Move cold value-symbol links out of the hot records

Most value symbols never need write types, computed names, or containing
types. These three pointers nevertheless occupy every checker link
record, which is the largest retained allocation on the VS Code
workload. Allocate that metadata only when it is used, shrinking the hot
record from 56 to 40 bytes without changing symbol identity or cache
semantics.

VS Code src, six samples versus compact inference mappers:

                   │ before  │             after              │
                   │ retained-B │ retained-B   vs base        │
ExternalProgram-20   5.029Gi       4.874Gi   -3.09% (p=0.002 n=6)

                   │ before  │             after              │
                   │ scan-B  │   scan-B       vs base         │
ExternalProgram-20  4.664Gi       4.508Gi    -3.33% (p=0.002 n=6)

                   │ before  │             after              │
                   │ B/op    │   B/op         vs base         │
ExternalProgram-20  6.200Gi       6.043Gi    -2.54% (p=0.002 n=6)

Allocation count is unchanged. All 321 semantic diagnostics and all
10284 files are identical. CLI diagnostic text and compiler counts match
exactly as well. No GC tuning is used.

Author: jakebailey

tsc/internal/checker/checker.go

@@ -685,6 +685,7 @@ type Checker struct {
 	indexedAccessTypeArena                      core.Arena[IndexedAccessType]
 	conditionalTypeArena                        core.Arena[ConditionalType]
 	typeListArena                               core.Arena[*Type]
+	valueSymbolExtraArena                       core.Arena[valueSymbolExtra]
 	freeTypeSets                                *temporaryTypeSet
 	mergedSymbols                               map[*ast.Symbol]*ast.Symbol
 	mergedExportsChecked                        collections.Set[*ast.Symbol]
@@ -12163,7 +12164,7 @@ func (c *Checker) forEachProperty(prop *ast.Symbol, callback func(p *ast.Symbol)
 	if prop.CheckFlags()&ast.CheckFlagsSynthetic == 0 {
 		return callback(prop)
 	}
-	for _, t := range c.valueSymbolLinks.Get(prop).containingType.Types() {
+	for _, t := range c.valueSymbolLinks.Get(prop).getContainingType().Types() {
 		p := c.getPropertyOfType(t, prop.Name())
 		if p != nil && c.forEachProperty(p, callback) {
 			return true
@@ -13505,7 +13506,7 @@ func (c *Checker) checkObjectLiteral(node *ast.Node, checkMode CheckMode) *Type
 			}
 			links := c.valueSymbolLinks.Get(prop)
 			if nameType != nil {
-				links.nameType = nameType
+				links.setNameType(c, nameType)
 			}
 			if inDestructuringPattern && c.hasDefaultValue(memberDecl) {
 				// If object literal is an assignment pattern and if the assignment pattern specifies a default value
@@ -13750,7 +13751,7 @@ func (c *Checker) getSpreadType(left *Type, right *Type, symbol *ast.Symbol, obj
 				c.spreadLinks.Get(result).leftSpread = leftProp
 				c.spreadLinks.Get(result).rightSpread = rightProp
 				result.SetDeclarations(declarations)
-				links.nameType = c.valueSymbolLinks.Get(leftProp).nameType
+				links.setNameType(c, c.valueSymbolLinks.Get(leftProp).getNameType())
 				members[leftProp.Name()] = result
 			}
 		} else {
@@ -13837,7 +13838,7 @@ func (c *Checker) tryMergeUnionOfObjectTypeAndEmptyObject(t *Type, readonly bool
 				links.resolvedType = c.addOptionalityEx(c.getTypeOfSymbol(prop), true /*isProperty*/, true /*isOptional*/)
 			}
 			result.SetDeclarations(prop.Declarations())
-			links.nameType = c.valueSymbolLinks.Get(prop).nameType
+			links.setNameType(c, c.valueSymbolLinks.Get(prop).getNameType())
 			c.mappedSymbolLinks.Get(result).syntheticOrigin = prop
 			members[prop.Name()] = result
 		}
@@ -13867,7 +13868,7 @@ func (c *Checker) getSpreadSymbol(prop *ast.Symbol, readonly bool) *ast.Symbol {
 		links.resolvedType = c.getTypeOfSymbol(prop)
 	}
 	result.SetDeclarations(prop.Declarations())
-	links.nameType = c.valueSymbolLinks.Get(prop).nameType
+	links.setNameType(c, c.valueSymbolLinks.Get(prop).getNameType())
 	c.mappedSymbolLinks.Get(result).syntheticOrigin = prop
 	return result
 }
@@ -16136,7 +16137,7 @@ func (c *Checker) createDefaultPropertyWrapperForModule(symbol *ast.Symbol, orig
 	memberTable := make(ast.SymbolTable)
 	newSymbol := c.newSymbol(ast.SymbolFlagsAlias, ast.InternalSymbolNameDefault)
 	newSymbol.SetParent(originalSymbol)
-	c.valueSymbolLinks.Get(newSymbol).nameType = c.getStringLiteralType("default")
+	c.valueSymbolLinks.Get(newSymbol).setNameType(c, c.getStringLiteralType("default"))
 	c.aliasSymbolLinks.Get(newSymbol).aliasTarget = c.resolveSymbol(symbol)
 	memberTable[ast.InternalSymbolNameDefault] = newSymbol
 	if anonymousSymbol == nil && originalSymbol != nil {
@@ -16482,7 +16483,7 @@ func (c *Checker) lateBindMember(parent *ast.Symbol, earlySymbols ast.SymbolTabl
 				}
 				lateSymbol = c.newSymbolEx(ast.SymbolFlagsNone, memberName, ast.CheckFlagsLate)
 			}
-			c.valueSymbolLinks.Get(lateSymbol).nameType = t
+			c.valueSymbolLinks.Get(lateSymbol).setNameType(c, t)
 			c.addDeclarationToLateBoundSymbol(lateSymbol, decl, symbolFlags)
 			if lateSymbol.Parent() == nil {
 				lateSymbol.SetParent(parent)
@@ -16841,19 +16842,19 @@ func (c *Checker) getTypeOfSymbolWithDeferredType(symbol *ast.Symbol) *Type {
 
