C++ JumpList::link方法代码示例

static void dispatch(CCallHelpers& jit, FTL::State* state, const B3::StackmapGenerationParams& params, DFG::Node* node, Box<CCallHelpers::JumpList> exceptions, CCallHelpers::JumpList from, OperationType operation, ResultType result, Arguments arguments, std::index_sequence<ArgumentsIndex...>)
{
    CCallHelpers::Label done = jit.label();
    params.addLatePath([=] (CCallHelpers& jit) {
        AllowMacroScratchRegisterUsage allowScratch(jit);

        from.link(&jit);
        callOperation(
            *state, params.unavailableRegisters(), jit, node->origin.semantic,
            exceptions.get(), operation, extractResult(result), std::get<ArgumentsIndex>(arguments)...);
        jit.jump().linkTo(done, &jit);
    });
}

void JSCallVarargs::emit(CCallHelpers& jit, State& state, int32_t spillSlotsOffset, int32_t osrExitFromGenericUnwindSpillSlots)
{
    // We are passed three pieces of information:
    // - The callee.
    // - The arguments object, if it's not a forwarding call.
    // - The "this" value, if it's a constructor call.

    CallVarargsData* data = m_node->callVarargsData();
    
    GPRReg calleeGPR = GPRInfo::argumentGPR0;
    
    GPRReg argumentsGPR = InvalidGPRReg;
    GPRReg thisGPR = InvalidGPRReg;
    
    bool forwarding = false;
    
    switch (m_node->op()) {
    case CallVarargs:
    case TailCallVarargs:
    case TailCallVarargsInlinedCaller:
    case ConstructVarargs:
        argumentsGPR = GPRInfo::argumentGPR1;
        thisGPR = GPRInfo::argumentGPR2;
        break;
    case CallForwardVarargs:
    case TailCallForwardVarargs:
    case TailCallForwardVarargsInlinedCaller:
    case ConstructForwardVarargs:
        thisGPR = GPRInfo::argumentGPR1;
        forwarding = true;
        break;
    default:
        RELEASE_ASSERT_NOT_REACHED();
        break;
    }
    
    const unsigned calleeSpillSlot = 0;
    const unsigned argumentsSpillSlot = 1;
    const unsigned thisSpillSlot = 2;
    const unsigned stackPointerSpillSlot = 3;
    
    // Get some scratch registers.
    RegisterSet usedRegisters;
    usedRegisters.merge(RegisterSet::stackRegisters());
    usedRegisters.merge(RegisterSet::reservedHardwareRegisters());
    usedRegisters.merge(RegisterSet::calleeSaveRegisters());
    usedRegisters.set(calleeGPR);
    if (argumentsGPR != InvalidGPRReg)
        usedRegisters.set(argumentsGPR);
    ASSERT(thisGPR);
    usedRegisters.set(thisGPR);
    ScratchRegisterAllocator allocator(usedRegisters);
    GPRReg scratchGPR1 = allocator.allocateScratchGPR();
    GPRReg scratchGPR2 = allocator.allocateScratchGPR();
    GPRReg scratchGPR3 = allocator.allocateScratchGPR();

    RELEASE_ASSERT(!allocator.numberOfReusedRegisters());
    
    auto computeUsedStack = [&] (GPRReg targetGPR, unsigned extra) {
        if (isARM64()) {
            // Have to do this the weird way because $sp on ARM64 means zero when used in a subtraction.
            jit.move(CCallHelpers::stackPointerRegister, targetGPR);
            jit.negPtr(targetGPR);
            jit.addPtr(GPRInfo::callFrameRegister, targetGPR);
        } else {
            jit.move(GPRInfo::callFrameRegister, targetGPR);
            jit.subPtr(CCallHelpers::stackPointerRegister, targetGPR);
        }
        if (extra)
            jit.subPtr(CCallHelpers::TrustedImm32(extra), targetGPR);
        jit.urshiftPtr(CCallHelpers::Imm32(3), targetGPR);
    };
    
