ssa: memory usage optimization around lastDefinitions

internal/engine/wazevo/backend/backend_test.go

@@ -458,11 +458,11 @@ L2 (SSA Block: blk2):
 			name: "single_predecessor_local_refs", m: testcases.SinglePredecessorLocalRefs.Module,
 			afterLoweringARM64: `
 L1 (SSA Block: blk0):
-	mov x132?, xzr
-	cbz w132?, (L2)
-L3 (SSA Block: blk1):
 	mov x131?, xzr
-	mov x0, x131?
+	cbz w131?, (L2)
+L3 (SSA Block: blk1):
+	mov x130?, xzr
+	mov x0, x130?
 	ret
 L2 (SSA Block: blk2):
 L4 (SSA Block: blk3):

internal/engine/wazevo/frontend/frontend.go

@@ -301,26 +301,7 @@ func (c *Compiler) declareWasmLocals(entry ssa.BasicBlock) {
 		st := WasmTypeToSSAType(typ)
 		variable := c.ssaBuilder.DeclareVariable(st)
 		c.setWasmLocalVariable(wasm.Index(i)+localCount, variable)
-
-		zeroInst := c.ssaBuilder.AllocateInstruction()
-		switch st {
-		case ssa.TypeI32:
-			zeroInst.AsIconst32(0)
-		case ssa.TypeI64:
-			zeroInst.AsIconst64(0)
-		case ssa.TypeF32:
-			zeroInst.AsF32const(0)
-		case ssa.TypeF64:
-			zeroInst.AsF64const(0)
-		case ssa.TypeV128:
-			zeroInst.AsVconst(0, 0)
-		default:
-			panic("TODO: " + wasm.ValueTypeName(typ))
-		}
-
-		c.ssaBuilder.InsertInstruction(zeroInst)
-		value := zeroInst.Return()
-		c.ssaBuilder.DefineVariable(variable, value, entry)
+		c.ssaBuilder.InsertZeroValue(st)
 	}
 }
 

internal/engine/wazevo/frontend/frontend_test.go

@@ -375,13 +375,11 @@ blk3: () <-- (blk1)
 			exp: `
 blk0: (exec_ctx:i64, module_ctx:i64)
 	v2:i32 = Iconst_32 0x0
-	v3:i32 = Iconst_32 0x0
-	v4:i32 = Iconst_32 0x0
 	Brz v2, blk2
 	Jump blk1
 
 blk1: () <-- (blk0)
-	Return v4
+	Return v2
 
 blk2: () <-- (blk0)
 	Jump blk3
@@ -392,12 +390,11 @@ blk3: () <-- (blk2)
 			expAfterPasses: `
 blk0: (exec_ctx:i64, module_ctx:i64)
 	v2:i32 = Iconst_32 0x0
-	v4:i32 = Iconst_32 0x0
 	Brz v2, blk2
 	Jump fallthrough
 
 blk1: () <-- (blk0)
-	Return v4
+	Return v2
 
 blk2: () <-- (blk0)
 	Jump fallthrough

internal/engine/wazevo/ssa/builder.go

@@ -127,7 +127,11 @@ type Builder interface {
 	// Idom returns the immediate dominator of the given BasicBlock.
 	Idom(blk BasicBlock) BasicBlock
 
+	// VarLengthPool returns the VarLengthPool of Value.
 	VarLengthPool() *wazevoapi.VarLengthPool[Value]
+
+	// InsertZeroValue inserts a zero value constant instruction of the given type.
+	InsertZeroValue(t Type)
 }
 
 // NewBuilder returns a new Builder implementation.
@@ -203,6 +207,32 @@ type builder struct {
 	donePostBlockLayoutPasses bool
 
 	currentSourceOffset SourceOffset
+
+	// zeros are the zero value constants for each type.
+	zeros [typeEnd]Value
+}
+
+// InsertZeroValue implements Builder.InsertZeroValue.
+func (b *builder) InsertZeroValue(t Type) {
+	if b.zeros[t].Valid() {
+		return
+	}
+	zeroInst := b.AllocateInstruction()
+	switch t {
+	case TypeI32:
+		zeroInst.AsIconst32(0)
+	case TypeI64:
+		zeroInst.AsIconst64(0)
+	case TypeF32:
+		zeroInst.AsF32const(0)
+	case TypeF64:
+		zeroInst.AsF64const(0)
+	case TypeV128:
+		zeroInst.AsVconst(0, 0)
+	default:
+		panic("TODO: " + t.String())
+	}
+	b.zeros[t] = zeroInst.Insert(b).Return()
 }
 
 func (b *builder) VarLengthPool() *wazevoapi.VarLengthPool[Value] {
@@ -218,6 +248,7 @@ func (b *builder) ReturnBlock() BasicBlock {
 func (b *builder) Init(s *Signature) {
 	b.nextVariable = 0
 	b.currentSignature = s
+	b.zeros = [typeEnd]Value{ValueInvalid, ValueInvalid, ValueInvalid, ValueInvalid, ValueInvalid, ValueInvalid}
 	resetBasicBlock(b.returnBlk)
 	b.instructionsPool.Reset()
 	b.basicBlocksPool.Reset()
@@ -486,6 +517,9 @@ func (b *builder) findValue(typ Type, variable Variable, blk *basicBlock) Value
 			value:    value,
 		})
 		return value
+	} else if blk.EntryBlock() {
+		// If this is the entry block, we reach the uninitialized variable which has zero value.
+		return b.zeros[b.definedVariableType(variable)]
 	}
 
 	if pred := blk.singlePred; pred != nil {

internal/engine/wazevo/ssa/type.go

@@ -21,6 +21,9 @@ const (
 
 	// TypeV128 represents 128-bit SIMD vectors.
 	TypeV128
+
+	// -- Do not add new types after this line. ----
+	typeEnd
 )
 
 // String implements fmt.Stringer.