 func (c *Checker) getWriteTypeOfSymbolWithDeferredType(symbol *ast.Symbol) *Type {
 	links := c.valueSymbolLinks.Get(symbol)
-	if links.writeType == nil {
+	if links.getWriteType() == nil {
 		deferred := c.deferredSymbolLinks.Get(symbol)
 		if len(deferred.writeConstituents) != 0 {
 			if deferred.parent.flags&TypeFlagsUnion != 0 {
-				links.writeType = c.getUnionType(deferred.writeConstituents)
+				links.setWriteType(c, c.getUnionType(deferred.writeConstituents))
 			} else {
-				links.writeType = c.getIntersectionType(deferred.writeConstituents)
+				links.setWriteType(c, c.getIntersectionType(deferred.writeConstituents))
 			}
 		} else {
-			links.writeType = c.getTypeOfSymbolWithDeferredType(symbol)
+			links.setWriteType(c, c.getTypeOfSymbolWithDeferredType(symbol))
 		}
 	}
-	return links.writeType
+	return links.getWriteType()
 }
 
 // Distinct write types come only from set accessors, but synthetic union and intersection
@@ -16865,7 +16866,7 @@ func (c *Checker) getWriteTypeOfSymbol(symbol *ast.Symbol) *Type {
 			return c.getWriteTypeOfSymbolWithDeferredType(symbol)
 		}
 		links := c.valueSymbolLinks.Get(symbol)
-		return core.OrElse(links.writeType, links.resolvedType)
+		return core.OrElse(links.getWriteType(), links.resolvedType)
 	}
 	if symbol.Flags()&ast.SymbolFlagsProperty != 0 {
 		return c.removeMissingType(c.getTypeOfSymbol(symbol), symbol.Flags()&ast.SymbolFlagsOptional != 0)
@@ -16963,10 +16964,10 @@ func (c *Checker) getTypeOfInstantiatedSymbol(symbol *ast.Symbol) *Type {
 
 func (c *Checker) getWriteTypeOfInstantiatedSymbol(symbol *ast.Symbol) *Type {
 	links := c.valueSymbolLinks.Get(symbol)
-	if links.writeType == nil {
-		links.writeType = c.instantiateType(c.getWriteTypeOfSymbol(links.target), links.mapper)
+	if links.getWriteType() == nil {
+		links.setWriteType(c, c.instantiateType(c.getWriteTypeOfSymbol(links.target), links.mapper))
 	}
-	return links.writeType
+	return links.getWriteType()
 }
 
 func (c *Checker) getTypeOfVariableOrParameterOrProperty(symbol *ast.Symbol) *Type {
@@ -19021,7 +19022,7 @@ func (c *Checker) getTypeOfAccessors(symbol *ast.Symbol) *Type {
 
