    AX,DI
0009 (main.go:5) JGE     $0,14
0010 (main.go:5) MOVL    (CX),BP
0011 (main.go:5) ADDQ    $4,CX
0012 (main.go:6) ADDL    BP,SI
0013 (main.go:5) JMP     ,7
0014 (main.go:8) MOVL    SI,.noname+16(FP)
0015 (main.go:8) RET     ,

The key difference is that
0011 (main.go:5) MOVQ    $4,BX
0012 (main.go:5) ADDQ    CX,BX
0013 (main.go:5) MOVQ    BX,CX
has changed to
0011 (main.go:5) ADDQ    $4,CX

R=rsc, dave, remyoudompheng
CC=golang-dev
https://golang.org/cl/6506089
This commit is contained in:
Nigel Tao 2012-09-11 19:45:28 +10:00
parent b64a99da88
commit 0184081eb9
1 changed files with 1 additions and 0 deletions
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1
src/cmd/gc/const.c
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@ -1207,6 +1207,7 @@ smallintconst(Node *n)
case TIDEAL:
case TINT64:
case TUINT64:
case TPTR64:
if(mpcmpfixfix(n->val.u.xval, minintval[TINT32]) < 0
|| mpcmpfixfix(n->val.u.xval, maxintval[TINT32]) > 0)
break;