code review 6506089: cmd/gc: recognize small TPTR64 values as small integer ...

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

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

[nigeltao, 2012-09-09 01:33:11]

Hello rsc@golang.org (cc: golang-dev@googlegroups.com,
remyoudompheng@gmail.com),

I'd like you to review this change to
https://go.googlecode.com/hg/

[dave_cheney.net, 2012-09-09 01:38:26]

Nice. Could you explain why ret is a TPTR64? With my limited understanding of
the compiler, ret := 0 means ret is of type int.

[remyoudompheng, 2012-09-09 05:19:46]

ret is not a PTR64. The range loop is tranformed by the frontend in:

for i, p := 0, &s[0]; i < len(s); i++, p += sizeof(s[0]) {
  x := *p
  sum += x
}

hence the constant 4.

[dave_cheney.net, 2012-09-09 05:37:49]

Ahh, thank you. To indulge me further, why does it matter if the size of the
slice element is smaller than 32 bits?

On 09/09/2012, at 15:19, remyoudompheng@gmail.com wrote:

> ret is not a PTR64. The range loop is tranformed by the frontend in:
> 
> for i, p := 0, &s[0]; i < len(s); i++, p += sizeof(s[0]) {
>  x := *p
>  sum += x
> }
> 
> hence the constant 4.
> 
> http://codereview.appspot.com/6506089/

[remyoudompheng, 2012-09-09 06:22:14]

On 2012/09/09 05:37:49, dfc wrote:
> Ahh, thank you. To indulge me further, why does it matter if the size of the
> slice element is smaller than 32 bits?

It's because (correct me if I'm wrong) although x86-64 is 64 bits, there is no
instruction to add a 64-bit constant to a register. You can only add a 32-bit
constant, or load a 64-bit constant into a separate register and then add it.

[dave_cheney.net, 2012-09-09 06:35:10]

> It's because (correct me if I'm wrong) although x86-64 is 64 bits, there is no
> instruction to add a 64-bit constant to a register. You can only add a 32-bit
> constant, or load a 64-bit constant into a separate register and then add it.

That would make sense, a 64bit constant would make for a very long instruction.
Reading the intel manual, the occurrence of imm64 is extremely rare, and not an
option with ADD/ADC.

[rsc, 2012-09-11 02:19:49]

LGTM

[nigeltao, 2012-09-11 09:44:44]

*** Submitted as http://code.google.com/p/go/source/detail?r=46e193d130ce ***

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

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
http://codereview.appspot.com/6506089