    auto callWithExceptionCheck = [&] (void* callee) {
        jit.move(CCallHelpers::TrustedImmPtr(callee), GPRInfo::nonPreservedNonArgumentGPR);
        jit.call(GPRInfo::nonPreservedNonArgumentGPR);
        m_exceptions.append(jit.emitExceptionCheck(AssemblyHelpers::NormalExceptionCheck, AssemblyHelpers::FarJumpWidth));
    };
    
    if (isARM64()) {
        jit.move(CCallHelpers::stackPointerRegister, scratchGPR1);
        jit.storePtr(scratchGPR1, CCallHelpers::addressFor(spillSlotsOffset + stackPointerSpillSlot));
    } else
        jit.storePtr(CCallHelpers::stackPointerRegister, CCallHelpers::addressFor(spillSlotsOffset + stackPointerSpillSlot));

    unsigned extraStack = sizeof(CallerFrameAndPC) +
        WTF::roundUpToMultipleOf(stackAlignmentBytes(), 5 * sizeof(void*));

    if (forwarding) {
        CCallHelpers::JumpList slowCase;
        computeUsedStack(scratchGPR2, 0);
        emitSetupVarargsFrameFastCase(jit, scratchGPR2, scratchGPR1, scratchGPR2, scratchGPR3, m_node->child2()->origin.semantic.inlineCallFrame, data->firstVarArgOffset, slowCase);
        
        CCallHelpers::Jump done = jit.jump();
        slowCase.link(&jit);
        jit.subPtr(CCallHelpers::TrustedImm32(extraStack), CCallHelpers::stackPointerRegister);
        jit.setupArgumentsExecState();
        callWithExceptionCheck(bitwise_cast<void*>(operationThrowStackOverflowForVarargs));
        jit.abortWithReason(DFGVarargsThrowingPathDidNotThrow);
        
//.........这里部分代码省略.........

//.........这里部分代码省略.........
    allocator.lock(static_cast<GPRReg>(stubInfo.patch.baseTagGPR));
#endif
    allocator.lock(valueRegs);
    allocator.lock(scratchGPR);

    GPRReg paramBaseGPR = InvalidGPRReg;
    GPRReg paramGlobalObjectGPR = InvalidGPRReg;
    JSValueRegs paramValueRegs = valueRegs;
    GPRReg remainingScratchGPR = InvalidGPRReg;

    // valueRegs and baseForGetGPR may be the same. For example, in Baseline JIT, we pass the same regT0 for baseGPR and valueRegs.
    // In FTL, there is no constraint that the baseForGetGPR interferes with the result. To make implementation simple in
    // Snippet, Snippet assumes that result registers always early interfere with input registers, in this case,
    // baseForGetGPR. So we move baseForGetGPR to the other register if baseForGetGPR == valueRegs.
    if (baseForGetGPR != valueRegs.payloadGPR()) {
        paramBaseGPR = baseForGetGPR;
        if (!snippet->requireGlobalObject)
            remainingScratchGPR = scratchGPR;
        else
            paramGlobalObjectGPR = scratchGPR;
    } else {
        jit.move(valueRegs.payloadGPR(), scratchGPR);
        paramBaseGPR = scratchGPR;
        if (snippet->requireGlobalObject)
            paramGlobalObjectGPR = allocator.allocateScratchGPR();
    }

    JSGlobalObject* globalObjectForDOMJIT = structure()->globalObject();

    regs.append(paramValueRegs);
    regs.append(paramBaseGPR);
    if (snippet->requireGlobalObject) {
        ASSERT(paramGlobalObjectGPR != InvalidGPRReg);
        regs.append(SnippetParams::Value(paramGlobalObjectGPR, globalObjectForDOMJIT));
    }

    if (snippet->numGPScratchRegisters) {
        unsigned i = 0;
        if (remainingScratchGPR != InvalidGPRReg) {
            gpScratch.append(remainingScratchGPR);
            ++i;
        }
        for (; i < snippet->numGPScratchRegisters; ++i)
            gpScratch.append(allocator.allocateScratchGPR());
    }

    for (unsigned i = 0; i < snippet->numFPScratchRegisters; ++i)
        fpScratch.append(allocator.allocateScratchFPR());

