cmd/compile: teach dse about equivalent LocalAddrs

This patch teaches DSE that two LocalAddrs of the same variable
are equal, even if they are from different memory states. This avoids
dependance on a store into the same LocalAddr being added to
loadUse even though the store is unnecessary and is in fact
shadowed.

Fixes #59021

Change-Id: I0ef128b783c4ad6fd2236fa5ff20345b4d31eddb
GitHub-Last-Rev: b80a6b28fb
GitHub-Pull-Request: golang/go#66793
Reviewed-on: https://go-review.googlesource.com/c/go/+/578376
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Joedian Reid <joedian@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -21,12 +21,18 @@ func dse(f *Func) {
 	defer f.retSparseSet(storeUse)
 	shadowed := f.newSparseMap(f.NumValues())
 	defer f.retSparseMap(shadowed)
+	// localAddrs maps from a local variable (the Aux field of a LocalAddr value) to an instance of a LocalAddr value for that variable in the current block.
+	localAddrs := map[any]*Value{}
 	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()
+		// TODO(deparker): use the 'clear' builtin once compiler bootstrap minimum version is raised to 1.21.
+		for k := range localAddrs {
+			delete(localAddrs, k)
+		}
 		stores = stores[:0]
 		for _, v := range b.Values {
 			if v.Op == OpPhi {
@@ -46,6 +52,13 @@ func dse(f *Func) {
 					}
 				}
 			} else {
+				if v.Op == OpLocalAddr {
+					if _, ok := localAddrs[v.Aux]; !ok {
+						localAddrs[v.Aux] = v
+					} else {
+						continue
+					}
+				}
 				for _, a := range v.Args {
 					if a.Block == b && a.Type.IsMemory() {
 						loadUse.add(a.ID)
@@ -100,6 +113,11 @@ func dse(f *Func) {
 			} else { // OpZero
 				sz = v.AuxInt
 			}
+			if ptr.Op == OpLocalAddr {
+				if la, ok := localAddrs[ptr.Aux]; ok {
+					ptr = la
+				}
+			}
 			sr := shadowRange(shadowed.get(ptr.ID))
 			if sr.contains(off, off+sz) {
 				// Modify the store/zero into a copy of the memory state,
@@ -146,6 +164,7 @@ type shadowRange int32
 func (sr shadowRange) lo() int64 {
 	return int64(sr & 0xffff)
 }
+
 func (sr shadowRange) hi() int64 {
 	return int64((sr >> 16) & 0xffff)
 }

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

@@ -6,6 +6,7 @@ package ssa
 
 import (
 	"cmd/compile/internal/types"
+	"cmd/internal/src"
 	"testing"
 )
 
@@ -44,6 +45,7 @@ func TestDeadStore(t *testing.T) {
 		t.Errorf("dead store (zero) not removed")
 	}
 }
+
 func TestDeadStorePhi(t *testing.T) {
 	// make sure we don't get into an infinite loop with phi values.
 	c := testConfig(t)
@@ -127,3 +129,46 @@ func TestDeadStoreUnsafe(t *testing.T) {
 		t.Errorf("store %s incorrectly removed", v)
 	}
 }
+
+func TestDeadStoreSmallStructInit(t *testing.T) {
+	c := testConfig(t)
+	ptrType := c.config.Types.BytePtr
+	typ := types.NewStruct([]*types.Field{
+		types.NewField(src.NoXPos, &types.Sym{Name: "A"}, c.config.Types.Int),
+		types.NewField(src.NoXPos, &types.Sym{Name: "B"}, c.config.Types.Int),
+	})
+	name := c.Temp(typ)
+	fun := c.Fun("entry",
+		Bloc("entry",
+			Valu("start", OpInitMem, types.TypeMem, 0, nil),
+			Valu("sp", OpSP, c.config.Types.Uintptr, 0, nil),
+			Valu("zero", OpConst64, c.config.Types.Int, 0, nil),
+			Valu("v6", OpLocalAddr, ptrType, 0, name, "sp", "start"),
+			Valu("v3", OpOffPtr, ptrType, 8, nil, "v6"),
+			Valu("v22", OpOffPtr, ptrType, 0, nil, "v6"),
+			Valu("zerostore1", OpStore, types.TypeMem, 0, c.config.Types.Int, "v22", "zero", "start"),
+			Valu("zerostore2", OpStore, types.TypeMem, 0, c.config.Types.Int, "v3", "zero", "zerostore1"),
+			Valu("v8", OpLocalAddr, ptrType, 0, name, "sp", "zerostore2"),
+			Valu("v23", OpOffPtr, ptrType, 8, nil, "v8"),
+			Valu("v25", OpOffPtr, ptrType, 0, nil, "v8"),
+			Valu("zerostore3", OpStore, types.TypeMem, 0, c.config.Types.Int, "v25", "zero", "zerostore2"),
+			Valu("zerostore4", OpStore, types.TypeMem, 0, c.config.Types.Int, "v23", "zero", "zerostore3"),
+			Goto("exit")),
+		Bloc("exit",
+			Exit("zerostore4")))
+
+	fun.f.Name = "smallstructinit"
+	CheckFunc(fun.f)
+	cse(fun.f)
+	dse(fun.f)
+	CheckFunc(fun.f)
+
+	v1 := fun.values["zerostore1"]
+	if v1.Op != OpCopy {
+		t.Errorf("dead store not removed")
+	}
+	v2 := fun.values["zerostore2"]
+	if v2.Op != OpCopy {
+		t.Errorf("dead store not removed")
+	}
+}