examples/dot: initial version

Implements dot product in avo.

Closes #4

examples/dot/asm.go

@@ -0,0 +1,93 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+	. "github.com/mmcloughlin/avo/reg"
+)
+
+var unroll = 6
+
+func main() {
+	TEXT("Dot", "func(x, y []float32) float32")
+	x := Mem{Base: Load(Param("x").Base(), GP64v())}
+	y := Mem{Base: Load(Param("y").Base(), GP64v())}
+	n := Load(Param("x").Len(), GP64v())
+
+	// Allocate accumulation registers.
+	acc := make([]VecVirtual, unroll)
+	for i := 0; i < unroll; i++ {
+		acc[i] = Yv()
+	}
+
+	// Zero initialization.
+	for i := 0; i < unroll; i++ {
+		VXORPS(acc[i], acc[i], acc[i])
+	}
+
+	// Loop over blocks and process them with vector instructions.
+	blockitems := 8 * unroll
+	blocksize := 4 * blockitems
+	LABEL("blockloop")
+	CMPQ(n, U32(blockitems))
+	JL(LabelRef("tail"))
+
+	// Load x.
+	xs := make([]VecVirtual, unroll)
+	for i := 0; i < unroll; i++ {
+		xs[i] = Yv()
+	}
+
+	for i := 0; i < unroll; i++ {
+		VMOVUPS(x.Offset(32*i), xs[i])
+	}
+
+	// The actual FMA.
+	for i := 0; i < unroll; i++ {
+		VFMADD231PS(y.Offset(32*i), xs[i], acc[i])
+	}
+
+	ADDQ(U32(blocksize), x.Base)
+	ADDQ(U32(blocksize), y.Base)
+	SUBQ(U32(blockitems), n)
+	JMP(LabelRef("blockloop"))
+
+	// Process any trailing entries.
+	LABEL("tail")
+	tail := Xv()
+	VXORPS(tail, tail, tail)
+
+	LABEL("tailloop")
+	CMPQ(n, U32(0))
+	JE(LabelRef("reduce"))
+
+	xt := Xv()
+	VMOVSS(x, xt)
+	VFMADD231SS(y, xt, tail)
+
+	ADDQ(U32(4), x.Base)
+	ADDQ(U32(4), y.Base)
+	DECQ(n)
+	JMP(LabelRef("tailloop"))
+
+	// Reduce the lanes to one.
+	LABEL("reduce")
+	for i := 1; i < unroll; i++ {
+		VADDPS(acc[0], acc[i], acc[0])
+	}
+
+	result := acc[0].AsX()
+	top := Xv()
+	VEXTRACTF128(U8(1), acc[0], top)
+	VADDPS(result, top, result)
+	VADDPS(result, tail, result)
+	VHADDPS(result, result, result)
+	VHADDPS(result, result, result)
+	Store(result, ReturnIndex(0))
+
+	RET()
+
+	Generate()
+}