    // Let's store the reused registers to the stack. After that, we can use allocated scratch registers.
    ScratchRegisterAllocator::PreservedState preservedState =
    allocator.preserveReusedRegistersByPushing(jit, ScratchRegisterAllocator::ExtraStackSpace::SpaceForCCall);

    if (verbose) {
        dataLog("baseGPR = ", baseGPR, "\n");
        dataLog("valueRegs = ", valueRegs, "\n");
        dataLog("scratchGPR = ", scratchGPR, "\n");
        dataLog("paramBaseGPR = ", paramBaseGPR, "\n");
        if (paramGlobalObjectGPR != InvalidGPRReg)
            dataLog("paramGlobalObjectGPR = ", paramGlobalObjectGPR, "\n");
        dataLog("paramValueRegs = ", paramValueRegs, "\n");
        for (unsigned i = 0; i < snippet->numGPScratchRegisters; ++i)
            dataLog("gpScratch[", i, "] = ", gpScratch[i], "\n");
    }

    if (snippet->requireGlobalObject)
        jit.move(CCallHelpers::TrustedImmPtr(globalObjectForDOMJIT), paramGlobalObjectGPR);

    // We just spill the registers used in Snippet here. For not spilled registers here explicitly,
    // they must be in the used register set passed by the callers (Baseline, DFG, and FTL) if they need to be kept.
    // Some registers can be locked, but not in the used register set. For example, the caller could make baseGPR
    // same to valueRegs, and not include it in the used registers since it will be changed.
    RegisterSet registersToSpillForCCall;
    for (auto& value : regs) {
        SnippetReg reg = value.reg();
        if (reg.isJSValueRegs())
            registersToSpillForCCall.set(reg.jsValueRegs());
        else if (reg.isGPR())
            registersToSpillForCCall.set(reg.gpr());
        else
            registersToSpillForCCall.set(reg.fpr());
    }
    for (GPRReg reg : gpScratch)
        registersToSpillForCCall.set(reg);
    for (FPRReg reg : fpScratch)
        registersToSpillForCCall.set(reg);
    registersToSpillForCCall.exclude(RegisterSet::registersToNotSaveForCCall());

    AccessCaseSnippetParams params(state.m_vm, WTFMove(regs), WTFMove(gpScratch), WTFMove(fpScratch));
    snippet->generator()->run(jit, params);
    allocator.restoreReusedRegistersByPopping(jit, preservedState);
    state.succeed();

    CCallHelpers::JumpList exceptions = params.emitSlowPathCalls(state, registersToSpillForCCall, jit);
    if (!exceptions.empty()) {
        exceptions.link(&jit);
        allocator.restoreReusedRegistersByPopping(jit, preservedState);
        state.emitExplicitExceptionHandler();
    }
}

static MacroAssemblerCodeRef virtualForThunkGenerator(
    VM* vm, CodeSpecializationKind kind, RegisterPreservationMode registers)
{
    // The callee is in regT0 (for JSVALUE32_64, the tag is in regT1).
    // The return address is on the stack, or in the link register. We will hence
    // jump to the callee, or save the return address to the call frame while we
    // make a C++ function call to the appropriate JIT operation.

    CCallHelpers jit(vm);
    
    CCallHelpers::JumpList slowCase;

    // FIXME: we should have a story for eliminating these checks. In many cases,
    // the DFG knows that the value is definitely a cell, or definitely a function.
    
