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diff --git a/js/src/jit/BaselineFrameInfo.h b/js/src/jit/BaselineFrameInfo.h
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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:
+ * 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_BaselineFrameInfo_h
+#define jit_BaselineFrameInfo_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+
+#include <new>
+
+#include "jit/BaselineFrame.h"
+#include "jit/BaselineJIT.h"
+#include "jit/FixedList.h"
+#include "jit/MacroAssembler.h"
+#include "jit/SharedICRegisters.h"
+
+namespace js {
+namespace jit {
+
+struct BytecodeInfo;
+class MacroAssembler;
+
+// [SMDOC] Baseline FrameInfo overview.
+//
+// FrameInfo is used by BaselineCodeGen to track values stored in the frame.
+// There are two implementations:
+//
+// InterpreterFrameInfo
+// --------------------
+// The InterpreterFrameInfo class is used by the interpreter generator and is
+// a very simple interface on top of the MacroAssembler, because the stack is
+// always synced.
+//
+// CompilerFrameInfo
+// -----------------
+// The CompilerFrameInfo class is more complicated because it maintains a
+// virtual stack to optimize some common stack operations. Locals and arguments
+// are always fully synced. Stack values can either be synced, stored as
+// constant, stored in a Value register or refer to a local slot. Syncing a
+// StackValue ensures it's stored on the stack, e.g. kind == Stack.
+//
+// To see how this works, consider the following statement:
+//
+//    var y = x + 9;
+//
+// Here two values are pushed: StackValue(LocalSlot(0)) and
+// StackValue(Int32Value(9)). Only when we reach the ADD op, code is generated
+// to load the operands directly into the right operand registers and sync all
+// other stack values.
+//
+// For stack values, the following invariants hold (and are checked between
+// ops):
+//
+// (1) If a value is synced (kind == Stack), all values below it must also be
+//     synced. In other words, values with kind other than Stack can only appear
+//     on top of the abstract stack.
+//
+// (2) When we call a stub or IC, all values still on the stack must be synced.
+
+// Represents a value pushed on the stack. Note that StackValue is not used for
+// locals or arguments since these are always fully synced.
+class StackValue {
+ public:
+  enum Kind {
+    Constant,
+    Register,
+    Stack,
+    LocalSlot,
+    ArgSlot,
+    ThisSlot,
+#ifdef DEBUG
+    // In debug builds, assert Kind is initialized.
+    Uninitialized,
+#endif
+  };
+
+ private:
+  MOZ_INIT_OUTSIDE_CTOR Kind kind_;
+
+  MOZ_INIT_OUTSIDE_CTOR union Data {
+    JS::Value constant;
+    ValueOperand reg;
+    uint32_t localSlot;
+    uint32_t argSlot;
+
+    // |constant| has a non-trivial constructor and therefore MUST be
+    // placement-new'd into existence.
+    MOZ_PUSH_DISABLE_NONTRIVIAL_UNION_WARNINGS
+    Data() {}
+    MOZ_POP_DISABLE_NONTRIVIAL_UNION_WARNINGS
+  } data;
+
+  MOZ_INIT_OUTSIDE_CTOR JSValueType knownType_;
+
+ public:
+  StackValue() { reset(); }
+
+  Kind kind() const { return kind_; }
+  bool hasKnownType() const { return knownType_ != JSVAL_TYPE_UNKNOWN; }
+  bool hasKnownType(JSValueType type) const {
+    MOZ_ASSERT(type != JSVAL_TYPE_UNKNOWN);
+    return knownType_ == type;
+  }
+  JSValueType knownType() const {
+    MOZ_ASSERT(hasKnownType());
+    return knownType_;
+  }
+  void reset() {
+#ifdef DEBUG
+    kind_ = Uninitialized;
+    knownType_ = JSVAL_TYPE_UNKNOWN;
+#endif
+  }
+  Value constant() const {
+    MOZ_ASSERT(kind_ == Constant);
+    return data.constant;
+  }
+  ValueOperand reg() const {
+    MOZ_ASSERT(kind_ == Register);
+    return data.reg;
+  }
+  uint32_t localSlot() const {
+    MOZ_ASSERT(kind_ == LocalSlot);
+    return data.localSlot;
+  }
+  uint32_t argSlot() const {
+    MOZ_ASSERT(kind_ == ArgSlot);
+    return data.argSlot;
+  }
+
+  void setConstant(const Value& v) {
+    kind_ = Constant;
+    new (&data.constant) Value(v);
+    knownType_ = v.isDouble() ? JSVAL_TYPE_DOUBLE : v.extractNonDoubleType();
+  }
+  void setRegister(const ValueOperand& val,
+                   JSValueType knownType = JSVAL_TYPE_UNKNOWN) {
+    kind_ = Register;
+    new (&data.reg) ValueOperand(val);
+    knownType_ = knownType;
+  }
+  void setLocalSlot(uint32_t slot) {
+    kind_ = LocalSlot;
