pass: add PruneSelfMoves cleanup pass (#80)

In some cases natural use of abstraction in avo programs can lead to redundant move instructions, specifically self-moves such as MOVQ AX, AX. This does not produce incorrect code but it is incorrect and inelegant.

This diff introduces a PruneSelfMoves pass that removes such unnecessary instructions.

Closes #76

pass/cleanup_test.go

@@ -0,0 +1,44 @@
+package pass_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/ir"
+	"github.com/mmcloughlin/avo/operand"
+	"github.com/mmcloughlin/avo/pass"
+	"github.com/mmcloughlin/avo/reg"
+)
+
+func TestPruneSelfMoves(t *testing.T) {
+	// Construct a function containing a self-move.
+	ctx := build.NewContext()
+	ctx.Function("add")
+	ctx.MOVQ(operand.U64(1), reg.RAX)
+	ctx.MOVQ(operand.U64(2), reg.RCX)
+	ctx.MOVQ(reg.RAX, reg.RAX) // self move
+	ctx.MOVQ(reg.RCX, reg.R8)
+	ctx.ADDQ(reg.R8, reg.RAX)
+
+	// Build the function without the pass and save the nodes.
+	fn := BuildFunction(t, ctx)
+	pre := append([]ir.Node{}, fn.Nodes...)
+
+	// Apply the pass.
+	if err := pass.PruneSelfMoves(fn); err != nil {
+		t.Fatal(err)
+	}
+
+	// Confirm the self-move was removed and everything else was untouched.
+	expect := []ir.Node{}
+	for i, n := range pre {
+		if i != 2 {
+			expect = append(expect, n)
+		}
+	}
+
+	if !reflect.DeepEqual(fn.Nodes, expect) {
+		t.Fatal("unexpected result from self-move pruning")
+	}
+}