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/* -*- 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 */
#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);
BailoutFrameInfo(const JitActivationIterator& activations, BailoutStack* sp);
BailoutFrameInfo(const JitActivationIterator& activations, InvalidationBailoutStack* sp);
BailoutFrameInfo(const JitActivationIterator& activations, const JitFrameIterator& frame);
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_;
ExceptionBailoutInfo(size_t frameNo, jsbytecode* resumePC, size_t numExprSlots)
: frameNo_(frameNo),
{ }
: frameNo_(0),
{ }
bool catchingException() const {
return !!resumePC_;
bool propagatingIonExceptionForDebugMode() const {
return !resumePC_;
size_t frameNo() const {
return frameNo_;
jsbytecode* resumePC() const {
return resumePC_;
size_t numExprSlots() const {
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 */