+    new (&data.localSlot) uint32_t(slot);
+    knownType_ = JSVAL_TYPE_UNKNOWN;
+  }
+  void setArgSlot(uint32_t slot) {
+    kind_ = ArgSlot;
+    new (&data.argSlot) uint32_t(slot);
+    knownType_ = JSVAL_TYPE_UNKNOWN;
+  }
+  void setThis() {
+    kind_ = ThisSlot;
+    knownType_ = JSVAL_TYPE_UNKNOWN;
+  }
+  void setStack() {
+    kind_ = Stack;
+    knownType_ = JSVAL_TYPE_UNKNOWN;
+  }
+};
+
+enum StackAdjustment { AdjustStack, DontAdjustStack };
+
+class FrameInfo {
+ protected:
+  MacroAssembler& masm;
+
+ public:
+  explicit FrameInfo(MacroAssembler& masm) : masm(masm) {}
+
+  Address addressOfLocal(size_t local) const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfLocal(local));
+  }
+  Address addressOfArg(size_t arg) const {
+    return Address(FramePointer, JitFrameLayout::offsetOfActualArg(arg));
+  }
+  Address addressOfThis() const {
+    return Address(FramePointer, JitFrameLayout::offsetOfThis());
+  }
+  Address addressOfCalleeToken() const {
+    return Address(FramePointer, JitFrameLayout::offsetOfCalleeToken());
+  }
+  Address addressOfEnvironmentChain() const {
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfEnvironmentChain());
+  }
+  Address addressOfICScript() const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfICScript());
+  }
+  Address addressOfFlags() const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfFlags());
+  }
+  Address addressOfReturnValue() const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfReturnValue());
+  }
+  Address addressOfArgsObj() const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfArgsObj());
+  }
+  Address addressOfScratchValue() const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfScratchValue());
+  }
+  Address addressOfScratchValueLow32() const {
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfScratchValueLow32());
+  }
+  Address addressOfScratchValueHigh32() const {
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfScratchValueHigh32());
+  }
+#ifdef DEBUG
+  Address addressOfDebugFrameSize() const {
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfDebugFrameSize());
+  }
+#endif
+};
+
+class CompilerFrameInfo : public FrameInfo {
+  friend class BaselinePerfSpewer;
+  JSScript* script;
+  FixedList<StackValue> stack;
+  size_t spIndex;
+
+ public:
+  CompilerFrameInfo(JSScript* script, MacroAssembler& masm)
+      : FrameInfo(masm), script(script), stack(), spIndex(0) {}
+  [[nodiscard]] bool init(TempAllocator& alloc);
+
+  size_t nlocals() const { return script->nfixed(); }
+  size_t nargs() const { return script->function()->nargs(); }
+
+ private:
+  inline StackValue* rawPush() {
+    StackValue* val = &stack[spIndex++];
+    val->reset();
+    return val;
+  }
+
+  inline StackValue* peek(int32_t index) const {
+    MOZ_ASSERT(index < 0);
+    return const_cast<StackValue*>(&stack[spIndex + index]);
+  }
+
+ public:
+  inline size_t stackDepth() const { return spIndex; }
+  inline void setStackDepth(uint32_t newDepth) {
+    if (newDepth <= stackDepth()) {
+      spIndex = newDepth;
+    } else {
+      uint32_t diff = newDepth - stackDepth();
+      for (uint32_t i = 0; i < diff; i++) {
+        StackValue* val = rawPush();
+        val->setStack();
+      }
+
+      MOZ_ASSERT(spIndex == newDepth);
+    }
+  }
+
+  void assertStackDepth(uint32_t depth) { MOZ_ASSERT(stackDepth() == depth); }
+  void incStackDepth(int32_t diff) { setStackDepth(stackDepth() + diff); }
+  bool hasKnownStackDepth(uint32_t depth) { return stackDepth() == depth; }
+
+  inline void pop(StackAdjustment adjust = AdjustStack);
+  inline void popn(uint32_t n, StackAdjustment adjust = AdjustStack);
+  inline void push(const Value& val) {
+    StackValue* sv = rawPush();
+    sv->setConstant(val);
+  }
+  inline void push(const ValueOperand& val,
+                   JSValueType knownType = JSVAL_TYPE_UNKNOWN) {
+    StackValue* sv = rawPush();
+    sv->setRegister(val, knownType);
+  }
+  inline void pushLocal(uint32_t local) {
+    MOZ_ASSERT(local < nlocals());
+    StackValue* sv = rawPush();
+    sv->setLocalSlot(local);
+  }
+  inline void pushArg(uint32_t arg) {
+    StackValue* sv = rawPush();
+    sv->setArgSlot(arg);
+  }
+  inline void pushThis() {
+    StackValue* sv = rawPush();
+    sv->setThis();
+  }
+
+  inline void pushScratchValue() {
+    masm.pushValue(addressOfScratchValue());
