[dev.ssa] cmd/internal/ssa: add deadstore pass

Eliminate dead stores.  Dead stores are those which are
unconditionally followed by another store to the same location, with
no intervening load.

Just a simple intra-block implementation for now.

Change-Id: I2bf54e3a342608fc4e01edbe1b429e83f24764ab
Reviewed-on: https://go-review.googlesource.com/10386
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -56,6 +56,7 @@ var passes = [...]pass{
 	{"opt", opt},
 	{"generic cse", cse},
 	{"generic deadcode", deadcode},
+	{"dse", dse},
 	{"fuse", fuse},
 	{"lower", lower},
 	{"lowered cse", cse},
@@ -76,6 +77,9 @@ type constraint struct {
 }
 
 var passOrder = [...]constraint{
+	// common-subexpression before dead-store elim, so that we recognize
+	// when two address expressions are the same.
+	{"generic cse", "dse"},
 	// don't layout blocks until critical edges have been removed
 	{"critical", "layout"},
 	// regalloc requires the removal of all critical edges

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

@@ -0,0 +1,103 @@
+// 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 "log"
+
+// dse does dead-store elimination on the Function.
+// Dead stores are those which are unconditionally followed by
+// another store to the same location, with no intervening load.
+// This implementation only works within a basic block.  TODO: use something more global.
+func dse(f *Func) {
+	var stores []*Value
+	loadUse := newSparseSet(f.NumValues())
+	storeUse := newSparseSet(f.NumValues())
+	shadowed := newSparseSet(f.NumValues())
+	for _, b := range f.Blocks {
+		// Find all the stores in this block.  Categorize their uses:
+		//  loadUse contains stores which are used by a subsequent load.
+		//  storeUse contains stores which are used by a subsequent store.
+		loadUse.clear()
+		storeUse.clear()
+		stores = stores[:0]
+		for _, v := range b.Values {
+			if v.Op == OpPhi {
+				// Ignore phis - they will always be first and can't be eliminated
+				continue
+			}
+			if v.Type.IsMemory() {
+				stores = append(stores, v)
+				for _, a := range v.Args {
+					if a.Block == b && a.Type.IsMemory() {
+						storeUse.add(a.ID)
+						if v.Op != OpStore {
+							// CALL, DUFFCOPY, etc. are both
+							// reads and writes.
+							loadUse.add(a.ID)
+						}
+					}
+				}
+			} else {
+				for _, a := range v.Args {
+					if a.Block == b && a.Type.IsMemory() {
+						loadUse.add(a.ID)
+					}
+				}
+			}
+		}
+		if len(stores) == 0 {
+			continue
+		}
+
+		// find last store in the block
+		var last *Value
+		for _, v := range stores {
+			if storeUse.contains(v.ID) {
+				continue
+			}
+			if last != nil {
+				log.Fatalf("two final stores - simultaneous live stores", last, v)
+			}
+			last = v
+		}
+		if last == nil {
+			log.Fatalf("no last store found - cycle?")
+		}
+
+		// Walk backwards looking for dead stores.  Keep track of shadowed addresses.
+		// An "address" is an SSA Value which encodes both the address and size of
+		// the write.  This code will not remove dead stores to the same address
+		// of different types.
+		shadowed.clear()
+		v := last
+
+	walkloop:
+		if loadUse.contains(v.ID) {
+			// Someone might be reading this memory state.
+			// Clear all shadowed addresses.
+			shadowed.clear()
+		}
+		if v.Op == OpStore {
+			if shadowed.contains(v.Args[0].ID) {
+				// Modify store into a copy
+				v.Op = OpCopy
+				v.Aux = nil
+				v.SetArgs1(v.Args[2])
+			} else {
+				shadowed.add(v.Args[0].ID)
+			}
+		}
+		// walk to previous store
+		if v.Op == OpPhi {
+			continue // At start of block.  Move on to next block.
+		}
+		for _, a := range v.Args {
+			if a.Block == b && a.Type.IsMemory() {
+				v = a
+				goto walkloop
+			}
+		}
+	}
+}

