1 files changed, 75 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/phielim.go b/src/cmd/compile/internal/ssa/phielim.go
new file mode 100644
index 0000000..4fc9423
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/phielim.go
@@ -0,0 +1,75 @@
+// 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
+
+// phielim eliminates redundant phi values from f.
+// A phi is redundant if its arguments are all equal. For
+// purposes of counting, ignore the phi itself. Both of
+// these phis are redundant:
+//
+//	v = phi(x,x,x)
+//	v = phi(x,v,x,v)
+//
+// We repeat this process to also catch situations like:
+//
+//	v = phi(x, phi(x, x), phi(x, v))
+//
+// TODO: Can we also simplify cases like:
+//
+//	v = phi(v, w, x)
+//	w = phi(v, w, x)
+//
+// and would that be useful?
+func phielim(f *Func) {
+	for {
+		change := false
+		for _, b := range f.Blocks {
+			for _, v := range b.Values {
+				copyelimValue(v)
+				change = phielimValue(v) || change
+			}
+		}
+		if !change {
+			break
+		}
+	}
+}
+
+// phielimValue tries to convert the phi v to a copy.
+func phielimValue(v *Value) bool {
+	if v.Op != OpPhi {
+		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 {
+		if x == v {
+			continue
+		}
+		if x == w {
+			continue
+		}
+		if w != nil {
+			return false
+		}
+		w = x
+	}
+
+	if w == nil {
+		// v references only itself. It must be in
+		// a dead code loop. Don't bother modifying it.
+		return false
+	}
+	v.Op = OpCopy
+	v.SetArgs1(w)
+	f := v.Block.Func
+	if f.pass.debug > 0 {
+		f.Warnl(v.Pos, "eliminated phi")
+	}
+	return true
+}