[JSC] Compact generator code's bytecode size
authorysuzuki@apple.com <ysuzuki@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Sun, 12 May 2019 19:43:16 +0000 (19:43 +0000)
committerysuzuki@apple.com <ysuzuki@apple.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc>
Sun, 12 May 2019 19:43:16 +0000 (19:43 +0000)
https://bugs.webkit.org/show_bug.cgi?id=197822

Reviewed by Michael Saboff.

op_put_to_scope's symbolTableOrScopeDepth is represented as int. This was OK for the old bytecode format since
VirtualRegister / scope depth can be represented by int anyway. But it is problematic now since only int8_t range
will be represented in narrow bytecode. When this field is used for symbol table constant index, it is always
larger than FirstConstantRegisterIndex. So it always exceeds the range of int8_t, and results in wide bytecode.
It makes all generator's op_put_to_scope wide bytecode.

In this patch, we introduce a new (logically) union type SymbolTableOrScopeDepth. It holds unsigned value, and we store the
SymbolTableConstantIndex - FirstConstantRegisterIndex in this unsigned value to make op_put_to_scope narrow bytecode.

* CMakeLists.txt:
* JavaScriptCore.xcodeproj/project.pbxproj:
* bytecode/BytecodeGeneratorification.cpp:
(JSC::BytecodeGeneratorification::run):
* bytecode/BytecodeList.rb:
* bytecode/CodeBlock.cpp:
(JSC::CodeBlock::finishCreation):
* bytecode/Fits.h:
* bytecompiler/BytecodeGenerator.cpp:
(JSC::BytecodeGenerator::BytecodeGenerator):
(JSC::BytecodeGenerator::emitProfileType):
(JSC::BytecodeGenerator::emitPutToScope):
(JSC::BytecodeGenerator::localScopeDepth const):
* bytecompiler/BytecodeGenerator.h:
* runtime/SymbolTableOrScopeDepth.h: Added.
(JSC::SymbolTableOrScopeDepth::symbolTable):
(JSC::SymbolTableOrScopeDepth::scopeDepth):
(JSC::SymbolTableOrScopeDepth::raw):
(JSC::SymbolTableOrScopeDepth::symbolTable const):
(JSC::SymbolTableOrScopeDepth::scopeDepth const):
(JSC::SymbolTableOrScopeDepth::raw const):
(JSC::SymbolTableOrScopeDepth::dump const):
(JSC::SymbolTableOrScopeDepth::SymbolTableOrScopeDepth):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@245213 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Source/JavaScriptCore/CMakeLists.txt
Source/JavaScriptCore/ChangeLog
Source/JavaScriptCore/JavaScriptCore.xcodeproj/project.pbxproj
Source/JavaScriptCore/bytecode/BytecodeGeneratorification.cpp
Source/JavaScriptCore/bytecode/BytecodeList.rb
Source/JavaScriptCore/bytecode/CodeBlock.cpp
Source/JavaScriptCore/bytecode/Fits.h
Source/JavaScriptCore/bytecompiler/BytecodeGenerator.cpp
Source/JavaScriptCore/bytecompiler/BytecodeGenerator.h
Source/JavaScriptCore/runtime/SymbolTableOrScopeDepth.h [new file with mode: 0644]

