src/third_party/mozjs-45/js/src/jit/Bailouts.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef jit_Bailouts_h
#define jit_Bailouts_h

#include "jstypes.h"

#include "jit/JitFrameIterator.h"
#include "jit/JitFrames.h"
#include "vm/Stack.h"

namespace js {
namespace jit {

// A "bailout" is a condition in which we need to recover an interpreter frame
// from an IonFrame. Bailouts can happen for the following reasons:
//   (1) A deoptimization guard, for example, an add overflows or a type check
//       fails.
//   (2) A check or assumption held by the JIT is invalidated by the VM, and
//       JIT code must be thrown away. This includes the GC possibly deciding
//       to evict live JIT code, or a Type Inference reflow.
//
// Note that bailouts as described here do not include normal Ion frame
// inspection, for example, if an exception must be built or the GC needs to
// scan an Ion frame for gcthings.
//
// The second type of bailout needs a different name - "deoptimization" or
// "deep bailout". Here we are concerned with eager (or maybe "shallow")
// bailouts, that happen from JIT code. These happen from guards, like:
//
//  cmp [obj + shape], 0x50M37TH1NG
//  jmp _bailout
//
// The bailout target needs to somehow translate the Ion frame (whose state
// will differ at each program point) to an interpreter frame. This state is
// captured into the IonScript's snapshot buffer, and for each bailout we know
// which snapshot corresponds to its state.
//
// Roughly, the following needs to happen at the bailout target.
//   (1) Move snapshot ID into a known stack location (registers cannot be
//       mutated).
//   (2) Spill all registers to the stack.
//   (3) Call a Bailout() routine, whose argument is the stack pointer.
//   (4) Bailout() will find the IonScript on the stack, use the snapshot ID
//       to find the structure of the frame, and then use the stack and spilled
//       registers to perform frame conversion.
//   (5) Bailout() returns, and the JIT must immediately return to the
//       interpreter (all frames are converted at once).
//
// (2) and (3) are implemented by a trampoline held in the compartment.
// Naively, we could implement (1) like:
//
//   _bailout_ID_1:
//     push 1
//     jmp _global_bailout_handler
//   _bailout_ID_2:
//     push 2
//     jmp _global_bailout_handler
//
// This takes about 10 extra bytes per guard. On some platforms, we can reduce
// this overhead to 4 bytes by creating a global jump table, shared again in
// the compartment:
//
//     call _global_bailout_handler
//     call _global_bailout_handler
//     call _global_bailout_handler
//     call _global_bailout_handler
//      ...
//    _global_bailout_handler:
//
// In the bailout handler, we can recompute which entry in the table was
// selected by subtracting the return addressed pushed by the call, from the
// start of the table, and then dividing by the size of a (call X) entry in the
// table. This gives us a number in [0, TableSize), which we call a
// "BailoutId".
//
// Then, we can provide a per-script mapping from BailoutIds to snapshots,
// which takes only four bytes per entry.
//
// This strategy does not work as given, because the bailout handler has no way
// to compute the location of an IonScript. Currently, we do not use frame
// pointers. To account for this we segregate frames into a limited set of
// "frame sizes", and create a table for each frame size. We also have the
// option of not using bailout tables, for platforms or situations where the
// 10 byte cost is more optimal than a bailout table. See JitFrames.h for more
// detail.

static const BailoutId INVALID_BAILOUT_ID = BailoutId(-1);

// Keep this arbitrarily small for now, for testing.
static const uint32_t BAILOUT_TABLE_SIZE = 16;

// Bailout return codes.
// N.B. the relative order of these values is hard-coded into ::GenerateBailoutThunk.
static const uint32_t BAILOUT_RETURN_OK = 0;
static const uint32_t BAILOUT_RETURN_FATAL_ERROR = 1;
static const uint32_t BAILOUT_RETURN_OVERRECURSED = 2;

// This address is a magic number made to cause crashes while indicating that we
// are making an attempt to mark the stack during a bailout.
static uint8_t * const FAKE_JIT_TOP_FOR_BAILOUT = reinterpret_cast<uint8_t*>(0xba1);

// BailoutStack is an architecture specific pointer to the stack, given by the
// bailout handler.
class BailoutStack;
class InvalidationBailoutStack;

// Must be implemented by each architecture.

// This structure is constructed before recovering the baseline frames for a
// bailout. It records all information extracted from the stack, and which are
// needed for the JitFrameIterator.
class BailoutFrameInfo
{
    MachineState machine_;
    uint8_t* framePointer_;
    size_t topFrameSize_;
    IonScript* topIonScript_;
    uint32_t snapshotOffset_;
    JitActivation* activation_;

    void attachOnJitActivation(const JitActivationIterator& activations);

  public:
    BailoutFrameInfo(const JitActivationIterator& activations, BailoutStack* sp);
    BailoutFrameInfo(const JitActivationIterator& activations, InvalidationBailoutStack* sp);
    BailoutFrameInfo(const JitActivationIterator& activations, const JitFrameIterator& frame);
    ~BailoutFrameInfo();

    uint8_t* fp() const {
        return framePointer_;
    }
    SnapshotOffset snapshotOffset() const {
        return snapshotOffset_;
    }
    const MachineState* machineState() const {
        return &machine_;
    }
    size_t topFrameSize() const {
        return topFrameSize_;
    }
    IonScript* ionScript() const {
        return topIonScript_;
    }
    JitActivation* activation() const {
        return activation_;
    }
};

bool EnsureHasScopeObjects(JSContext* cx, AbstractFramePtr fp);

struct BaselineBailoutInfo;

// Called from a bailout thunk. Returns a BAILOUT_* error code.
uint32_t Bailout(BailoutStack* sp, BaselineBailoutInfo** info);

// Called from the invalidation thunk. Returns a BAILOUT_* error code.
uint32_t InvalidationBailout(InvalidationBailoutStack* sp, size_t* frameSizeOut,
                             BaselineBailoutInfo** info);

class ExceptionBailoutInfo
{
    size_t frameNo_;
    jsbytecode* resumePC_;
    size_t numExprSlots_;

  public:
    ExceptionBailoutInfo(size_t frameNo, jsbytecode* resumePC, size_t numExprSlots)
      : frameNo_(frameNo),
        resumePC_(resumePC),
        numExprSlots_(numExprSlots)
    { }

    ExceptionBailoutInfo()
      : frameNo_(0),
        resumePC_(nullptr),
        numExprSlots_(0)
    { }

    bool catchingException() const {
        return !!resumePC_;
    }
    bool propagatingIonExceptionForDebugMode() const {
        return !resumePC_;
    }

    size_t frameNo() const {
        MOZ_ASSERT(catchingException());
        return frameNo_;
    }
    jsbytecode* resumePC() const {
        MOZ_ASSERT(catchingException());
        return resumePC_;
    }
    size_t numExprSlots() const {
        MOZ_ASSERT(catchingException());
        return numExprSlots_;
    }
};

// Called from the exception handler to enter a catch or finally block.
// Returns a BAILOUT_* error code.
uint32_t ExceptionHandlerBailout(JSContext* cx, const InlineFrameIterator& frame,
                                 ResumeFromException* rfe,
                                 const ExceptionBailoutInfo& excInfo,
                                 bool* overrecursed);

uint32_t FinishBailoutToBaseline(BaselineBailoutInfo* bailoutInfo);

bool CheckFrequentBailouts(JSContext* cx, JSScript* script, BailoutKind bailoutKind);

} // namespace jit
} // namespace js

#endif /* jit_Bailouts_h */