#if USE(JSVALUE64)
    jit.move(CCallHelpers::TrustedImm64(TagMask), GPRInfo::regT2);
    
    slowCase.append(
        jit.branchTest64(
            CCallHelpers::NonZero, GPRInfo::regT0, GPRInfo::regT2));
#else
    slowCase.append(
        jit.branch32(
            CCallHelpers::NotEqual, GPRInfo::regT1,
            CCallHelpers::TrustedImm32(JSValue::CellTag)));
#endif
    AssemblyHelpers::emitLoadStructure(jit, GPRInfo::regT0, GPRInfo::regT2, GPRInfo::regT1);
    slowCase.append(
        jit.branchPtr(
            CCallHelpers::NotEqual,
            CCallHelpers::Address(GPRInfo::regT2, Structure::classInfoOffset()),
            CCallHelpers::TrustedImmPtr(JSFunction::info())));
    
    // Now we know we have a JSFunction.
    
    jit.loadPtr(
        CCallHelpers::Address(GPRInfo::regT0, JSFunction::offsetOfExecutable()),
        GPRInfo::regT2);
    jit.loadPtr(
        CCallHelpers::Address(
            GPRInfo::regT2, ExecutableBase::offsetOfJITCodeWithArityCheckFor(kind, registers)),
        GPRInfo::regT2);
    slowCase.append(jit.branchTestPtr(CCallHelpers::Zero, GPRInfo::regT2));
    
    // Now we know that we have a CodeBlock, and we're committed to making a fast
    // call.
    
    jit.loadPtr(
        CCallHelpers::Address(GPRInfo::regT0, JSFunction::offsetOfScopeChain()),
        GPRInfo::regT1);
#if USE(JSVALUE64)
    jit.emitPutToCallFrameHeaderBeforePrologue(GPRInfo::regT1, JSStack::ScopeChain);
#else
    jit.emitPutPayloadToCallFrameHeaderBeforePrologue(GPRInfo::regT1, JSStack::ScopeChain);
    jit.emitPutTagToCallFrameHeaderBeforePrologue(CCallHelpers::TrustedImm32(JSValue::CellTag),
        JSStack::ScopeChain);
#endif
    
    // Make a tail call. This will return back to JIT code.
    emitPointerValidation(jit, GPRInfo::regT2);
    jit.jump(GPRInfo::regT2);

    slowCase.link(&jit);
    
    // Here we don't know anything, so revert to the full slow path.
    
    slowPathFor(jit, vm, operationVirtualFor(kind, registers));
    
    LinkBuffer patchBuffer(*vm, &jit, GLOBAL_THUNK_ID);
    return FINALIZE_CODE(
        patchBuffer,
        ("Virtual %s%s slow path thunk", kind == CodeForCall ? "call" : "construct", registers == MustPreserveRegisters ? " that preserves registers" : ""));
}

static MacroAssemblerCodeRef virtualForThunkGenerator(
    JSGlobalData* globalData, CodeSpecializationKind kind)
{
    // The return address is on the stack, or in the link register. We will hence
    // jump to the callee, or save the return address to the call frame while we
    // make a C++ function call to the appropriate DFG operation.

    CCallHelpers jit(globalData);
    
    CCallHelpers::JumpList slowCase;

    // FIXME: we should have a story for eliminating these checks. In many cases,
    // the DFG knows that the value is definitely a cell, or definitely a function.
    
