[dev.ssa] cmd/compile: short-circuit empty blocks

Empty blocks are introduced to remove critical edges.
After regalloc, we can remove any of the added blocks
that are still empty.

Change-Id: I0b40e95ac3a6cc1e632a479443479532b6c5ccd9
Reviewed-on: https://go-review.googlesource.com/18833
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/trim.go

@@ -0,0 +1,37 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssa
+
+// trim removes blocks with no code in them.
+// These blocks were inserted to remove critical edges.
+func trim(f *Func) {
+	i := 0
+	for _, b := range f.Blocks {
+		if b.Kind != BlockPlain || len(b.Values) != 0 || len(b.Preds) != 1 {
+			f.Blocks[i] = b
+			i++
+			continue
+		}
+		// TODO: handle len(b.Preds)>1 case.
+
+		// Splice b out of the graph.
+		pred := b.Preds[0]
+		succ := b.Succs[0]
+		for j, s := range pred.Succs {
+			if s == b {
+				pred.Succs[j] = succ
+			}
+		}
+		for j, p := range succ.Preds {
+			if p == b {
+				succ.Preds[j] = pred
+			}
+		}
+	}
+	for j := i; j < len(f.Blocks); j++ {
+		f.Blocks[j] = nil
+	}
+	f.Blocks = f.Blocks[:i]
+}