 func (c *Checker) getWriteTypeOfAccessors(symbol *ast.Symbol) *Type {
 	links := c.valueSymbolLinks.Get(symbol)
-	if links.writeType == nil {
+	if links.getWriteType() == nil {
 		if !c.pushTypeResolution(symbol, TypeSystemPropertyNameWriteType) {
 			return c.errorType
 		}
@@ -19040,15 +19041,15 @@ func (c *Checker) getWriteTypeOfAccessors(symbol *ast.Symbol) *Type {
 			writeType = c.anyType
 		}
 		// Absent an explicit setter type annotation we use the read type of the accessor.
-		if links.writeType == nil {
+		if links.getWriteType() == nil {
 			if writeType != nil {
-				links.writeType = writeType
+				links.setWriteType(c, writeType)
 			} else {
-				links.writeType = c.getTypeOfAccessors(symbol)
+				links.setWriteType(c, c.getTypeOfAccessors(symbol))
 			}
 		}
 	}
-	return links.writeType
+	return links.getWriteType()
 }
 
 func (c *Checker) getTypeOfAlias(symbol *ast.Symbol) *Type {
@@ -19268,7 +19269,7 @@ func (c *Checker) typeResolutionHasProperty(r *TypeResolution) bool {
 	case TypeSystemPropertyNameInitializerIsUndefined:
 		return c.nodeLinks.Get(r.target.(*ast.Node)).flags&NodeCheckFlagsInitializerIsUndefinedComputed != 0
 	case TypeSystemPropertyNameWriteType:
-		return c.valueSymbolLinks.Get(r.target.(*ast.Symbol)).writeType != nil
+		return c.valueSymbolLinks.Get(r.target.(*ast.Symbol)).getWriteType() != nil
 	case TypeSystemPropertyNameAliasTarget:
 		return c.aliasSymbolLinks.Get(r.target.(*ast.Symbol)).aliasTarget != nil
 	}
@@ -21218,7 +21219,7 @@ func (c *Checker) instantiateSymbol(symbol *ast.Symbol, m *TypeMapper) *ast.Symb
 			return symbol
 		}
 		// If we're a setter, check writeType.
-		if links.writeType != nil && !c.couldContainTypeVariables(links.writeType) {
+		if links.getWriteType() != nil && !c.couldContainTypeVariables(links.getWriteType()) {
 			return symbol
 		}
 	}
@@ -21235,7 +21236,7 @@ func (c *Checker) instantiateSymbol(symbol *ast.Symbol, m *TypeMapper) *ast.Symb
 	resultLinks := c.valueSymbolLinks.Get(result)
 	resultLinks.target = symbol
 	resultLinks.mapper = m
-	resultLinks.nameType = links.nameType
+	resultLinks.setNameType(c, links.getNameType())
 	return result
 }
 
