package main

func f() int {
buf := make([]byte, 1024)
return len(buf)
}

go tool 6g -S x.go

    000000 00000 (x.go:3)   TEXT    "".f+0(SB),$1056-8
    000000 00000 (x.go:3)   MOVQ    2208(GS),CX
    0x0009 00009 (x.go:3)   LEAQ    -928(SP),AX
    0x0011 00017 (x.go:3)   CMPQ    AX,(CX)
    0x0014 00020 (x.go:3)   JHI ,29
    0x0016 00022 (x.go:3)   CALL    ,runtime.morestack8(SB)
    0x001b 00027 (x.go:3)   JMP ,0
    0x001d 00029 (x.go:3)   SUBQ    $1056,SP
    0x0024 00036 (x.go:3)   FUNCDATA    $2,"".gcargs·0+0(SB)
    0x0024 00036 (x.go:3)   FUNCDATA    $3,"".gclocals·1+0(SB)
    0x0024 00036 (x.go:4)   LEAQ    "".autotmp_0000+0(SP),DI
    0x0028 00040 (x.go:4)   MOVQ    $0,AX
    0x002b 00043 (x.go:4)   MOVQ    $128,CX
    0x0032 00050 (x.go:4)   REP ,
    0x0033 00051 (x.go:4)   STOSQ   ,
    0x0035 00053 (x.go:4)   LEAQ    "".autotmp_0000+0(SP),BX
    0x0039 00057 (x.go:4)   MOVQ    BX,"".autotmp_0001+1024(SP)
    0x0041 00065 (x.go:4)   MOVQ    $1024,"".buf+1040(SP)
    0x004d 00077 (x.go:4)   MOVQ    $1024,"".buf+1048(SP)
    0x0059 00089 (x.go:4)   MOVQ    "".autotmp_0001+1024(SP),BX
    0x0061 00097 (x.go:4)   MOVQ    BX,"".buf+1032(SP)
    0x0069 00105 (x.go:4)   CMPQ    "".buf+1032(SP),$0
    0x0072 00114 (x.go:4)   JEQ $1,140
    0x0074 00116 (x.go:5)   MOVQ    "".buf+1040(SP),BX
    0x007c 00124 (x.go:5)   MOVQ    BX,"".~anon0+1064(FP)
    0x0084 00132 (x.go:5)   ADDQ    $1056,SP
    0x008b 00139 (x.go:5)   RET ,
    0x008c 00140 (x.go:4)   MOVL    AX,(NONE)
    0x0093 00147 (x.go:5)   JMP ,116

the CMPQ at 0x0069 is a nil check during len(buf), which is bogus.

also, why is the initialization of the header not registerizing better and then being
optimized away?
i would expect this code to be something like

SUBQ $1056, SP
LEAQ autotmp+0(SP), DI
MOVQ $0, AX
MOVQ $128, CX
REP
STOSQ
MOVQ $1024, result+1064(FP)
RET