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SHA-1 in avo.

Compare to the crypto/sha1 assembly.

func main() {
	TEXT("block", 0, "func(h *[5]uint32, m []byte)")
	Doc("block SHA-1 hashes the 64-byte message m into the running state h.")
	h := Mem{Base: Load(Param("h"), GP64())}
	m := Mem{Base: Load(Param("m").Base(), GP64())}

	// Store message values on the stack.
	w := AllocLocal(64)
	W := func(r int) Mem { return w.Offset((r % 16) * 4) }

	Comment("Load initial hash.")
	hash := [5]Register{GP32(), GP32(), GP32(), GP32(), GP32()}
	for i, r := range hash {
		MOVL(h.Offset(4*i), r)

	Comment("Initialize registers.")
	a, b, c, d, e := GP32(), GP32(), GP32(), GP32(), GP32()
	for i, r := range []Register{a, b, c, d, e} {
		MOVL(hash[i], r)

	// Generate round updates.
	quarter := []struct {
		F func(Register, Register, Register) Register
		K uint32
		{choose, 0x5a827999},
		{xor, 0x6ed9eba1},
		{majority, 0x8f1bbcdc},
		{xor, 0xca62c1d6},

	for r := 0; r < 80; r++ {
		Commentf("Round %d.", r)
		q := quarter[r/20]

		// Load message value.
		u := GP32()
		if r < 16 {
			MOVL(m.Offset(4*r), u)
		} else {
			MOVL(W(r-3), u)
			XORL(W(r-8), u)
			XORL(W(r-14), u)
			XORL(W(r-16), u)
			ROLL(U8(1), u)
		MOVL(u, W(r))

		// Compute the next state register.
		t := GP32()
		MOVL(a, t)
		ROLL(U8(5), t)
		ADDL(q.F(b, c, d), t)
		ADDL(e, t)
		ADDL(U32(q.K), t)
		ADDL(u, t)

		// Update registers.
		ROLL(Imm(30), b)
		a, b, c, d, e = t, a, b, c, d

	Comment("Final add.")
	for i, r := range []Register{a, b, c, d, e} {
		ADDL(r, hash[i])

	Comment("Store results back.")
	for i, r := range hash {
		MOVL(r, h.Offset(4*i))

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