#if USE(JSVALUE64)
    slowCase.append(
        jit.branchTestPtr(
            CCallHelpers::NonZero, GPRInfo::nonArgGPR0, GPRInfo::tagMaskRegister));
#else
    slowCase.append(
        jit.branch32(
            CCallHelpers::NotEqual, GPRInfo::nonArgGPR1,
            CCallHelpers::TrustedImm32(JSValue::CellTag)));
#endif
    jit.loadPtr(CCallHelpers::Address(GPRInfo::nonArgGPR0, JSCell::structureOffset()), GPRInfo::nonArgGPR2);
    slowCase.append(
        jit.branchPtr(
            CCallHelpers::NotEqual,
            CCallHelpers::Address(GPRInfo::nonArgGPR2, Structure::classInfoOffset()),
            CCallHelpers::TrustedImmPtr(&JSFunction::s_info)));
    
    // Now we know we have a JSFunction.
    
    jit.loadPtr(
        CCallHelpers::Address(GPRInfo::nonArgGPR0, JSFunction::offsetOfExecutable()),
        GPRInfo::nonArgGPR2);
    slowCase.append(
        jit.branch32(
            CCallHelpers::LessThan,
            CCallHelpers::Address(
                GPRInfo::nonArgGPR2, ExecutableBase::offsetOfNumParametersFor(kind)),
            CCallHelpers::TrustedImm32(0)));
    
    // Now we know that we have a CodeBlock, and we're committed to making a fast
    // call.
    
    jit.loadPtr(
        CCallHelpers::Address(GPRInfo::nonArgGPR0, JSFunction::offsetOfScopeChain()),
        GPRInfo::nonArgGPR1);
#if USE(JSVALUE64)
    jit.storePtr(
        GPRInfo::nonArgGPR1,
        CCallHelpers::Address(
            GPRInfo::callFrameRegister,
            static_cast<ptrdiff_t>(sizeof(Register)) * RegisterFile::ScopeChain));
#else
    jit.storePtr(
        GPRInfo::nonArgGPR1,
        CCallHelpers::Address(
            GPRInfo::callFrameRegister,
            static_cast<ptrdiff_t>(sizeof(Register)) * RegisterFile::ScopeChain +
            OBJECT_OFFSETOF(EncodedValueDescriptor, asBits.payload)));
    jit.store32(
        CCallHelpers::TrustedImm32(JSValue::CellTag),
        CCallHelpers::Address(
            GPRInfo::callFrameRegister,
            static_cast<ptrdiff_t>(sizeof(Register)) * RegisterFile::ScopeChain +
            OBJECT_OFFSETOF(EncodedValueDescriptor, asBits.tag)));
#endif
    
    jit.loadPtr(
        CCallHelpers::Address(GPRInfo::nonArgGPR2, ExecutableBase::offsetOfJITCodeWithArityCheckFor(kind)),
        GPRInfo::regT0);
    
    // Make a tail call. This will return back to DFG code.
    emitPointerValidation(jit, GPRInfo::regT0);
    jit.jump(GPRInfo::regT0);

    slowCase.link(&jit);
    
    // Here we don't know anything, so revert to the full slow path.
    
    slowPathFor(jit, globalData, kind == CodeForCall ? operationVirtualCall : operationVirtualConstruct);
    
    LinkBuffer patchBuffer(*globalData, &jit, GLOBAL_THUNK_ID);
    return FINALIZE_CODE(
        patchBuffer,
        ("DFG virtual %s slow path thunk", kind == CodeForCall ? "call" : "construct"));
}

static void fixFunctionBasedOnStackMaps(
    State& state, CodeBlock* codeBlock, JITCode* jitCode, GeneratedFunction generatedFunction,
    StackMaps::RecordMap& recordMap)
{
    Graph& graph = state.graph;
    VM& vm = graph.m_vm;
    StackMaps stackmaps = jitCode->stackmaps;
    
    int localsOffset = offsetOfStackRegion(recordMap, state.capturedStackmapID) + graph.m_nextMachineLocal;
    int varargsSpillSlotsOffset = offsetOfStackRegion(recordMap, state.varargsSpillSlotsStackmapID);
    
    for (unsigned i = graph.m_inlineVariableData.size(); i--;) {
        InlineCallFrame* inlineCallFrame = graph.m_inlineVariableData[i].inlineCallFrame;
        
