Compile the following program:

package footest

import (
    "bytes"
)

func foo() {
    var buf bytes.Buffer
    var b [4]byte
    buf.Write(b[:])
}

This produces the following assembly for the buf.Write line:

    0x0049 00073 (test.go:10)   MOVQ    $4,DX
    0x0050 00080 (test.go:10)   MOVQ    $4,CX
    0x0057 00087 (test.go:10)   MOVQ    BX,"".autotmp_0002+64(SP)
    0x005c 00092 (test.go:10)   MOVQ    BX,8(SP)
    0x0061 00097 (test.go:10)   MOVQ    DX,"".autotmp_0002+72(SP)
    0x0066 00102 (test.go:10)   MOVQ    DX,16(SP)
    0x006b 00107 (test.go:10)   MOVQ    CX,"".autotmp_0002+80(SP)
    0x0070 00112 (test.go:10)   MOVQ    CX,24(SP)
    0x0075 00117 (test.go:10)   PCDATA  $0,$0
    0x0075 00117 (test.go:10)   CALL    ,bytes.(*Buffer).Write(SB)

Note that after this, autotmp_0002 is never used again.

regopt gets passed this as its input (obtained via -R -v):

00000 (test.go:7)   TEXT    "".foo+0(SB),$0-0
00001 (test.go:7)   FUNCDATA    $0,"".gcargs·0+0(SB)
00002 (test.go:7)   FUNCDATA    $1,"".gclocals·1+0(SB)
00003 (test.go:7)   TYPE    "".b+0(SP),$4
00004 (test.go:7)   TYPE    "".&buf+0(SP),$8
00005 (test.go:7)   TYPE    "".autotmp_0001+0(SP),$8
00006 (test.go:8)   MOVQ    $type.bytes.Buffer+0(SB),BX
00007 (test.go:8)   MOVQ    BX,(SP)
00008 (test.go:8)   CALL    ,runtime.newobject+0(SB)
00009 (test.go:8)   MOVQ    8(SP),BX
00010 (test.go:8)   MOVQ    BX,"".&buf+0(SP)
00011 (test.go:9)   VARDEF  ,"".b+0(SP)
00012 (test.go:9)   LEAQ    "".b+0(SP),BX
00013 (test.go:9)   MOVL    $0,(BX)
00014 (test.go:10)  LEAQ    "".b+0(SP),BX
00015 (test.go:10)  MOVQ    BX,"".autotmp_0001+0(SP)
00016 (test.go:10)  MOVQ    "".&buf+0(SP),BX
00017 (test.go:10)  MOVQ    BX,(SP)
00018 (test.go:10)  MOVQ    "".autotmp_0001+0(SP),BX
00019 (test.go:10)  MOVQ    BX,"".autotmp_0003+0(SP)
00020 (test.go:10)  CHECKNIL    "".autotmp_0003+0(SP),
00021 (test.go:10)  VARDEF  ,"".autotmp_0002+0(SP)
00022 (test.go:10)  MOVQ    $4,"".autotmp_0004+0(SP)
00023 (test.go:10)  MOVQ    $4,"".autotmp_0005+0(SP)
00024 (test.go:10)  MOVQ    "".autotmp_0003+0(SP),BX
00025 (test.go:10)  MOVQ    BX,"".autotmp_0002+0(SP)
00026 (test.go:10)  MOVQ    "".autotmp_0004+0(SP),BX
00027 (test.go:10)  MOVQ    BX,"".autotmp_0002+8(SP)
00028 (test.go:10)  MOVQ    "".autotmp_0005+0(SP),BX
00029 (test.go:10)  MOVQ    BX,"".autotmp_0002+16(SP)
00030 (test.go:10)  MOVQ    "".autotmp_0002+0(SP),BX
00031 (test.go:10)  MOVQ    BX,8(SP)
00032 (test.go:10)  MOVQ    "".autotmp_0002+8(SP),BX
00033 (test.go:10)  MOVQ    BX,16(SP)
00034 (test.go:10)  MOVQ    "".autotmp_0002+16(SP),BX
00035 (test.go:10)  MOVQ    BX,24(SP)
00036 (test.go:10)  CALL    ,bytes.(*Buffer).Write+0(SB)
00037 (test.go:11)  RET ,

Note that autotmp_0002 has a vardef which was inserted during cgen_slice, but no
matching varkill instruction.

Do we need to insert vardefs for slices? We don't care about their cap or len anymore.