index fb6210c..fde79e2 100644 (file)
@@ -963,6 +963,7 @@ set(JavaScriptCore_PRIVATE_FRAMEWORK_HEADERS
     runtime/Symbol.h
     runtime/SymbolPrototype.h
     runtime/SymbolTable.h
+    runtime/SymbolTableOrScopeDepth.h
     runtime/TemplateObjectDescriptor.h
     runtime/TestRunnerUtils.h
     runtime/ThrowScope.h
index b3ecb66..adea1fc 100644 (file)
@@ -1,3 +1,43 @@
+2019-05-12  Yusuke Suzuki  <ysuzuki@apple.com>
+
+        [JSC] Compact generator code's bytecode size
+        https://bugs.webkit.org/show_bug.cgi?id=197822
+
+        Reviewed by Michael Saboff.
+
+        op_put_to_scope's symbolTableOrScopeDepth is represented as int. This was OK for the old bytecode format since
+        VirtualRegister / scope depth can be represented by int anyway. But it is problematic now since only int8_t range
+        will be represented in narrow bytecode. When this field is used for symbol table constant index, it is always
+        larger than FirstConstantRegisterIndex. So it always exceeds the range of int8_t, and results in wide bytecode.
+        It makes all generator's op_put_to_scope wide bytecode.
+
+        In this patch, we introduce a new (logically) union type SymbolTableOrScopeDepth. It holds unsigned value, and we store the
+        SymbolTableConstantIndex - FirstConstantRegisterIndex in this unsigned value to make op_put_to_scope narrow bytecode.
+
+        * CMakeLists.txt:
+        * JavaScriptCore.xcodeproj/project.pbxproj:
+        * bytecode/BytecodeGeneratorification.cpp:
+        (JSC::BytecodeGeneratorification::run):
+        * bytecode/BytecodeList.rb:
+        * bytecode/CodeBlock.cpp:
+        (JSC::CodeBlock::finishCreation):
+        * bytecode/Fits.h:
+        * bytecompiler/BytecodeGenerator.cpp:
+        (JSC::BytecodeGenerator::BytecodeGenerator):
+        (JSC::BytecodeGenerator::emitProfileType):
+        (JSC::BytecodeGenerator::emitPutToScope):
+        (JSC::BytecodeGenerator::localScopeDepth const):
+        * bytecompiler/BytecodeGenerator.h:
+        * runtime/SymbolTableOrScopeDepth.h: Added.
+        (JSC::SymbolTableOrScopeDepth::symbolTable):
+        (JSC::SymbolTableOrScopeDepth::scopeDepth):
+        (JSC::SymbolTableOrScopeDepth::raw):
+        (JSC::SymbolTableOrScopeDepth::symbolTable const):
+        (JSC::SymbolTableOrScopeDepth::scopeDepth const):
+        (JSC::SymbolTableOrScopeDepth::raw const):
+        (JSC::SymbolTableOrScopeDepth::dump const):
+        (JSC::SymbolTableOrScopeDepth::SymbolTableOrScopeDepth):
+
 2019-05-10  Saam barati  <sbarati@apple.com>
 