+    StackValue* sv = rawPush();
+    sv->setStack();
+  }
+
+  Address addressOfLocal(size_t local) const {
+    MOZ_ASSERT(local < nlocals());
+    return FrameInfo::addressOfLocal(local);
+  }
+  Address addressOfArg(size_t arg) const {
+    MOZ_ASSERT(arg < nargs());
+    return FrameInfo::addressOfArg(arg);
+  }
+
+  Address addressOfStackValue(int32_t depth) const {
+    const StackValue* value = peek(depth);
+    MOZ_ASSERT(value->kind() == StackValue::Stack);
+    size_t slot = value - &stack[0];
+    MOZ_ASSERT(slot < stackDepth());
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfLocal(nlocals() + slot));
+  }
+
+  void popValue(ValueOperand dest);
+
+  void sync(StackValue* val);
+  void syncStack(uint32_t uses);
+  uint32_t numUnsyncedSlots();
+  void popRegsAndSync(uint32_t uses);
+
+  void assertSyncedStack() const {
+    MOZ_ASSERT_IF(stackDepth() > 0, peek(-1)->kind() == StackValue::Stack);
+  }
+
+  bool stackValueHasKnownType(int32_t depth, JSValueType type) const {
+    return peek(depth)->hasKnownType(type);
+  }
+
+  mozilla::Maybe<Value> knownStackValue(int32_t depth) const {
+    StackValue* val = peek(depth);
+    if (val->kind() == StackValue::Constant) {
+      return mozilla::Some(val->constant());
+    }
+    return mozilla::Nothing();
+  }
+
+  void storeStackValue(int32_t depth, const Address& dest,
+                       const ValueOperand& scratch);
+
+  uint32_t frameSize() const {
+    return BaselineFrame::frameSizeForNumValueSlots(nlocals() + stackDepth());
+  }
+
+#ifdef DEBUG
+  // Assert the state is valid before excuting "pc".
+  void assertValidState(const BytecodeInfo& info);
+#else
+  inline void assertValidState(const BytecodeInfo& info) {}
+#endif
+};
+
+class InterpreterFrameInfo : public FrameInfo {
+ public:
+  explicit InterpreterFrameInfo(MacroAssembler& masm) : FrameInfo(masm) {}
+
+  // These methods are no-ops in the interpreter, because we don't have a
+  // virtual stack there.
+  void syncStack(uint32_t uses) {}
+  void assertSyncedStack() const {}
+  void assertStackDepth(uint32_t depth) {}
+  void incStackDepth(int32_t diff) {}
+  bool hasKnownStackDepth(uint32_t depth) { return false; }
+  uint32_t numUnsyncedSlots() { return 0; }
+
+  bool stackValueHasKnownType(int32_t depth, JSValueType type) const {
+    return false;
+  }
+
+  mozilla::Maybe<Value> knownStackValue(int32_t depth) const {
+    return mozilla::Nothing();
+  }
+
+  Address addressOfStackValue(int depth) const {
+    MOZ_ASSERT(depth < 0);
+    return Address(masm.getStackPointer(),
+                   masm.framePushed() + size_t(-(depth + 1)) * sizeof(Value));
+  }
+
+  BaseIndex addressOfStackValue(Register index, int32_t offset = 0) const {
+    return BaseIndex(masm.getStackPointer(), index, ValueScale, offset);
+  }
+
+  void popRegsAndSync(uint32_t uses);
+
+  inline void pop();
+
+  inline void popn(uint32_t n);
+
+  void popn(Register reg) {
+    // sp := sp + reg * sizeof(Value)
+    Register spReg = AsRegister(masm.getStackPointer());
+    masm.computeEffectiveAddress(BaseValueIndex(spReg, reg), spReg);
+    // On arm64, SP may be < PSP now (that's OK).
+    // eg testcase: tests/arguments/strict-args-generator-flushstack.js
+  }
+
+  void popValue(ValueOperand dest) { masm.popValue(dest); }
+
+  void push(const ValueOperand& val,
+            JSValueType knownType = JSVAL_TYPE_UNKNOWN) {
+    masm.pushValue(val);
+  }
+  void push(const Value& val) { masm.pushValue(val); }
+
+  void pushThis() { masm.pushValue(addressOfThis()); }
+  void pushScratchValue() { masm.pushValue(addressOfScratchValue()); }
+
+  void storeStackValue(int32_t depth, const Address& dest,
+                       const ValueOperand& scratch) {
+    masm.loadValue(addressOfStackValue(depth), scratch);
+    masm.storeValue(scratch, dest);
+  }
+
+  void bumpInterpreterICEntry();
+
+  Address addressOfInterpreterScript() const {
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfInterpreterScript());
+  }
+  Address addressOfInterpreterPC() const {
+    return Address(FramePointer, BaselineFrame::reverseOffsetOfInterpreterPC());
+  }
+  Address addressOfInterpreterICEntry() const {
+    return Address(FramePointer,
+                   BaselineFrame::reverseOffsetOfInterpreterICEntry());
+  }
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_BaselineFrameInfo_h */