[dev.ssa] cmd/compile : replace load of store with a copy

Loads of stores from the same pointer with compatible types
can be replaced with a copy.

Change-Id: I514b3ed8e5b6a9c432946880eac67a51b1607932
Reviewed-on: https://go-review.googlesource.com/19743
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -388,6 +388,11 @@
 (EqSlice x y)  -> (EqPtr  (SlicePtr x) (SlicePtr y))
 (NeqSlice x y) -> (NeqPtr (SlicePtr x) (SlicePtr y))
 
+
+// Load of store of same address, with compatibly typed value and same size
+(Load <t1> p1 (Store [w] p2 x _)) && isSamePtr(p1,p2) && t1.Compare(x.Type)==CMPeq && w == t1.Size() -> x
+
+
 // indexing operations
 // Note: bounds check has already been done
 (ArrayIndex (Load ptr mem) idx) && b == v.Args[0].Block -> (Load (PtrIndex <v.Type.PtrTo()> ptr idx) mem)

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

@@ -202,6 +202,15 @@ func uaddOvf(a, b int64) bool {
 	return uint64(a)+uint64(b) < uint64(a)
 }
 
+// isSamePtr reports whether p1 and p2 point to the same address.
+func isSamePtr(p1, p2 *Value) bool {
+	// Aux isn't used  in OffPtr, and AuxInt isn't currently used in
+	// Addr, but this still works as the values will be null/0
+	return (p1.Op == OpOffPtr || p1.Op == OpAddr) && p1.Op == p2.Op &&
+		p1.Aux == p2.Aux && p1.AuxInt == p2.AuxInt &&
+		p1.Args[0] == p2.Args[0]
+}
+
 // DUFFZERO consists of repeated blocks of 4 MOVUPSs + ADD,
 // See runtime/mkduff.go.
 const (

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

@@ -2901,6 +2901,26 @@ func rewriteValuegeneric_OpLess8U(v *Value, config *Config) bool {
 func rewriteValuegeneric_OpLoad(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b
+	// match: (Load <t1> p1 (Store [w] p2 x _))
+	// cond: isSamePtr(p1,p2) && t1.Compare(x.Type)==CMPeq && w == t1.Size()
+	// result: x
+	for {
+		t1 := v.Type
+		p1 := v.Args[0]
+		if v.Args[1].Op != OpStore {
+			break
+		}
+		w := v.Args[1].AuxInt
+		p2 := v.Args[1].Args[0]
+		x := v.Args[1].Args[1]
+		if !(isSamePtr(p1, p2) && t1.Compare(x.Type) == CMPeq && w == t1.Size()) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
 	// match: (Load <t> _ _)
 	// cond: t.IsStruct() && t.NumFields() == 0 && config.fe.CanSSA(t)
 	// result: (StructMake0)