         Call to JSToWasmICCallee::createStructure passes in wrong prototype value
index e7dc887..fa0c844 100644 (file)
                DE26E9031CB5DD0500D2BE82 /* BuiltinExecutableCreator.h in Headers */ = {isa = PBXBuildFile; fileRef = DE26E9021CB5DD0500D2BE82 /* BuiltinExecutableCreator.h */; };
                DEA7E2451BBC677F00D78440 /* JSTypedArrayViewPrototype.h in Headers */ = {isa = PBXBuildFile; fileRef = 53917E7C1B791106000EBD33 /* JSTypedArrayViewPrototype.h */; settings = {ATTRIBUTES = (Private, ); }; };
                E124A8F70E555775003091F1 /* OpaqueJSString.h in Headers */ = {isa = PBXBuildFile; fileRef = E124A8F50E555775003091F1 /* OpaqueJSString.h */; settings = {ATTRIBUTES = (Private, ); }; };
+               E31179AA2288386100514B2C /* SymbolTableOrScopeDepth.h in Headers */ = {isa = PBXBuildFile; fileRef = E31179A92288385D00514B2C /* SymbolTableOrScopeDepth.h */; settings = {ATTRIBUTES = (Private, ); }; };
                E31618131EC5FE170006A218 /* DOMAnnotation.h in Headers */ = {isa = PBXBuildFile; fileRef = E31618101EC5FE080006A218 /* DOMAnnotation.h */; settings = {ATTRIBUTES = (Private, ); }; };
                E31618151EC5FE270006A218 /* DOMAttributeGetterSetter.h in Headers */ = {isa = PBXBuildFile; fileRef = E31618121EC5FE080006A218 /* DOMAttributeGetterSetter.h */; settings = {ATTRIBUTES = (Private, ); }; };
                E318CBC11B8AEF5100A2929D /* JSModuleNamespaceObject.h in Headers */ = {isa = PBXBuildFile; fileRef = E318CBBF1B8AEF5100A2929D /* JSModuleNamespaceObject.h */; settings = {ATTRIBUTES = (Private, ); }; };
                E18E3A560DF9278C00D90B34 /* VM.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; lineEnding = 0; path = VM.h; sourceTree = "<group>"; xcLanguageSpecificationIdentifier = xcode.lang.objcpp; };
                E18E3A570DF9278C00D90B34 /* VM.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; lineEnding = 0; path = VM.cpp; sourceTree = "<group>"; xcLanguageSpecificationIdentifier = xcode.lang.cpp; };
                E30677971B8BC6F5003F87F0 /* ModuleLoader.js */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.javascript; path = ModuleLoader.js; sourceTree = "<group>"; };
+               E31179A92288385D00514B2C /* SymbolTableOrScopeDepth.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = SymbolTableOrScopeDepth.h; sourceTree = "<group>"; };
                E31618101EC5FE080006A218 /* DOMAnnotation.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = DOMAnnotation.h; sourceTree = "<group>"; };
                E31618111EC5FE080006A218 /* DOMAttributeGetterSetter.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = DOMAttributeGetterSetter.cpp; sourceTree = "<group>"; };
                E31618121EC5FE080006A218 /* DOMAttributeGetterSetter.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = DOMAttributeGetterSetter.h; sourceTree = "<group>"; };
                                705B41AA1A6E501E00716757 /* SymbolPrototype.h */,
                                0F919D2715856770004A4E7D /* SymbolTable.cpp */,
                                14A396A60CD2933100B5B4FF /* SymbolTable.h */,
+                               E31179A92288385D00514B2C /* SymbolTableOrScopeDepth.h */,
                                BDB4B5E099CD4C1BB3C1CF05 /* TemplateObjectDescriptor.cpp */,
                                70ECA6041AFDBEA200449739 /* TemplateObjectDescriptor.h */,
                                0FA2C17917D7CF84009D015F /* TestRunnerUtils.cpp */,
                                705B41B21A6E501E00716757 /* SymbolPrototype.h in Headers */,
                                996B73281BDA08EF00331B84 /* SymbolPrototype.lut.h in Headers */,
                                BC18C46B0E16F5CD00B34460 /* SymbolTable.h in Headers */,
+                               E31179AA2288386100514B2C /* SymbolTableOrScopeDepth.h in Headers */,
                                0FD79A2D1EBBBDBB00DA88D3 /* Synchronousness.h in Headers */,
                                0F1FB38F1E173A6700A9BE50 /* SynchronousStopTheWorldMutatorScheduler.h in Headers */,
                                A784A26411D16622005776AC /* SyntaxChecker.h in Headers */,
index 2e6dcc8..81ab758 100644 (file)
@@ -252,7 +252,7 @@ void BytecodeGeneratorification::run()
                     storage.identifierIndex, // identifier
                     operand, // value
                     GetPutInfo(DoNotThrowIfNotFound, LocalClosureVar, InitializationMode::NotInitialization), // info
-                    m_generatorFrameSymbolTableIndex, // symbol table constant index
+                    SymbolTableOrScopeDepth::symbolTable(VirtualRegister { m_generatorFrameSymbolTableIndex }), // symbol table constant index
                     storage.scopeOffset.offset() // scope offset
                 );
             });
index 8ca0da9..0695a25 100644 (file)
@@ -50,6 +50,7 @@ types [
     :StructureID,
     :StructureChain,
     :SymbolTable,
+    :SymbolTableOrScopeDepth,
     :ToThisStatus,
     :TypeLocation,
     :WatchpointSet,
@@ -881,7 +882,7 @@ op :put_to_scope,
         value: VirtualRegister, # offset 3
         # $begin: :private,
         getPutInfo: GetPutInfo,
-        symbolTableOrScopeDepth: int,
+        symbolTableOrScopeDepth: SymbolTableOrScopeDepth,
         offset: unsigned,
     },
     metadata: {
@@ -977,7 +978,7 @@ op :end,
 op :profile_type,
     args: {
         targetVirtualRegister: VirtualRegister,
-        symbolTableOrScopeDepth: int,
+        symbolTableOrScopeDepth: SymbolTableOrScopeDepth,
         flag: ProfileTypeBytecodeFlag,
         identifier?: unsigned,
         resolveType: ResolveType,
index db28f67..ecc1a95 100644 (file)
@@ -643,7 +643,7 @@ bool CodeBlock::finishCreation(VM& vm, ScriptExecutable* ownerExecutable, Unlink
             if (bytecode.m_getPutInfo.resolveType() == LocalClosureVar) {
                 // Only do watching if the property we're putting to is not anonymous.
                 if (bytecode.m_var != UINT_MAX) {
-                    SymbolTable* symbolTable = jsCast<SymbolTable*>(getConstant(bytecode.m_symbolTableOrScopeDepth));
+                    SymbolTable* symbolTable = jsCast<SymbolTable*>(getConstant(bytecode.m_symbolTableOrScopeDepth.symbolTable().offset()));
                     const Identifier& ident = identifier(bytecode.m_var);
                     ConcurrentJSLocker locker(symbolTable->m_lock);
                     auto iter = symbolTable->find(locker, ident.impl());
@@ -657,7 +657,7 @@ bool CodeBlock::finishCreation(VM& vm, ScriptExecutable* ownerExecutable, Unlink
 
