DFGOSREntry.cpp   [plain text]

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#include "config.h"
#include "DFGOSREntry.h"

#if ENABLE(DFG_JIT)

#include "CallFrame.h"
#include "CodeBlock.h"
#include "DFGJITCode.h"
#include "DFGNode.h"
#include "JIT.h"
#include "JSStackInlines.h"
#include "JSCInlines.h"
#include <wtf/CommaPrinter.h>

namespace JSC { namespace DFG {

void OSREntryData::dumpInContext(PrintStream& out, DumpContext* context) const
{
    out.print("bc#", m_bytecodeIndex, ", machine code offset = ", m_machineCodeOffset);
    out.print(", stack rules = [");
    
    auto printOperand = [&] (VirtualRegister reg) {
        out.print(inContext(m_expectedValues.operand(reg), context), " (");
        VirtualRegister toReg;
        bool overwritten = false;
        for (OSREntryReshuffling reshuffling : m_reshufflings) {
            if (reg == VirtualRegister(reshuffling.fromOffset)) {
                toReg = VirtualRegister(reshuffling.toOffset);
                break;
            }
            if (reg == VirtualRegister(reshuffling.toOffset))
                overwritten = true;
        }
        if (!overwritten && !toReg.isValid())
            toReg = reg;
        if (toReg.isValid()) {
            if (toReg.isLocal() && !m_machineStackUsed.get(toReg.toLocal()))
                out.print("ignored");
            else
                out.print("maps to ", toReg);
        } else
            out.print("overwritten");
        if (reg.isLocal() && m_localsForcedDouble.get(reg.toLocal()))
            out.print(", forced double");
        if (reg.isLocal() && m_localsForcedMachineInt.get(reg.toLocal()))
            out.print(", forced machine int");
        out.print(")");
    };
    
    CommaPrinter comma;
    for (size_t argumentIndex = m_expectedValues.numberOfArguments(); argumentIndex--;) {
        out.print(comma, "arg", argumentIndex, ":");
        printOperand(virtualRegisterForArgument(argumentIndex));
    }
    for (size_t localIndex = 0; localIndex < m_expectedValues.numberOfLocals(); ++localIndex) {
        out.print(comma, "loc", localIndex, ":");
        printOperand(virtualRegisterForLocal(localIndex));
    }
    
    out.print("], machine stack used = ", m_machineStackUsed);
}

void OSREntryData::dump(PrintStream& out) const
{
    dumpInContext(out, nullptr);
}

SUPPRESS_ASAN
void* prepareOSREntry(ExecState* exec, CodeBlock* codeBlock, unsigned bytecodeIndex)
{
    ASSERT(JITCode::isOptimizingJIT(codeBlock->jitType()));
    ASSERT(codeBlock->alternative());
    ASSERT(codeBlock->alternative()->jitType() == JITCode::BaselineJIT);
    ASSERT(!codeBlock->jitCodeMap());

    if (!Options::enableOSREntryToDFG())
        return 0;

    if (Options::verboseOSR()) {
        dataLog(
            "DFG OSR in ", *codeBlock->alternative(), " -> ", *codeBlock,
            " from bc#", bytecodeIndex, "\n");
    }
    
    VM* vm = &exec->vm();

    sanitizeStackForVM(vm);
    
    if (bytecodeIndex)
        codeBlock->ownerExecutable()->setDidTryToEnterInLoop(true);
    
    if (codeBlock->jitType() != JITCode::DFGJIT) {
        RELEASE_ASSERT(codeBlock->jitType() == JITCode::FTLJIT);
        
        // When will this happen? We could have:
        //
        // - An exit from the FTL JIT into the baseline JIT followed by an attempt
        //   to reenter. We're fine with allowing this to fail. If it happens
        //   enough we'll just reoptimize. It basically means that the OSR exit cost
        //   us dearly and so reoptimizing is the right thing to do.
        //
        // - We have recursive code with hot loops. Consider that foo has a hot loop
        //   that calls itself. We have two foo's on the stack, lets call them foo1
        //   and foo2, with foo1 having called foo2 from foo's hot loop. foo2 gets
        //   optimized all the way into the FTL. Then it returns into foo1, and then
        //   foo1 wants to get optimized. It might reach this conclusion from its
        //   hot loop and attempt to OSR enter. And we'll tell it that it can't. It
        //   might be worth addressing this case, but I just think this case will
        //   be super rare. For now, if it does happen, it'll cause some compilation
        //   thrashing.
        
