[dev.ssa] cmd/compile/internal/ssa: factor out copyelimValue and phielimValue

* Merge copyelim into phielim.
* Add phielimValue to rewrite. cgoIsGoPointer is, for example, 2
instructions smaller now.

Change-Id: I8baeb206d1b3ef8aba4a6e3bcdc432959bcae2d5
Reviewed-on: https://go-review.googlesource.com/19462Reviewed-by: David Chase <drchase@google.com>

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

@@ -8,15 +8,7 @@ package ssa
 func copyelim(f *Func) {
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
-			for i, w := range v.Args {
-				x := w
-				for x.Op == OpCopy {
-					x = x.Args[0]
-				}
-				if x != w {
-					v.Args[i] = x
-				}
-			}
+			copyelimValue(v)
 		}
 		v := b.Control
 		if v != nil {
@@ -41,3 +33,28 @@ func copyelim(f *Func) {
 		}
 	}
 }
+
+func copyelimValue(v *Value) {
+	// elide any copies generated during rewriting
+	for i, a := range v.Args {
+		if a.Op != OpCopy {
+			continue
+		}
+		// Rewriting can generate OpCopy loops.
+		// They are harmless (see removePredecessor),
+		// but take care to stop if we find a cycle.
+		slow := a // advances every other iteration
+		var advance bool
+		for a.Op == OpCopy {
+			a = a.Args[0]
+			if slow == a {
+				break
+			}
+			if advance {
+				slow = slow.Args[0]
+			}
+			advance = !advance
+		}
+		v.Args[i] = a
+	}
+}

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

@@ -18,21 +18,29 @@ package ssa
 // and would that be useful?
 func phielim(f *Func) {
 	for {
-		changed := false
+		change := false
 		for _, b := range f.Blocks {
-		nextv:
 			for _, v := range b.Values {
+				copyelimValue(v)
+				change = phielimValue(v) || change
+			}
+		}
+		if !change {
+			break
+		}
+	}
+}
+
+func phielimValue(v *Value) bool {
 	if v.Op != OpPhi {
-					continue
+		return false
 	}
+
 	// If there are two distinct args of v which
 	// are not v itself, then the phi must remain.
 	// Otherwise, we can replace it with a copy.
 	var w *Value
 	for _, x := range v.Args {
-					for x.Op == OpCopy {
-						x = x.Args[0]
-					}
 		if x == v {
 			continue
 		}
@@ -40,22 +48,17 @@ func phielim(f *Func) {
 			continue
 		}
 		if w != nil {
-						continue nextv
+			return false
 		}
 		w = x
 	}
+
 	if w == nil {
 		// v references only itself.  It must be in
 		// a dead code loop.  Don't bother modifying it.
-					continue
+		return false
 	}
 	v.Op = OpCopy
 	v.SetArgs1(w)
-				changed = true
-			}
-		}
-		if !changed {
-			break
-		}
-	}
+	return true
 }

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

@@ -40,28 +40,8 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
 			}
 			curb = nil
 			for _, v := range b.Values {
-				// elide any copies generated during rewriting
-				for i, a := range v.Args {
-					if a.Op != OpCopy {
-						continue
-					}
-					// Rewriting can generate OpCopy loops.
-					// They are harmless (see removePredecessor),
-					// but take care to stop if we find a cycle.
-					slow := a // advances every other iteration
-					var advance bool
-					for a.Op == OpCopy {
-						a = a.Args[0]
-						if slow == a {
-							break
-						}
-						if advance {
-							slow = a
-						}
-						advance = !advance
-					}
-					v.Args[i] = a
-				}
+				copyelimValue(v)
+				change = phielimValue(v) || change
 
 				// apply rewrite function
 				curv = v