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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* Copyright 2021 Mozilla Foundation
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
* Wasm-internal ABI.
* The *Wasm-internal ABI* is the ABI a wasm function assumes when it is
* entered, and the one it assumes when it is making a call to what it believes
* is another wasm function.
* We pass the first function arguments in registers (GPR and FPU both) and the
* rest on the stack, generally according to platform ABI conventions (which can
* be hairy). On x86-32 there are no register arguments.
* We have no callee-saves registers in the wasm-internal ABI, regardless of the
* platform ABI conventions, though see below about InstanceReg or HeapReg.
* We return the last return value in the first return register, according to
* platform ABI conventions. If there is more than one return value, an area is
* allocated in the caller's frame to receive the other return values, and the
* address of this area is passed to the callee as the last argument. Return
* values except the last are stored in ascending order within this area. Also
* see below about alignment of this area and the values in it.
* When a function is entered, there are two incoming register values in
* addition to the function's declared parameters: InstanceReg must have the
* correct instance pointer, and HeapReg the correct memoryBase, for the
* function. (On x86-32 there is no HeapReg.) From the instance we can get to
* the JSContext, the instance, the MemoryBase, and many other things. The
* instance maps one-to-one with an instance.
* HeapReg and InstanceReg are not parameters in the usual sense, nor are they
* callee-saves registers. Instead they constitute global register state, the
* purpose of which is to bias the call ABI in favor of intra-instance calls,
* the predominant case where the caller and the callee have the same
* InstanceReg and HeapReg values.
* With this global register state, literally no work needs to take place to
* save and restore the instance and MemoryBase values across intra-instance
* call boundaries.
* For inter-instance calls, in contrast, there must be an instance switch at
* the call boundary: Before the call, the callee's instance must be loaded
* (from a closure or from the import table), and from the instance we load the
* callee's MemoryBase, the realm, and the JSContext. The caller's and callee's
* instance values must be stored into the frame (to aid unwinding), the
* callee's realm must be stored into the JSContext, and the callee's instance
* and MemoryBase values must be moved to appropriate registers. After the
* call, the caller's instance must be loaded, and from it the caller's
* MemoryBase and realm, and the JSContext. The realm must be stored into the
* JSContext and the caller's instance and MemoryBase values must be moved to
* appropriate registers.
* Direct calls to functions within the same module are always intra-instance,
* while direct calls to imported functions are always inter-instance. Indirect
* calls -- call_indirect in the MVP, future call_ref and call_funcref -- may or
* may not be intra-instance.
* call_indirect, and future call_funcref, also pass a signature value in a
* register (even on x86-32), this is a small integer or a pointer value
* denoting the caller's expected function signature. The callee must compare
* it to the value or pointer that denotes its actual signature, and trap on
* mismatch.
* This is what the stack looks like during a call, after the callee has
* completed the prologue:
* | |
* +-----------------------------------+ <-+
* | ... | |
* | Caller's private frame | |
* +-----------------------------------+ |
* | Multi-value return (optional) | |
* | ... | |
* +-----------------------------------+ |
* | Stack args (optional) | |
* | ... | |
* +-----------------------------------+ -+|
* | Caller instance slot | \
* | Callee instance slot | | \
* +-----------------------------------+ | \
* | Shadowstack area (Win64) | | wasm::FrameWithInstances
* | (32 bytes) | | /
* +-----------------------------------+ | / <= SP "Before call"
* | Return address | // <= SP "After call"
* | Saved FP ----|--+/
* +-----------------------------------+ -+ <= FP (a wasm::Frame*)
* | DebugFrame, Locals, spills, etc |
* | (i.e., callee's private frame) |
* | .... |
* +-----------------------------------+ <= SP
* The FrameWithInstances is a struct with four fields: the saved FP, the return
* address, and the two instance slots; the shadow stack area is there only on
* Win64 and is unused by wasm but is part of the native ABI, with which the
* wasm ABI is mostly compatible. The slots for caller and callee instance are
* only populated by the instance switching code in inter-instance calls so that
* stack unwinding can keep track of the correct instance value for each frame,
* the instance not being obtainable from anywhere else. Nothing in the frame
* itself indicates directly whether the instance slots are valid - for that,
* the return address must be used to look up a CallSite structure that carries
* that information.
* The stack area above the return address is owned by the caller, which may
* deallocate the area on return or choose to reuse it for subsequent calls.
* (The baseline compiler allocates and frees the stack args area and the
* multi-value result area per call. Ion reuses the areas and allocates them as
* part of the overall activation frame when the procedure is entered; indeed,
* the multi-value return area can be anywhere within the caller's private
* frame, not necessarily directly above the stack args.)
* If the stack args area contain references, it is up to the callee's stack map
* to name the locations where those references exist, and the caller's stack
* map must not (redundantly) name those locations. (The callee's ownership of
* this area will be crucial for making tail calls work, as the types of the
* locations can change if the callee makes a tail call.) If pointer values are
* spilled by anyone into the Shadowstack area they will not be traced.
* References in the multi-return area are covered by the caller's map, as these
* slots outlive the call.