        if (Options::verboseOSR())
            dataLog("    OSR failed because the target code block is not DFG.\n");
        return 0;
    }
    
    JITCode* jitCode = codeBlock->jitCode()->dfg();
    OSREntryData* entry = jitCode->osrEntryDataForBytecodeIndex(bytecodeIndex);
    
    if (!entry) {
        if (Options::verboseOSR())
            dataLogF("    OSR failed because the entrypoint was optimized out.\n");
        return 0;
    }
    
    ASSERT(entry->m_bytecodeIndex == bytecodeIndex);
    
    // The code below checks if it is safe to perform OSR entry. It may find
    // that it is unsafe to do so, for any number of reasons, which are documented
    // below. If the code decides not to OSR then it returns 0, and it's the caller's
    // responsibility to patch up the state in such a way as to ensure that it's
    // both safe and efficient to continue executing baseline code for now. This
    // should almost certainly include calling either codeBlock->optimizeAfterWarmUp()
    // or codeBlock->dontOptimizeAnytimeSoon().
    
    // 1) Verify predictions. If the predictions are inconsistent with the actual
    //    values, then OSR entry is not possible at this time. It's tempting to
    //    assume that we could somehow avoid this case. We can certainly avoid it
    //    for first-time loop OSR - that is, OSR into a CodeBlock that we have just
    //    compiled. Then we are almost guaranteed that all of the predictions will
    //    check out. It would be pretty easy to make that a hard guarantee. But
    //    then there would still be the case where two call frames with the same
    //    baseline CodeBlock are on the stack at the same time. The top one
    //    triggers compilation and OSR. In that case, we may no longer have
    //    accurate value profiles for the one deeper in the stack. Hence, when we
    //    pop into the CodeBlock that is deeper on the stack, we might OSR and
    //    realize that the predictions are wrong. Probably, in most cases, this is
    //    just an anomaly in the sense that the older CodeBlock simply went off
    //    into a less-likely path. So, the wisest course of action is to simply not
    //    OSR at this time.
    
    for (size_t argument = 0; argument < entry->m_expectedValues.numberOfArguments(); ++argument) {
        if (argument >= exec->argumentCountIncludingThis()) {
            if (Options::verboseOSR()) {
                dataLogF("    OSR failed because argument %zu was not passed, expected ", argument);
                entry->m_expectedValues.argument(argument).dump(WTF::dataFile());
                dataLogF(".\n");
            }
            return 0;
        }
        
        JSValue value;
        if (!argument)
            value = exec->thisValue();
        else
            value = exec->argument(argument - 1);
        
        if (!entry->m_expectedValues.argument(argument).validate(value)) {
            if (Options::verboseOSR()) {
                dataLog(
                    "    OSR failed because argument ", argument, " is ", value,
                    ", expected ", entry->m_expectedValues.argument(argument), ".\n");
            }
            return 0;
        }
    }
    
    for (size_t local = 0; local < entry->m_expectedValues.numberOfLocals(); ++local) {
        int localOffset = virtualRegisterForLocal(local).offset();
        if (entry->m_localsForcedDouble.get(local)) {
            if (!exec->registers()[localOffset].asanUnsafeJSValue().isNumber()) {
                if (Options::verboseOSR()) {
                    dataLog(
                        "    OSR failed because variable ", localOffset, " is ",
                        exec->registers()[localOffset].asanUnsafeJSValue(), ", expected number.\n");
                }
                return 0;
            }
            continue;
        }
        if (entry->m_localsForcedMachineInt.get(local)) {
            if (!exec->registers()[localOffset].asanUnsafeJSValue().isMachineInt()) {
                if (Options::verboseOSR()) {
                    dataLog(
                        "    OSR failed because variable ", localOffset, " is ",
                        exec->registers()[localOffset].asanUnsafeJSValue(), ", expected ",
                        "machine int.\n");
                }
                return 0;
            }
            continue;
        }
        if (!entry->m_expectedValues.local(local).validate(exec->registers()[localOffset].asanUnsafeJSValue())) {
            if (Options::verboseOSR()) {
                dataLog(
                    "    OSR failed because variable ", localOffset, " is ",
                    exec->registers()[localOffset].asanUnsafeJSValue(), ", expected ",
                    entry->m_expectedValues.local(local), ".\n");
            }
            return 0;
        }
    }

