pass: allow consecutive labels (#123)

Fixes #122

tests/fixedbugs/issue122/asm.go

@@ -0,0 +1,42 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Triangle", NOSPLIT, "func(n uint64) uint64")
+	Doc("Triangle computes the nth triangle number.")
+	n := Load(Param("n"), GP64())
+
+	Comment("Initialize sum register to zero.")
+	s := GP64()
+	XORQ(s, s)
+
+	// Use two labels for the top of the loop.
+	Label("loop_even")
+	Label("loop_odd")
+	Comment("Loop until n is zero.")
+	CMPQ(n, Imm(0))
+	JE(LabelRef("done"))
+
+	Comment("Add n to sum.")
+	ADDQ(n, s)
+
+	Comment("Decrement n.")
+	DECQ(n)
+
+	Comment("Jump to one of the loop labels depending on parity.")
+	TESTQ(U32(1), n)
+	JZ(LabelRef("loop_even"))
+	JMP(LabelRef("loop_odd"))
+
+	Label("done")
+	Comment("Store sum to return value.")
+	Store(s, ReturnIndex(0))
+	RET()
+	Generate()
+}

tests/fixedbugs/issue122/doc.go

@@ -0,0 +1,2 @@
+// Package issue122 tests consecutive labels.
+package issue122

tests/fixedbugs/issue122/issue122.s

@@ -0,0 +1,32 @@
+// Code generated by command: go run asm.go -out issue122.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func Triangle(n uint64) uint64
+TEXT ·Triangle(SB), NOSPLIT, $0-16
+	MOVQ n+0(FP), AX
+
+	// Initialize sum register to zero.
+	XORQ CX, CX
+
+loop_even:
+loop_odd:
+	// Loop until n is zero.
+	CMPQ AX, $0x00
+	JE   done
+
+	// Add n to sum.
+	ADDQ AX, CX
+
+	// Decrement n.
+	DECQ AX
+
+	// Jump to one of the loop labels depending on parity.
+	TESTQ $0x00000001, AX
+	JZ    loop_even
+	JMP   loop_odd
+
+done:
+	// Store sum to return value.
+	MOVQ CX, ret+8(FP)
+	RET

tests/fixedbugs/issue122/issue122_test.go

@@ -0,0 +1,16 @@
+package issue122
+
+import (
+	"testing"
+)
+
+//go:generate go run asm.go -out issue122.s -stubs stub.go
+
+func TestTriangle(t *testing.T) {
+	expect := func(n uint64) uint64 { return n * (n + 1) / 2 }
+	for n := uint64(1); n < 42; n++ {
+		if got := Triangle(n); expect(n) != got {
+			t.Fatalf("Triangle(%v) = %v; expect %v", n, got, expect(n))
+		}
+	}
+}

tests/fixedbugs/issue122/stub.go

@@ -0,0 +1,6 @@
+// Code generated by command: go run asm.go -out issue122.s -stubs stub.go. DO NOT EDIT.
+
+package issue122
+
+// Triangle computes the nth triangle number.
+func Triangle(n uint64) uint64