@@ -21367,7 +21368,7 @@ func (c *Checker) resolveMappedTypeMembers(t *Type) {
 			// property symbol's name type be the union of those enum member types.
 			if existingProp := members[propName]; existingProp != nil {
 				valueLinks := c.valueSymbolLinks.Get(existingProp)
-				valueLinks.nameType = c.getUnionType([]*Type{valueLinks.nameType, propNameType})
+				valueLinks.setNameType(c, c.getUnionType([]*Type{valueLinks.getNameType(), propNameType}))
 				mappedLinks := c.mappedSymbolLinks.Get(existingProp)
 				mappedLinks.keyType = c.getUnionType([]*Type{mappedLinks.keyType, keyType})
 			} else {
@@ -21385,8 +21386,8 @@ func (c *Checker) resolveMappedTypeMembers(t *Type) {
 				prop := c.newSymbol(ast.SymbolFlagsProperty|core.IfElse(isOptional, ast.SymbolFlagsOptional, 0), propName)
 				prop.SetCheckFlags(lateFlag | ast.CheckFlagsMapped | core.IfElse(isReadonly, ast.CheckFlagsReadonly, 0) | core.IfElse(stripOptional, ast.CheckFlagsStripOptional, 0))
 				valueLinks := c.valueSymbolLinks.Get(prop)
-				valueLinks.containingType = t
-				valueLinks.nameType = propNameType
+				valueLinks.setContainingType(c, t)
+				valueLinks.setNameType(c, propNameType)
 				mappedLinks := c.mappedSymbolLinks.Get(prop)
 				mappedLinks.keyType = keyType
 				if modifiersProp != nil {
@@ -21433,7 +21434,7 @@ func (c *Checker) resolveMappedTypeMembers(t *Type) {
 func (c *Checker) getTypeOfMappedSymbol(symbol *ast.Symbol) *Type {
 	links := c.valueSymbolLinks.Get(symbol)
 	if links.resolvedType == nil {
-		mappedType := links.containingType
+		mappedType := links.getContainingType()
 		if !c.pushTypeResolution(symbol, TypeSystemPropertyNameType) {
 			mappedType.AsMappedType().containsError = true
 			return c.errorType
@@ -22047,9 +22048,9 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 			clone.SetParent(singleProp.ValueDeclaration().Symbol().Parent())
 		}
 		links := c.valueSymbolLinks.Get(clone)
-		links.containingType = containingType
+		links.setContainingType(c, containingType)
 		links.mapper = singlePropMapper
-		links.writeType = c.getWriteTypeOfSymbol(singleProp)
+		links.setWriteType(c, c.getWriteTypeOfSymbol(singleProp))
 		return clone
 	}
 	if propSet.Size() == 0 {
@@ -22074,7 +22075,7 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 		t := c.getTypeOfSymbol(prop)
 		if firstType == nil {
 			firstType = t
-			nameType = c.valueSymbolLinks.Get(prop).nameType
+			nameType = c.valueSymbolLinks.Get(prop).getNameType()
 		}
 		writeType := c.getWriteTypeOfSymbol(prop)
 		if writeTypes != nil || writeType != t {
@@ -22103,8 +22104,8 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 		result.SetParent(firstValueDeclaration.Symbol().Parent())
 	}
 	links := c.valueSymbolLinks.Get(result)
-	links.containingType = containingType
-	links.nameType = nameType
+	links.setContainingType(c, containingType)
+	links.setNameType(c, nameType)
 	if len(propTypes) > 2 {
 		// When `propTypes` has the potential to explode in size when normalized, defer normalization until absolutely needed
 		result.SetCheckFlags(result.CheckFlags() | ast.CheckFlagsDeferredType)
@@ -22121,9 +22122,9 @@ func (c *Checker) createUnionOrIntersectionProperty(containingType *Type, name s
 	}
 	if writeTypes != nil {
 		if isUnion {
-			links.writeType = c.getUnionType(writeTypes)
+			links.setWriteType(c, c.getUnionType(writeTypes))
 		} else {
-			links.writeType = c.getIntersectionType(writeTypes)
+			links.setWriteType(c, c.getIntersectionType(writeTypes))
 		}
 	}
 	return result
@@ -22178,7 +22179,7 @@ func (c *Checker) createSymbolWithType(source *ast.Symbol, t *Type) *ast.Symbol
 	links := c.valueSymbolLinks.Get(symbol)
 	links.resolvedType = t
 	links.target = source
-	links.nameType = c.valueSymbolLinks.Get(source).nameType
+	links.setNameType(c, c.valueSymbolLinks.Get(source).getNameType())
 	return symbol
 }
 
@@ -27349,7 +27350,7 @@ func (c *Checker) getLiteralTypeFromProperties(t *Type, include TypeFlags, inclu
 
 func (c *Checker) getLiteralTypeFromProperty(prop *ast.Symbol, include TypeFlags, includeNonPublic bool) *Type {
 	if includeNonPublic || getDeclarationModifierFlagsFromSymbol(prop)&ast.ModifierFlagsNonPublicAccessibilityModifier == 0 {
-		t := c.valueSymbolLinks.Get(c.getLateBoundSymbol(prop)).nameType
+		t := c.valueSymbolLinks.Get(c.getLateBoundSymbol(prop)).getNameType()
 		if t == nil {
 			if prop.Name() == ast.InternalSymbolNameDefault {
 				t = c.getStringLiteralType("default")
@@ -30572,7 +30573,7 @@ func (c *Checker) getContextualTypeForObjectLiteralElement(element *ast.Node, co
 			// in the type. It will just be "__computed", which does not appear in any
 			// SymbolTable.
 			symbol := c.getSymbolOfDeclaration(element)
-			return c.getTypeOfPropertyOfContextualTypeEx(t, symbol.Name(), c.valueSymbolLinks.Get(symbol).nameType)
+			return c.getTypeOfPropertyOfContextualTypeEx(t, symbol.Name(), c.valueSymbolLinks.Get(symbol).getNameType())
 		}
 		if ast.HasDynamicName(element) {
 			name := ast.GetNameOfDeclaration(element)

tsc/internal/checker/exports.go

@@ -83,7 +83,7 @@ func (c *Checker) GetUnionType(types []*Type) *Type {
 
 func (c *Checker) GetNameTypeOfSymbol(symbol *ast.Symbol) *Type {
 	if links := c.valueSymbolLinks.TryGet(symbol); links != nil {
-		return links.nameType
+		return links.getNameType()
 	}
 	return nil
 }