        if (inlineCallFrame->argumentCountRegister.isValid())
            inlineCallFrame->argumentCountRegister += localsOffset;
        
        for (unsigned argument = inlineCallFrame->arguments.size(); argument-- > 1;) {
            inlineCallFrame->arguments[argument] =
                inlineCallFrame->arguments[argument].withLocalsOffset(localsOffset);
        }
        
        if (inlineCallFrame->isClosureCall) {
            inlineCallFrame->calleeRecovery =
                inlineCallFrame->calleeRecovery.withLocalsOffset(localsOffset);
        }

        if (graph.hasDebuggerEnabled())
            codeBlock->setScopeRegister(codeBlock->scopeRegister() + localsOffset);
    }
    
    MacroAssembler::Label stackOverflowException;

    {
        CCallHelpers checkJIT(&vm, codeBlock);
        
        // At this point it's perfectly fair to just blow away all state and restore the
        // JS JIT view of the universe.
        checkJIT.copyCalleeSavesToVMCalleeSavesBuffer();
        checkJIT.move(MacroAssembler::TrustedImmPtr(&vm), GPRInfo::argumentGPR0);
        checkJIT.move(GPRInfo::callFrameRegister, GPRInfo::argumentGPR1);
        MacroAssembler::Call callLookupExceptionHandler = checkJIT.call();
        checkJIT.jumpToExceptionHandler();

        stackOverflowException = checkJIT.label();
        checkJIT.copyCalleeSavesToVMCalleeSavesBuffer();
        checkJIT.move(MacroAssembler::TrustedImmPtr(&vm), GPRInfo::argumentGPR0);
        checkJIT.move(GPRInfo::callFrameRegister, GPRInfo::argumentGPR1);
        MacroAssembler::Call callLookupExceptionHandlerFromCallerFrame = checkJIT.call();
        checkJIT.jumpToExceptionHandler();

        auto linkBuffer = std::make_unique<LinkBuffer>(
            vm, checkJIT, codeBlock, JITCompilationCanFail);
        if (linkBuffer->didFailToAllocate()) {
            state.allocationFailed = true;
            return;
        }
        linkBuffer->link(callLookupExceptionHandler, FunctionPtr(lookupExceptionHandler));
        linkBuffer->link(callLookupExceptionHandlerFromCallerFrame, FunctionPtr(lookupExceptionHandlerFromCallerFrame));

        state.finalizer->handleExceptionsLinkBuffer = WTF::move(linkBuffer);
    }

    ExitThunkGenerator exitThunkGenerator(state);
    exitThunkGenerator.emitThunks();
    if (exitThunkGenerator.didThings()) {
        RELEASE_ASSERT(state.finalizer->osrExit.size());
        
        auto linkBuffer = std::make_unique<LinkBuffer>(
            vm, exitThunkGenerator, codeBlock, JITCompilationCanFail);
        if (linkBuffer->didFailToAllocate()) {
            state.allocationFailed = true;
            return;
        }
        
        RELEASE_ASSERT(state.finalizer->osrExit.size() == state.jitCode->osrExit.size());
        
        for (unsigned i = 0; i < state.jitCode->osrExit.size(); ++i) {
            OSRExitCompilationInfo& info = state.finalizer->osrExit[i];
            OSRExit& exit = jitCode->osrExit[i];
            
            if (verboseCompilationEnabled())
                dataLog("Handling OSR stackmap #", exit.m_stackmapID, " for ", exit.m_codeOrigin, "\n");

            auto iter = recordMap.find(exit.m_stackmapID);
            if (iter == recordMap.end()) {
                // It was optimized out.
                continue;
            }
            
            info.m_thunkAddress = linkBuffer->locationOf(info.m_thunkLabel);
            exit.m_patchableCodeOffset = linkBuffer->offsetOf(info.m_thunkJump);
            
            for (unsigned j = exit.m_values.size(); j--;)
                exit.m_values[j] = exit.m_values[j].withLocalsOffset(localsOffset);
            for (ExitTimeObjectMaterialization* materialization : exit.m_materializations)
                materialization->accountForLocalsOffset(localsOffset);
            
            if (verboseCompilationEnabled()) {
                DumpContext context;
//.........这里部分代码省略.........

