cmd/compile: improve map iteration

[cherrymui]

Currently compiling this code

type T struct {
        a, b, c, d, e, f, g, h int
}

func f(m map[int]T) {
        for _, e := range m {
                g(e.f)
        }
}

//go:noinline
func g(int) {}

results

	0x0051 00081 (/tmp/x.go:8)	LEAQ	type.map[int]"".T(SB), AX
	0x0058 00088 (/tmp/x.go:8)	MOVQ	AX, (SP)
	0x005c 00092 (/tmp/x.go:8)	MOVQ	"".m+264(FP), AX
	0x0064 00100 (/tmp/x.go:8)	MOVQ	AX, 8(SP)
	0x0069 00105 (/tmp/x.go:8)	LEAQ	""..autotmp_1+152(SP), AX
	0x0071 00113 (/tmp/x.go:8)	MOVQ	AX, 16(SP)
	0x0076 00118 (/tmp/x.go:8)	PCDATA	$0, $1
	0x0076 00118 (/tmp/x.go:8)	CALL	runtime.mapiterinit(SB)
	0x007b 00123 (/tmp/x.go:8)	MOVQ	""..autotmp_1+152(SP), AX
	0x0083 00131 (/tmp/x.go:8)	TESTQ	AX, AX
	0x0086 00134 (/tmp/x.go:8)	JEQ	$0, 253
	0x0088 00136 (/tmp/x.go:8)	MOVQ	""..autotmp_1+160(SP), SI
	0x0090 00144 (/tmp/x.go:8)	TESTB	AL, (SI)
	0x0092 00146 (/tmp/x.go:8)	LEAQ	""..autotmp_2+88(SP), DI
	0x0097 00151 (/tmp/x.go:8)	DUFFCOPY	$840
	0x00aa 00170 (/tmp/x.go:8)	MOVQ	""..autotmp_1+152(SP), AX
	0x00b2 00178 (/tmp/x.go:8)	TESTB	AL, (AX)
	0x00b4 00180 (/tmp/x.go:8)	LEAQ	"".e+24(SP), DI
	0x00b9 00185 (/tmp/x.go:8)	LEAQ	""..autotmp_2+88(SP), SI
	0x00be 00190 (/tmp/x.go:8)	DUFFCOPY	$840
	0x00d1 00209 (/tmp/x.go:9)	MOVQ	"".e+64(SP), AX
	0x00d6 00214 (/tmp/x.go:9)	MOVQ	AX, (SP)
	0x00da 00218 (/tmp/x.go:9)	PCDATA	$0, $1
	0x00da 00218 (/tmp/x.go:9)	CALL	"".g(SB)
	0x00df 00223 (/tmp/x.go:8)	LEAQ	""..autotmp_1+152(SP), AX
	0x00e7 00231 (/tmp/x.go:8)	MOVQ	AX, (SP)
	0x00eb 00235 (/tmp/x.go:8)	PCDATA	$0, $1
	0x00eb 00235 (/tmp/x.go:8)	CALL	runtime.mapiternext(SB)
	0x00f0 00240 (/tmp/x.go:8)	MOVQ	""..autotmp_1+152(SP), AX
	0x00f8 00248 (/tmp/x.go:8)	TESTQ	AX, AX
	0x00fb 00251 (/tmp/x.go:8)	JNE	$0, 136

Note that there are two DUFFCOPYs. It first copies the data pointed by the pointer that mapiterinit/mapiternext returns to autotmp_2, then copies from autotmp_2 to e.

I think one copy should be enough.

It might be able to do zero copy in some circumstances (but sounds a little crazy for now). Maybe no function call before the variable die, no address-taken, read only?
(Currently, if T is SSA-able (i.e. smaller), it generates no copy.)