tsc/internal/checker/inference.go

@@ -1132,7 +1132,7 @@ func (c *Checker) resolveReverseMappedTypeMembers(t *Type) {
 		checkFlags := ast.CheckFlagsReverseMapped | core.IfElse(readonlyMask && c.isReadonlySymbol(prop), ast.CheckFlagsReadonly, 0)
 		inferredProp := c.newSymbolEx(ast.SymbolFlagsProperty|prop.Flags()&optionalMask, prop.Name(), checkFlags)
 		inferredProp.SetDeclarations(prop.Declarations())
-		c.valueSymbolLinks.Get(inferredProp).nameType = c.valueSymbolLinks.Get(prop).nameType
+		c.valueSymbolLinks.Get(inferredProp).setNameType(c, c.valueSymbolLinks.Get(prop).getNameType())
 		links := c.ReverseMappedSymbolLinks.Get(inferredProp)
 		links.propertyType = c.getTypeOfSymbol(prop)
 		constraintTarget := r.constraintType.AsIndexType().target

tsc/internal/checker/nodebuilderimpl.go

@@ -591,7 +591,7 @@ func (b *NodeBuilderImpl) symbolToNode(symbol *ast.Symbol, meaning ast.SymbolFla
 			}
 		}
 		if b.ch.valueSymbolLinks.Has(symbol) {
-			nameType := b.ch.valueSymbolLinks.Get(symbol).nameType
+			nameType := b.ch.valueSymbolLinks.Get(symbol).getNameType()
 			if nameType != nil && nameType.flags&(TypeFlagsEnumLiteral|TypeFlagsUniqueESSymbol) != 0 {
 				oldEnclosing := b.ctx.enclosingDeclaration
 				b.ctx.enclosingDeclaration = nameType.symbol.ValueDeclaration()
@@ -936,7 +936,7 @@ func isDefaultBindingContext(location *ast.Node) bool {
 
 func (b *NodeBuilderImpl) getNameOfSymbolFromNameType(symbol *ast.Symbol) string {
 	if b.ch.valueSymbolLinks.Has(symbol) {
-		nameType := b.ch.valueSymbolLinks.Get(symbol).nameType
+		nameType := b.ch.valueSymbolLinks.Get(symbol).getNameType()
 		if nameType == nil {
 			return ""
 		}
@@ -993,7 +993,7 @@ func (b *NodeBuilderImpl) getNameOfSymbolAsWritten(symbol *ast.Symbol) string {
 			// 	return symbol.Name
 			// }
 			if ast.IsComputedPropertyName(name) && symbol.CheckFlags()&ast.CheckFlagsLate == 0 {
-				if b.ch.valueSymbolLinks.Has(symbol) && b.ch.valueSymbolLinks.Get(symbol).nameType != nil && b.ch.valueSymbolLinks.Get(symbol).nameType.flags&TypeFlagsStringOrNumberLiteral != 0 {
+				if b.ch.valueSymbolLinks.Has(symbol) && b.ch.valueSymbolLinks.Get(symbol).getNameType() != nil && b.ch.valueSymbolLinks.Get(symbol).getNameType().flags&TypeFlagsStringOrNumberLiteral != 0 {
 					result := b.getNameOfSymbolFromNameType(symbol)
 					if len(result) > 0 {
 						return result
@@ -2544,7 +2544,7 @@ func (b *NodeBuilderImpl) getPropertyNameNodeForSymbolFromNameType(symbol *ast.S
 	if !b.ch.valueSymbolLinks.Has(symbol) {
 		return nil
 	}
-	nameType := b.ch.valueSymbolLinks.TryGet(symbol).nameType
+	nameType := b.ch.valueSymbolLinks.TryGet(symbol).getNameType()
 	if nameType == nil {
 		return nil
 	}

tsc/internal/checker/services.go

@@ -413,15 +413,15 @@ func (c *Checker) GetRootSymbols(symbol *ast.Symbol) []*ast.Symbol {
 
 func (c *Checker) GetMappedTypeSymbolOfProperty(symbol *ast.Symbol) *ast.Symbol {
 	if valueLinks := c.valueSymbolLinks.TryGet(symbol); valueLinks != nil {
-		return valueLinks.containingType.symbol
+		return valueLinks.getContainingType().symbol
 	}
 	return nil
 }
 
 func (c *Checker) getImmediateRootSymbols(symbol *ast.Symbol) []*ast.Symbol {
 	if symbol.CheckFlags()&ast.CheckFlagsSynthetic != 0 {
 		return core.MapNonNil(
-			c.valueSymbolLinks.Get(symbol).containingType.Types(),
+			c.valueSymbolLinks.Get(symbol).getContainingType().Types(),
 			func(t *Type) *ast.Symbol {
 				return c.getPropertyOfType(t, symbol.Name())
 			},