[dev.ssa] Merge remote-tracking branch 'origin/master' into mergebranch

Semi-regular merge of tip to dev.ssa.

Complicated a bit by the move of cmd/internal/* to cmd/compile/internal/*.

Change-Id: I1c66d3c29bb95cce4a53c5a3476373aa5245303d

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

@@ -0,0 +1,163 @@
+// Copyright 2015 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
+
+import "sort"
+
+// cse does common-subexpression elimination on the Function.
+// Values are just relinked, nothing is deleted.  A subsequent deadcode
+// pass is required to actually remove duplicate expressions.
+func cse(f *Func) {
+	// Two values are equivalent if they satisfy the following definition:
+	// equivalent(v, w):
+	//   v.op == w.op
+	//   v.type == w.type
+	//   v.aux == w.aux
+	//   len(v.args) == len(w.args)
+	//   equivalent(v.args[i], w.args[i]) for i in 0..len(v.args)-1
+
+	// The algorithm searches for a partition of f's values into
+	// equivalence classes using the above definition.
+	// It starts with a coarse partition and iteratively refines it
+	// until it reaches a fixed point.
+
+	// Make initial partition based on opcode/type/aux/nargs
+	// TODO(khr): types are not canonical, so we may split unnecessarily.  Fix that.
+	type key struct {
+		op    Op
+		typ   Type
+		aux   interface{}
+		nargs int
+	}
+	m := map[key]eqclass{}
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			k := key{v.Op, v.Type, v.Aux, len(v.Args)}
+			m[k] = append(m[k], v)
+		}
+	}
+
+	// A partition is a set of disjoint eqclasses.
+	var partition []eqclass
+	for _, v := range m {
+		partition = append(partition, v)
+	}
+
+	// map from value id back to eqclass id
+	valueEqClass := make([]int, f.NumValues())
+	for i, e := range partition {
+		for _, v := range e {
+			valueEqClass[v.ID] = i
+		}
+	}
+
+	// Find an equivalence class where some members of the class have
+	// non-equivalent arguments.  Split the equivalence class appropriately.
+	// Repeat until we can't find any more splits.
+	for {
+		changed := false
+
+		for i, e := range partition {
+			v := e[0]
+			// all values in this equiv class that are not equivalent to v get moved
+			// into another equiv class q.
+			var q eqclass
+		eqloop:
+			for j := 1; j < len(e); {
+				w := e[j]
+				for i := 0; i < len(v.Args); i++ {
+					if valueEqClass[v.Args[i].ID] != valueEqClass[w.Args[i].ID] {
+						// w is not equivalent to v.
+						// remove w from e
+						e, e[j] = e[:len(e)-1], e[len(e)-1]
+						// add w to q
+						q = append(q, w)
+						valueEqClass[w.ID] = len(partition)
+						changed = true
+						continue eqloop
+					}
+				}
+				// v and w are equivalent.  Keep w in e.
+				j++
+			}
+			partition[i] = e
+			if q != nil {
+				partition = append(partition, q)
+			}
+		}
+
+		if !changed {
+			break
+		}
+	}
+
+	// Compute dominator tree
+	idom := dominators(f)
+
+	// Compute substitutions we would like to do.  We substitute v for w
+	// if v and w are in the same equivalence class and v dominates w.
+	rewrite := make([]*Value, f.NumValues())
+	for _, e := range partition {
+		sort.Sort(e) // ensure deterministic ordering
+		for len(e) > 1 {
+			// Find a maximal dominant element in e
+			v := e[0]
+			for _, w := range e[1:] {
+				if dom(w.Block, v.Block, idom) {
+					v = w
+				}
+			}
+
+			// Replace all elements of e which v dominates
+			for i := 0; i < len(e); {
+				w := e[i]
+				if w == v {
+					e, e[i] = e[:len(e)-1], e[len(e)-1]
+				} else if dom(v.Block, w.Block, idom) {
+					rewrite[w.ID] = v
+					e, e[i] = e[:len(e)-1], e[len(e)-1]
+				} else {
+					i++
+				}
+			}
+			// TODO(khr): if value is a control value, do we need to keep it block-local?
+		}
+	}
+
+	// Apply substitutions
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			for i, w := range v.Args {
+				if x := rewrite[w.ID]; x != nil {
+					v.SetArg(i, x)
+				}
+			}
+		}
+	}
+}
+
+// returns true if b dominates c.
+// TODO(khr): faster
+func dom(b, c *Block, idom []*Block) bool {
+	// Walk up from c in the dominator tree looking for b.
+	for c != nil {
+		if c == b {
+			return true
+		}
+		c = idom[c.ID]
+	}
+	// Reached the entry block, never saw b.
+	return false
+}
+
+// An eqclass approximates an equivalence class.  During the
+// algorithm it may represent the union of several of the
+// final equivalence classes.
+type eqclass []*Value
+
+// Sort an equivalence class by value ID.
+func (e eqclass) Len() int           { return len(e) }
+func (e eqclass) Swap(i, j int)      { e[i], e[j] = e[j], e[i] }
+func (e eqclass) Less(i, j int) bool { return e[i].ID < e[j].ID }