examples: add sum example (its not pretty)

examples/sum/asm.go

@@ -0,0 +1,27 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Sum", "func(xs []uint64) uint64")
+	ptr := Load(Param("xs").Base(), GP64v())
+	n := Load(Param("xs").Len(), GP64v())
+	s := GP64v()
+	XORQ(s, s)
+	LABEL("loop")
+	CMPQ(n, operand.Imm(0))
+	JE(operand.LabelRef("done"))
+	ADDQ(operand.Mem{Base: ptr}, s)
+	ADDQ(operand.Imm(8), ptr)
+	DECQ(n)
+	JMP(operand.LabelRef("loop"))
+	LABEL("done")
+	Store(s, ReturnIndex(0))
+	RET()
+	Generate()
+}

examples/sum/stub.go

@@ -0,0 +1,3 @@
+package sum
+
+func Sum(xs []uint64) uint64

examples/sum/sum.s

@@ -0,0 +1,18 @@
+
+#include "textflag.h"
+
+// func Sum(xs []uint64) uint64
+TEXT ·Sum(SB),0,$0-32
+	MOVQ	xs_base(FP), DX
+	MOVQ	xs_len+8(FP), CX
+	XORQ	AX, AX
+loop:
+	CMPQ	CX, $0x0
+	JE	done
+	ADDQ	(DX), AX
+	ADDQ	$0x8, DX
+	DECQ	CX
+	JMP	loop
+done:
+	MOVQ	AX, ret+24(FP)
+	RET	

examples/sum/sum_test.go

@@ -0,0 +1,20 @@
+package sum
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out sum.s -stubs stub.go
+
+func expect(xs []uint64) uint64 {
+	var s uint64
+	for _, x := range xs {
+		s += x
+	}
+	return s
+}
+
+func TestSum(t *testing.T) {
+	quick.CheckEqual(Sum, expect, nil)
+}