    // 2) Check the stack height. The DFG JIT may require a taller stack than the
    //    baseline JIT, in some cases. If we can't grow the stack, then don't do
    //    OSR right now. That's the only option we have unless we want basic block
    //    boundaries to start throwing RangeErrors. Although that would be possible,
    //    it seems silly: you'd be diverting the program to error handling when it
    //    would have otherwise just kept running albeit less quickly.
    
    unsigned frameSizeForCheck = jitCode->common.requiredRegisterCountForExecutionAndExit();
    if (!vm->interpreter->stack().ensureCapacityFor(&exec->registers()[virtualRegisterForLocal(frameSizeForCheck - 1).offset()])) {
        if (Options::verboseOSR())
            dataLogF("    OSR failed because stack growth failed.\n");
        return 0;
    }
    
    if (Options::verboseOSR())
        dataLogF("    OSR should succeed.\n");

    // At this point we're committed to entering. We will do some work to set things up,
    // but we also rely on our caller recognizing that when we return a non-null pointer,
    // that means that we're already past the point of no return and we must succeed at
    // entering.
    
    // 3) Set up the data in the scratch buffer and perform data format conversions.

    unsigned frameSize = jitCode->common.frameRegisterCount;
    unsigned baselineFrameSize = entry->m_expectedValues.numberOfLocals();
    unsigned maxFrameSize = std::max(frameSize, baselineFrameSize);

    Register* scratch = bitwise_cast<Register*>(vm->scratchBufferForSize(sizeof(Register) * (2 + JSStack::CallFrameHeaderSize + maxFrameSize))->dataBuffer());
    
    *bitwise_cast<size_t*>(scratch + 0) = frameSize;
    
    void* targetPC = codeBlock->jitCode()->executableAddressAtOffset(entry->m_machineCodeOffset);
    if (Options::verboseOSR())
        dataLogF("    OSR using target PC %p.\n", targetPC);
    RELEASE_ASSERT(targetPC);
    *bitwise_cast<void**>(scratch + 1) = targetPC;
    
    Register* pivot = scratch + 2 + JSStack::CallFrameHeaderSize;
    
    for (int index = -JSStack::CallFrameHeaderSize; index < static_cast<int>(baselineFrameSize); ++index) {
        VirtualRegister reg(-1 - index);
        
        if (reg.isLocal()) {
            if (entry->m_localsForcedDouble.get(reg.toLocal())) {
                *bitwise_cast<double*>(pivot + index) = exec->registers()[reg.offset()].asanUnsafeJSValue().asNumber();
                continue;
            }
            
            if (entry->m_localsForcedMachineInt.get(reg.toLocal())) {
                *bitwise_cast<int64_t*>(pivot + index) = exec->registers()[reg.offset()].asanUnsafeJSValue().asMachineInt() << JSValue::int52ShiftAmount;
                continue;
            }
        }
        
        pivot[index] = exec->registers()[reg.offset()].asanUnsafeJSValue();
    }
    
    // 4) Reshuffle those registers that need reshuffling.
    Vector<JSValue> temporaryLocals(entry->m_reshufflings.size());
    for (unsigned i = entry->m_reshufflings.size(); i--;)
        temporaryLocals[i] = pivot[VirtualRegister(entry->m_reshufflings[i].fromOffset).toLocal()].asanUnsafeJSValue();
    for (unsigned i = entry->m_reshufflings.size(); i--;)
        pivot[VirtualRegister(entry->m_reshufflings[i].toOffset).toLocal()] = temporaryLocals[i];
    
    // 5) Clear those parts of the call frame that the DFG ain't using. This helps GC on
    //    some programs by eliminating some stale pointer pathologies.
    for (unsigned i = frameSize; i--;) {
        if (entry->m_machineStackUsed.get(i))
            continue;
        pivot[i] = JSValue();
    }
    
    // 6) Fix the call frame to have the right code block.
    
    *bitwise_cast<CodeBlock**>(pivot - 1 - JSStack::CodeBlock) = codeBlock;
    
    if (Options::verboseOSR())
        dataLogF("    OSR returning data buffer %p.\n", scratch);
    return scratch;
}

} } // namespace JSC::DFG

#endif // ENABLE(DFG_JIT)