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

@@ -0,0 +1,87 @@
+// 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 (
+	"testing"
+)
+
+func TestDeadStore(t *testing.T) {
+	c := NewConfig("amd64", DummyFrontend{})
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("start", OpArg, TypeMem, ".mem"),
+			Valu("v", OpConst, TypeBool, true),
+			Valu("addr1", OpGlobal, ptrType, nil),
+			Valu("addr2", OpGlobal, ptrType, nil),
+			Valu("store1", OpStore, TypeMem, nil, "addr1", "v", "start"),
+			Valu("store2", OpStore, TypeMem, nil, "addr2", "v", "store1"),
+			Valu("store3", OpStore, TypeMem, nil, "addr1", "v", "store2"),
+			Goto("exit")),
+		Bloc("exit",
+			Exit("store3")))
+
+	CheckFunc(fun.f)
+	dse(fun.f)
+	CheckFunc(fun.f)
+
+	v := fun.values["store1"]
+	if v.Op != OpCopy {
+		t.Errorf("dead store not removed")
+	}
+}
+func TestDeadStorePhi(t *testing.T) {
+	// make sure we don't get into an infinite loop with phi values.
+	c := NewConfig("amd64", DummyFrontend{})
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("start", OpArg, TypeMem, ".mem"),
+			Valu("v", OpConst, TypeBool, true),
+			Valu("addr", OpGlobal, ptrType, nil),
+			Goto("loop")),
+		Bloc("loop",
+			Valu("phi", OpPhi, TypeMem, nil, "start", "store"),
+			Valu("store", OpStore, TypeMem, nil, "addr", "v", "phi"),
+			If("v", "loop", "exit")),
+		Bloc("exit",
+			Exit("store")))
+
+	CheckFunc(fun.f)
+	dse(fun.f)
+	CheckFunc(fun.f)
+}
+
+func TestDeadStoreTypes(t *testing.T) {
+	// Make sure a narrow store can't shadow a wider one.  We test an even
+	// stronger restriction, that one store can't shadow another unless the
+	// types of the address fields are identical (where identicalness is
+	// decided by the CSE pass).
+	c := NewConfig("amd64", DummyFrontend{})
+	t1 := &TypeImpl{Size_: 8, Ptr: true, Name: "t1"}
+	t2 := &TypeImpl{Size_: 4, Ptr: true, Name: "t2"}
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("start", OpArg, TypeMem, ".mem"),
+			Valu("v", OpConst, TypeBool, true),
+			Valu("addr1", OpGlobal, t1, nil),
+			Valu("addr2", OpGlobal, t2, nil),
+			Valu("store1", OpStore, TypeMem, nil, "addr1", "v", "start"),
+			Valu("store2", OpStore, TypeMem, nil, "addr2", "v", "store1"),
+			Goto("exit")),
+		Bloc("exit",
+			Exit("store2")))
+
+	CheckFunc(fun.f)
+	cse(fun.f)
+	dse(fun.f)
+	CheckFunc(fun.f)
+
+	v := fun.values["store1"]
+	if v.Op == OpCopy {
+		t.Errorf("store %s incorrectly removed", v)
+	}
+}

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

@@ -70,8 +70,8 @@ const (
 	OpStringPtr  // ptr(arg0)
 	OpStringLen  // len(arg0)
 
-	OpLoad       // Load from arg0+aux.(int64).  arg1=memory
-	OpStore      // Store arg1 to arg0+aux.(int64).  arg2=memory.  Returns memory.
+	OpLoad       // Load from arg0.  arg1=memory
+	OpStore      // Store arg1 to arg0.  arg2=memory.  Returns memory.
 	OpArrayIndex // arg0=array, arg1=index.  Returns a[i]
 	OpPtrIndex   // arg0=ptr, arg1=index. Computes ptr+sizeof(*v.type)*index, where index is extended to ptrwidth type
 	OpIsNonNil   // arg0 != nil