// FIXME: We should distinguish between a megamorphic virtual call vs. a slow
// path virtual call so that we can enable fast tail calls for megamorphic
// virtual calls by using the shuffler.
// https://bugs.webkit.org/show_bug.cgi?id=148831
MacroAssemblerCodeRef virtualThunkFor(VM* vm, CallLinkInfo& callLinkInfo)
{
    // The callee is in regT0 (for JSVALUE32_64, the tag is in regT1).
    // The return address is on the stack, or in the link register. We will hence
    // jump to the callee, or save the return address to the call frame while we
    // make a C++ function call to the appropriate JIT operation.

    CCallHelpers jit(vm);
    
    CCallHelpers::JumpList slowCase;
    
    // This is a slow path execution, and regT2 contains the CallLinkInfo. Count the
    // slow path execution for the profiler.
    jit.add32(
        CCallHelpers::TrustedImm32(1),
        CCallHelpers::Address(GPRInfo::regT2, CallLinkInfo::offsetOfSlowPathCount()));

    // FIXME: we should have a story for eliminating these checks. In many cases,
    // the DFG knows that the value is definitely a cell, or definitely a function.
    
#if USE(JSVALUE64)
    jit.move(CCallHelpers::TrustedImm64(TagMask), GPRInfo::regT4);
    
    slowCase.append(
        jit.branchTest64(
            CCallHelpers::NonZero, GPRInfo::regT0, GPRInfo::regT4));
#else
    slowCase.append(
        jit.branch32(
            CCallHelpers::NotEqual, GPRInfo::regT1,
            CCallHelpers::TrustedImm32(JSValue::CellTag)));
#endif
    AssemblyHelpers::emitLoadStructure(jit, GPRInfo::regT0, GPRInfo::regT4, GPRInfo::regT1);
    slowCase.append(
        jit.branchPtr(
            CCallHelpers::NotEqual,
            CCallHelpers::Address(GPRInfo::regT4, Structure::classInfoOffset()),
            CCallHelpers::TrustedImmPtr(JSFunction::info())));
    
    // Now we know we have a JSFunction.
    
    jit.loadPtr(
        CCallHelpers::Address(GPRInfo::regT0, JSFunction::offsetOfExecutable()),
        GPRInfo::regT4);
    jit.loadPtr(
        CCallHelpers::Address(
            GPRInfo::regT4, ExecutableBase::offsetOfJITCodeWithArityCheckFor(
                callLinkInfo.specializationKind())),
        GPRInfo::regT4);
    slowCase.append(jit.branchTestPtr(CCallHelpers::Zero, GPRInfo::regT4));
    
    // Now we know that we have a CodeBlock, and we're committed to making a fast
    // call.
    
    // Make a tail call. This will return back to JIT code.
    emitPointerValidation(jit, GPRInfo::regT4);
    if (callLinkInfo.isTailCall()) {
        jit.preserveReturnAddressAfterCall(GPRInfo::regT0);
        jit.prepareForTailCallSlow(GPRInfo::regT4);
    }
    jit.jump(GPRInfo::regT4);

    slowCase.link(&jit);
    
    // Here we don't know anything, so revert to the full slow path.
    
    slowPathFor(jit, vm, operationVirtualCall);
    
    LinkBuffer patchBuffer(*vm, jit, GLOBAL_THUNK_ID);
    return FINALIZE_CODE(
        patchBuffer,
        ("Virtual %s slow path thunk",
        callLinkInfo.callMode() == CallMode::Regular ? "call" : callLinkInfo.callMode() == CallMode::Tail ? "tail call" : "construct"));
}