             const Identifier& ident = identifier(bytecode.m_var);
             metadata.m_watchpointSet = nullptr;
-            ResolveOp op = JSScope::abstractResolve(m_globalObject->globalExec(), bytecode.m_symbolTableOrScopeDepth, scope, ident, Put, bytecode.m_getPutInfo.resolveType(), bytecode.m_getPutInfo.initializationMode());
+            ResolveOp op = JSScope::abstractResolve(m_globalObject->globalExec(), bytecode.m_symbolTableOrScopeDepth.scopeDepth(), scope, ident, Put, bytecode.m_getPutInfo.resolveType(), bytecode.m_getPutInfo.initializationMode());
             RETURN_IF_EXCEPTION(throwScope, false);
 
             metadata.m_getPutInfo = GetPutInfo(bytecode.m_getPutInfo.resolveMode(), op.type, bytecode.m_getPutInfo.initializationMode());
@@ -687,7 +687,7 @@ bool CodeBlock::finishCreation(VM& vm, ScriptExecutable* ownerExecutable, Unlink
             switch (bytecode.m_flag) {
             case ProfileTypeBytecodeClosureVar: {
                 const Identifier& ident = identifier(bytecode.m_identifier);
-                unsigned localScopeDepth = bytecode.m_symbolTableOrScopeDepth;
+                unsigned localScopeDepth = bytecode.m_symbolTableOrScopeDepth.scopeDepth();
                 // Even though type profiling may be profiling either a Get or a Put, we can always claim a Get because
                 // we're abstractly "read"ing from a JSScope.
                 ResolveOp op = JSScope::abstractResolve(m_globalObject->globalExec(), localScopeDepth, scope, ident, Get, bytecode.m_resolveType, InitializationMode::NotInitialization);
@@ -711,7 +711,7 @@ bool CodeBlock::finishCreation(VM& vm, ScriptExecutable* ownerExecutable, Unlink
                 break;
             }
             case ProfileTypeBytecodeLocallyResolved: {
-                int symbolTableIndex = bytecode.m_symbolTableOrScopeDepth;
+                int symbolTableIndex = bytecode.m_symbolTableOrScopeDepth.symbolTable().offset();
                 SymbolTable* symbolTable = jsCast<SymbolTable*>(getConstant(symbolTableIndex));
                 const Identifier& ident = identifier(bytecode.m_identifier);
                 ConcurrentJSLocker locker(symbolTable->m_lock);
index 207e885..24d7757 100644 (file)
@@ -33,6 +33,7 @@
 #include "PutByIdFlags.h"
 #include "ResultType.h"
 #include "SpecialPointer.h"
+#include "SymbolTableOrScopeDepth.h"
 #include "VirtualRegister.h"
 #include <type_traits>
 
@@ -133,6 +134,25 @@ struct Fits<VirtualRegister, OpcodeSize::Narrow> {
 };
 