* The address "Before call", ie the part of the FrameWithInstances above the
* Frame, must be aligned to WasmStackAlignment, and everything follows from
* that, with padding inserted for alignment as required for stack arguments. In
* turn WasmStackAlignment is at least as large as the largest parameter type.
* The address of the multiple-results area is currently 8-byte aligned by Ion
* and its alignment in baseline is uncertain, see bug 1747787. Result values
* are stored packed within the area in fields whose size is given by
* ResultStackSize(ValType), this breaks alignment too. This all seems
* underdeveloped.
* In the wasm-internal ABI, the ARM64 PseudoStackPointer (PSP) is garbage on
* entry but must be synced with the real SP at the point the function returns.
* The Wasm Builtin ABIs.
* Also see `[SMDOC] Process-wide builtin thunk set` in WasmBuiltins.cpp.
* The *Wasm-builtin ABIs* comprise the ABIs used when wasm makes calls directly
* to the C++ runtime (but not to the JS interpreter), including instance
* methods, helpers for operations such as 64-bit division on 32-bit systems,
* allocation and writer barriers, conversions to/from JS values, special
* fast-path JS imports, and trap handling.
* The callee of a builtin call will always assume the C/C++ ABI. Therefore
* every volatile (caller-saves) register that wasm uses must be saved across
* the call, the stack must be aligned as for a C/C++-ABI call before the call,
* and any ABI registers the callee expect to have specific values must be set
* up (eg the frame pointer, if the C/C++ ABI assumes it is set).
* Most builtin calls are straightforward: the wasm caller knows that it is
* performing a call, and so it saves live registers, moves arguments into ABI
* locations, etc, before calling. Abstractions in the masm make sure to pass
* the instance pointer to an instance "method" call and to restore the
* InstanceReg and HeapReg after the call. In these straightforward cases,
* calling the builtin additionally amounts to:
* - exiting the wasm activation
* - adjusting parameter values to account for platform weirdness (FP arguments
* are handled differently in the C/C++ ABIs on ARM and x86-32 than in the
* Wasm ABI)
* - copying stack arguments into place for the C/C++ ABIs
* - making the call
* - adjusting the return values on return
* - re-entering the wasm activation and returning to the wasm caller
* The steps above are performed by the *builtin thunk* for the builtin and the
* builtin itself is said to be *thunked*. Going via the thunk is simple and,
* except for always having to copy stack arguments on x86-32 and the extra call
* in the thunk, close to as fast as we can make it without heroics. Except for
* the arithmetic helpers on 32-bit systems, most builtins are rarely used, are
* asm.js-specific, or are expensive anyway, and the overhead of the extra call
* doesn't matter.
* A few builtins for special purposes are *unthunked* and fall into two
* classes: they would normally be thunked but are used in circumstances where
* the VM is in an unusual state; or they do their work within the activation.
* In the former class, we find the debug trap handler, which must preserve all
* live registers because it is called in contexts where live registers have not
* been saved; argument coercion functions, which are called while a call frame
* is being built for a JS->Wasm or Wasm->JS call; and other routines that have
* special needs for constructing the call. These all exit the activation, but
* handle the exit specially.
* In the latter class, we find two functions that abandon the VM state and
* unwind the activation, HandleThrow and HandleTrap; and some debug print
* functions that do not affect the VM state at all.
* To summarize, when wasm calls a builtin thunk the stack will end up looking
* like this from within the C++ code:
* | |
* +-------------------------+
* | Wasm frame |
* +-------------------------+
* | Thunk frame (exit) |
* +-------------------------+
* | Builtin frame (C++) |
* +-------------------------+ <= SP
* There is an assumption in the profiler (in initFromExitFP) that an exit has
* left precisely one frame on the stack for the thunk itself. There may be
* additional assumptions elsewhere, not yet found.
* Very occasionally, Wasm will call C++ without going through the builtin
* thunks, and this can be a source of problems. The one case I know about
* right now is that the JS pre-barrier filtering code is called directly from
* Wasm, see bug 1464157.
* Wasm stub ABIs.
* Also see `[SMDOC] Exported wasm functions and the jit-entry stubs` in
* WasmJS.cpp.
* The "stub ABIs" are not properly speaking ABIs themselves, but ABI
* converters. An "entry" stub calls in to wasm and an "exit" stub calls out
* from wasm. The entry stubs must convert from whatever data formats the
* caller has to wasm formats (and in the future must provide some kind of type
* checking for pointer types); the exit stubs convert from wasm formats to the
* callee's expected format.
* There are different entry paths from the JS interpreter (using the C++ ABI
* and data formats) and from jitted JS code (using the JIT ABI and data
* formats); indeed there is a "normal" JitEntry path ("JitEntry") that will
* perform argument and return value conversion, and the "fast" JitEntry path
* ("DirectCallFromJit") that is only used when it is known that the JIT will
* only pass and receive wasm-compatible data and no conversion is needed.
* Similarly, there are different exit paths to the interpreter (using the C++
* ABI and data formats) and to JS JIT code (using the JIT ABI and data
* formats). Also, builtin calls described above are themselves a type of exit,
* and builtin thunks are properly a type of exit stub.
