1 files changed, 79 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/zcse.go b/src/cmd/compile/internal/ssa/zcse.go
new file mode 100644
index 0000000..e08272c
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/zcse.go
@@ -0,0 +1,79 @@
+// 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
+
+import "cmd/compile/internal/types"
+
+// zcse does an initial pass of common-subexpression elimination on the
+// function for values with zero arguments to allow the more expensive cse
+// to begin with a reduced number of values. Values are just relinked,
+// nothing is deleted. A subsequent deadcode pass is required to actually
+// remove duplicate expressions.
+func zcse(f *Func) {
+	vals := make(map[vkey]*Value)
+
+	for _, b := range f.Blocks {
+		for i := 0; i < len(b.Values); i++ {
+			v := b.Values[i]
+			if opcodeTable[v.Op].argLen == 0 {
+				key := vkey{v.Op, keyFor(v), v.Aux, v.Type}
+				if vals[key] == nil {
+					vals[key] = v
+					if b != f.Entry {
+						// Move v to the entry block so it will dominate every block
+						// where we might use it. This prevents the need for any dominator
+						// calculations in this pass.
+						v.Block = f.Entry
+						f.Entry.Values = append(f.Entry.Values, v)
+						last := len(b.Values) - 1
+						b.Values[i] = b.Values[last]
+						b.Values[last] = nil
+						b.Values = b.Values[:last]
+
+						i-- // process b.Values[i] again
+					}
+				}
+			}
+		}
+	}
+
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			for i, a := range v.Args {
+				if opcodeTable[a.Op].argLen == 0 {
+					key := vkey{a.Op, keyFor(a), a.Aux, a.Type}
+					if rv, ok := vals[key]; ok {
+						v.SetArg(i, rv)
+					}
+				}
+			}
+		}
+	}
+}
+
+// vkey is a type used to uniquely identify a zero arg value.
+type vkey struct {
+	op Op
+	ai int64       // aux int
+	ax Aux         // aux
+	t  *types.Type // type
+}
+
+// keyFor returns the AuxInt portion of a  key structure uniquely identifying a
+// zero arg value for the supported ops.
+func keyFor(v *Value) int64 {
+	switch v.Op {
+	case OpConst64, OpConst64F, OpConst32F:
+		return v.AuxInt
+	case OpConst32:
+		return int64(int32(v.AuxInt))
+	case OpConst16:
+		return int64(int16(v.AuxInt))
+	case OpConst8, OpConstBool:
+		return int64(int8(v.AuxInt))
+	default:
+		return v.AuxInt
+	}
+}