 template<>
+struct Fits<SymbolTableOrScopeDepth, OpcodeSize::Narrow> {
+    static bool check(SymbolTableOrScopeDepth u)
+    {
+        return u.raw() <= UINT8_MAX;
+    }
+
+    static uint8_t convert(SymbolTableOrScopeDepth u)
+    {
+        ASSERT(check(u));
+        return static_cast<uint8_t>(u.raw());
+    }
+
+    static SymbolTableOrScopeDepth convert(uint8_t u)
+    {
+        return SymbolTableOrScopeDepth::raw(u);
+    }
+};
+
+template<>
 struct Fits<Special::Pointer, OpcodeSize::Narrow> : Fits<int, OpcodeSize::Narrow> {
     using Base = Fits<int, OpcodeSize::Narrow>;
     static bool check(Special::Pointer sp) { return Base::check(static_cast<int>(sp)); }
index 661cb49..667dac0 100644 (file)
@@ -551,7 +551,7 @@ BytecodeGenerator::BytecodeGenerator(VM& vm, FunctionNode* functionNode, Unlinke
                     entry.disableWatching(*m_vm);
                     functionSymbolTable->set(NoLockingNecessary, name, entry);
                 }
-                OpPutToScope::emit(this, m_lexicalEnvironmentRegister, UINT_MAX, virtualRegisterForArgument(1 + i), GetPutInfo(ThrowIfNotFound, LocalClosureVar, InitializationMode::NotInitialization), symbolTableConstantIndex, offset.offset());
+                OpPutToScope::emit(this, m_lexicalEnvironmentRegister, UINT_MAX, virtualRegisterForArgument(1 + i), GetPutInfo(ThrowIfNotFound, LocalClosureVar, InitializationMode::NotInitialization), SymbolTableOrScopeDepth::symbolTable(VirtualRegister { symbolTableConstantIndex }), offset.offset());
             }
             
             // This creates a scoped arguments object and copies the overflow arguments into the
@@ -589,7 +589,7 @@ BytecodeGenerator::BytecodeGenerator(VM& vm, FunctionNode* functionNode, Unlinke
                 static_cast<const BindingNode*>(parameters.at(i).first)->boundProperty();
             functionSymbolTable->set(NoLockingNecessary, name, SymbolTableEntry(VarOffset(offset)));
             
-            OpPutToScope::emit(this, m_lexicalEnvironmentRegister, addConstant(ident), virtualRegisterForArgument(1 + i), GetPutInfo(ThrowIfNotFound, LocalClosureVar, InitializationMode::NotInitialization), symbolTableConstantIndex, offset.offset());
+            OpPutToScope::emit(this, m_lexicalEnvironmentRegister, addConstant(ident), virtualRegisterForArgument(1 + i), GetPutInfo(ThrowIfNotFound, LocalClosureVar, InitializationMode::NotInitialization), SymbolTableOrScopeDepth::symbolTable(VirtualRegister { symbolTableConstantIndex }), offset.offset());
         }
     }
     
@@ -1803,7 +1803,7 @@ void BytecodeGenerator::emitProfileType(RegisterID* registerToProfile, ProfileTy
     if (!registerToProfile)
         return;
 
-    OpProfileType::emit(this, registerToProfile, 0, flag, { }, resolveType());
+    OpProfileType::emit(this, registerToProfile, { }, flag, { }, resolveType());
 
     // Don't emit expression info for this version of profile type. This generally means
     // we're profiling information for something that isn't in the actual text of a JavaScript
@@ -1823,7 +1823,7 @@ void BytecodeGenerator::emitProfileType(RegisterID* registerToProfile, ProfileTy
     if (!registerToProfile)
         return;
 
-    OpProfileType::emit(this, registerToProfile, 0,  flag, { }, resolveType());
+    OpProfileType::emit(this, registerToProfile, { },  flag, { }, resolveType());
     emitTypeProfilerExpressionInfo(startDivot, endDivot);
 }
 
@@ -1836,14 +1836,14 @@ void BytecodeGenerator::emitProfileType(RegisterID* registerToProfile, const Var
         return;
 
     ProfileTypeBytecodeFlag flag;
-    int symbolTableOrScopeDepth;
+    SymbolTableOrScopeDepth symbolTableOrScopeDepth;
     if (var.local() || var.offset().isScope()) {
         flag = ProfileTypeBytecodeLocallyResolved;
         ASSERT(var.symbolTableConstantIndex());
-        symbolTableOrScopeDepth = var.symbolTableConstantIndex();
+        symbolTableOrScopeDepth = SymbolTableOrScopeDepth::symbolTable(VirtualRegister { var.symbolTableConstantIndex() });
     } else {
         flag = ProfileTypeBytecodeClosureVar;
-        symbolTableOrScopeDepth = localScopeDepth();
+        symbolTableOrScopeDepth = SymbolTableOrScopeDepth::scopeDepth(localScopeDepth());
     }
 