* Data conversions are difficult because the VM is in an intermediate state
* when they happen, we want them to be fast when possible, and some conversions
* can re-enter both JS code and wasm code.
#ifndef wasm_frame_h
#define wasm_frame_h
#include "mozilla/Assertions.h"
#include <stddef.h>
#include <stdint.h>
#include <type_traits>
#include "jit/Registers.h" // For js::jit::ShadowStackSpace
namespace js {
namespace wasm {
class Instance;
// Bit tag set when exiting wasm code in JitActivation's exitFP.
constexpr uintptr_t ExitFPTag = 0x1;
// wasm::Frame represents the bytes pushed by the call instruction and the
// fixed prologue generated by wasm::GenerateCallablePrologue.
// Across all architectures it is assumed that, before the call instruction, the
// stack pointer is WasmStackAlignment-aligned. Thus after the prologue, and
// before the function has made its stack reservation, the stack alignment is
// sizeof(Frame) % WasmStackAlignment.
// During MacroAssembler code generation, the bytes pushed after the wasm::Frame
// are counted by masm.framePushed. Thus, the stack alignment at any point in
// time is (sizeof(wasm::Frame) + masm.framePushed) % WasmStackAlignment.
class Frame {
// See GenerateCallableEpilogue for why this must be
// the first field of wasm::Frame (in a downward-growing stack).
// It's either the caller's Frame*, for wasm callers, or the JIT caller frame
// plus a tag otherwise.
uint8_t* callerFP_;
// The return address pushed by the call (in the case of ARM/MIPS the return
// address is pushed by the first instruction of the prologue).
void* returnAddress_;
static constexpr uint32_t callerFPOffset() {
return offsetof(Frame, callerFP_);
static constexpr uint32_t returnAddressOffset() {
return offsetof(Frame, returnAddress_);
uint8_t* returnAddress() const {
return reinterpret_cast<uint8_t*>(returnAddress_);
void** addressOfReturnAddress() {
return reinterpret_cast<void**>(&returnAddress_);
uint8_t* rawCaller() const { return callerFP_; }
Frame* wasmCaller() const { return reinterpret_cast<Frame*>(callerFP_); }
uint8_t* jitEntryCaller() const { return callerFP_; }
static const Frame* fromUntaggedWasmExitFP(const void* savedFP) {
return reinterpret_cast<const Frame*>(savedFP);
static bool isExitFP(const void* fp) {
return reinterpret_cast<uintptr_t>(fp) & ExitFPTag;
static uint8_t* untagExitFP(const void* fp) {
return reinterpret_cast<uint8_t*>(reinterpret_cast<uintptr_t>(fp) &
static uint8_t* addExitFPTag(const Frame* fp) {
return reinterpret_cast<uint8_t*>(reinterpret_cast<uintptr_t>(fp) |
static_assert(!std::is_polymorphic_v<Frame>, "Frame doesn't need a vtable.");
static_assert(sizeof(Frame) == 2 * sizeof(void*),
"Frame is a two pointer structure");
class FrameWithShadowStackSpace : public Frame {
// `ShadowStackSpace` bytes will be allocated here on Win64, at higher
// addresses than Frame and at lower addresses than the instance fields.
uint8_t shadowStackSpace_[js::jit::ShadowStackSpace];
class FrameWithInstances
: public std::conditional_t<js::jit::ShadowStackSpace >= 1,
FrameWithShadowStackSpace, Frame> {
// The instance area MUST be two pointers exactly.
Instance* calleeInstance_;
Instance* callerInstance_;
Instance* calleeInstance() { return calleeInstance_; }
Instance* callerInstance() { return callerInstance_; }
constexpr static uint32_t sizeOfInstanceFields() {
return sizeof(wasm::FrameWithInstances) - sizeof(wasm::Frame) -
constexpr static uint32_t sizeOfInstanceFieldsAndShadowStack() {
return sizeof(wasm::FrameWithInstances) - sizeof(wasm::Frame);
constexpr static uint32_t calleeInstanceOffset() {
return offsetof(FrameWithInstances, calleeInstance_);
constexpr static uint32_t calleeInstanceOffsetWithoutFrame() {
return calleeInstanceOffset() - sizeof(wasm::Frame);
constexpr static uint32_t callerInstanceOffset() {
return offsetof(FrameWithInstances, callerInstance_);
constexpr static uint32_t callerInstanceOffsetWithoutFrame() {
return callerInstanceOffset() - sizeof(wasm::Frame);
static_assert(FrameWithInstances::calleeInstanceOffsetWithoutFrame() ==
"Callee instance stored right above the return address.");
static_assert(FrameWithInstances::callerInstanceOffsetWithoutFrame() ==
js::jit::ShadowStackSpace + sizeof(void*),
"Caller instance stored right above the callee instance.");
static_assert(FrameWithInstances::sizeOfInstanceFields() == 2 * sizeof(void*),
"There are only two additional slots");
#if defined(JS_CODEGEN_ARM64)
static_assert(sizeof(Frame) % 16 == 0, "frame is aligned");
} // namespace wasm
} // namespace js
#endif // wasm_frame_h