     OpProfileType::emit(this, registerToProfile, symbolTableOrScopeDepth, flag, addConstant(var.ident()), resolveType());
@@ -2512,18 +2512,18 @@ RegisterID* BytecodeGenerator::emitPutToScope(RegisterID* scope, const Variable&
     case VarKind::Scope:
     case VarKind::Invalid: {
         GetPutInfo getPutInfo(0);
-        int scopeDepth;
+        SymbolTableOrScopeDepth symbolTableOrScopeDepth;
         ScopeOffset offset;
         if (variable.offset().isScope()) {
             offset = variable.offset().scopeOffset();
             getPutInfo = GetPutInfo(resolveMode, LocalClosureVar, initializationMode);
-            scopeDepth = variable.symbolTableConstantIndex();
+            symbolTableOrScopeDepth = SymbolTableOrScopeDepth::symbolTable(VirtualRegister { variable.symbolTableConstantIndex() });
         } else {
             ASSERT(resolveType() != LocalClosureVar);
             getPutInfo = GetPutInfo(resolveMode, resolveType(), initializationMode);
-            scopeDepth = localScopeDepth();
+            symbolTableOrScopeDepth = SymbolTableOrScopeDepth::scopeDepth(localScopeDepth());
         }
-        OpPutToScope::emit(this, scope, addConstant(variable.ident()), value, getPutInfo, scopeDepth, !!offset ? offset.offset() : 0);
+        OpPutToScope::emit(this, scope, addConstant(variable.ident()), value, getPutInfo, symbolTableOrScopeDepth, !!offset ? offset.offset() : 0);
         m_codeBlock->addPropertyAccessInstruction(m_lastInstruction.offset());
         return value;
     } }
@@ -3743,7 +3743,7 @@ RegisterID* BytecodeGenerator::emitArgumentCount(RegisterID* dst)
     return dst;
 }
 
-int BytecodeGenerator::localScopeDepth() const
+unsigned BytecodeGenerator::localScopeDepth() const
 {
     return m_localScopeDepth;
 }
index 95341a9..1c90313 100644 (file)
@@ -1235,10 +1235,10 @@ namespace JSC {
         SegmentedVector<Label, 32> m_labels;
         SegmentedVector<LabelScope, 32> m_labelScopes;
         unsigned m_finallyDepth { 0 };
-        int m_localScopeDepth { 0 };
+        unsigned m_localScopeDepth { 0 };
         const CodeType m_codeType;
 
-        int localScopeDepth() const;
+        unsigned localScopeDepth() const;
         void pushLocalControlFlowScope();
         void popLocalControlFlowScope();
 
diff --git a/Source/JavaScriptCore/runtime/SymbolTableOrScopeDepth.h b/Source/JavaScriptCore/runtime/SymbolTableOrScopeDepth.h
new file mode 100644 (file)
index 0000000..91f5e8f
--- /dev/null
@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2019 Apple Inc. All Rights Reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "VirtualRegister.h"
+#include <wtf/PrintStream.h>
+
+namespace JSC {
+
+class SymbolTableOrScopeDepth {
+public:
+    SymbolTableOrScopeDepth() = default;
+
+    static SymbolTableOrScopeDepth symbolTable(VirtualRegister reg)
+    {
+        ASSERT(reg.isConstant());
+        return SymbolTableOrScopeDepth(reg.offset() - FirstConstantRegisterIndex);
+    }
+
+    static SymbolTableOrScopeDepth scopeDepth(unsigned scopeDepth)
+    {
+        return SymbolTableOrScopeDepth(scopeDepth);
+    }
+
+    static SymbolTableOrScopeDepth raw(unsigned value)
+    {
+        return SymbolTableOrScopeDepth(value);
+    }
+
+    VirtualRegister symbolTable() const
+    {
+        return VirtualRegister(m_raw + FirstConstantRegisterIndex);
+    }
+
+    unsigned scopeDepth() const
+    {
+        return m_raw;
+    }
+
+    unsigned raw() const { return m_raw; }
+
+    void dump(PrintStream& out) const
+    {
+        out.print(m_raw);
+    }
+
+private:
+    SymbolTableOrScopeDepth(unsigned value)
+        : m_raw(value)
+    { }
+
+    unsigned m_raw { 0 };
+};
+
+} // namespace JSC