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diff --git a/js/src/wasm/WasmCompileArgs.h b/js/src/wasm/WasmCompileArgs.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:
+ *
+ * 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
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_compile_args_h
+#define wasm_compile_args_h
+
+#include "mozilla/RefPtr.h"
+#include "mozilla/TypedEnumBits.h"
+
+#include "js/Utility.h"
+#include "js/WasmFeatures.h"
+#include "wasm/WasmConstants.h"
+#include "wasm/WasmShareable.h"
+
+namespace js {
+namespace wasm {
+
+enum class Shareable { False, True };
+
+// Code can be compiled either with the Baseline compiler or the Ion compiler,
+// and tier-variant data are tagged with the Tier value.
+//
+// A tier value is used to request tier-variant aspects of code, metadata, or
+// linkdata.  The tiers are normally explicit (Baseline and Ion); implicit tiers
+// can be obtained through accessors on Code objects (eg, stableTier).
+
+enum class Tier {
+  Baseline,
+  Debug = Baseline,
+  Optimized,
+  Serialized = Optimized
+};
+
+// Iterator over tiers present in a tiered data structure.
+
+class Tiers {
+  Tier t_[2];
+  uint32_t n_;
+
+ public:
+  explicit Tiers() { n_ = 0; }
+  explicit Tiers(Tier t) {
+    t_[0] = t;
+    n_ = 1;
+  }
+  explicit Tiers(Tier t, Tier u) {
+    MOZ_ASSERT(t != u);
+    t_[0] = t;
+    t_[1] = u;
+    n_ = 2;
+  }
+
+  Tier* begin() { return t_; }
+  Tier* end() { return t_ + n_; }
+};
+
+// Describes per-compilation settings that are controlled by an options bag
+// passed to compilation and validation functions. (Nonstandard extension
+// available under prefs.)
+
+struct FeatureOptions {
+  FeatureOptions() : isBuiltinModule(false), jsStringBuiltins(false) {}
+
+  // Enables builtin module opcodes, only set in WasmBuiltinModule.cpp.
+  bool isBuiltinModule;
+  // Enable JS String builtins for this module, only available if the feature
+  // is also enabled.
+  bool jsStringBuiltins;
+
+  // Parse the compile options bag.
+  [[nodiscard]] bool init(JSContext* cx, HandleValue val);
+};
+
+// Describes the features that control wasm compilation.
+
+struct FeatureArgs {
+  FeatureArgs()
+      :
+#define WASM_FEATURE(NAME, LOWER_NAME, ...) LOWER_NAME(false),
+        JS_FOR_WASM_FEATURES(WASM_FEATURE)
+#undef WASM_FEATURE
+            sharedMemory(Shareable::False),
+        simd(false),
+        isBuiltinModule(false) {
+  }
+  FeatureArgs(const FeatureArgs&) = default;
+  FeatureArgs& operator=(const FeatureArgs&) = default;
+  FeatureArgs(FeatureArgs&&) = default;
+
+  static FeatureArgs build(JSContext* cx, const FeatureOptions& options);
+
+#define WASM_FEATURE(NAME, LOWER_NAME, ...) bool LOWER_NAME;
+  JS_FOR_WASM_FEATURES(WASM_FEATURE)
+#undef WASM_FEATURE
+
+  Shareable sharedMemory;
+  bool simd;
+  // Whether this module is a wasm builtin module (see WasmBuiltinModule.h) and
+  // can contain special opcodes in function bodies.
+  bool isBuiltinModule;
+  // The set of builtin modules that are imported by this module.
+  BuiltinModuleIds builtinModules;
+};
+
+// Observed feature usage for a compiled module. Intended to be used for use
+// counters.
+enum class FeatureUsage : uint8_t {
+  None = 0x0,
+  LegacyExceptions = 0x1,
+};
+
+void SetUseCountersForFeatureUsage(JSContext* cx, JSObject* object,
+                                   FeatureUsage usage);
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(FeatureUsage);
+
+// Describes the JS scripted caller of a request to compile a wasm module.
+
+struct ScriptedCaller {
+  UniqueChars filename;  // UTF-8 encoded
+  bool filenameIsURL;
+  uint32_t line;
+
+  ScriptedCaller() : filenameIsURL(false), line(0) {}
+};
+
+// Describes the reasons we cannot compute compile args
+
+enum class CompileArgsError {
+  OutOfMemory,
+  NoCompiler,
+};
+
+// Describes all the parameters that control wasm compilation.
+
+struct CompileArgs;
+using MutableCompileArgs = RefPtr<CompileArgs>;
+using SharedCompileArgs = RefPtr<const CompileArgs>;
+
+struct CompileArgs : ShareableBase<CompileArgs> {
+  ScriptedCaller scriptedCaller;
+  UniqueChars sourceMapURL;
+
+  bool baselineEnabled;
+  bool ionEnabled;
+  bool debugEnabled;
+  bool forceTiering;
+
+  FeatureArgs features;
+
+  // CompileArgs has several constructors:
+  //
+  // - two through factory functions `build`/`buildAndReport`, which checks
+  //   that flags are consistent with each other, and optionally reports any
+  //   errors.
+  // - the 'buildForAsmJS' one, which uses the appropriate configuration for
+  //   legacy asm.js code.
+  // - one that gives complete access to underlying fields.
+  //
+  // You should use the factory functions in general, unless you have a very
+  // good reason (i.e. no JSContext around and you know which flags have been
+  // used).
+
+  static SharedCompileArgs build(JSContext* cx, ScriptedCaller&& scriptedCaller,
+                                 const FeatureOptions& options,
+                                 CompileArgsError* error);
+  static SharedCompileArgs buildForAsmJS(ScriptedCaller&& scriptedCaller);
+  static SharedCompileArgs buildAndReport(JSContext* cx,
+                                          ScriptedCaller&& scriptedCaller,
+                                          const FeatureOptions& options,
+                                          bool reportOOM = false);
+
+  explicit CompileArgs(ScriptedCaller&& scriptedCaller)
+      : scriptedCaller(std::move(scriptedCaller)),
+        baselineEnabled(false),
+        ionEnabled(false),
+        debugEnabled(false),
+        forceTiering(false) {}
+};
+
+// CompilerEnvironment holds any values that will be needed to compute
+// compilation parameters once the module's feature opt-in sections have been
+// parsed.
+//
+// Subsequent to construction a computeParameters() call will compute the final
+// compilation parameters, and the object can then be queried for their values.
+
+struct CompileArgs;
+class Decoder;
+
+struct CompilerEnvironment {
+  // The object starts in one of two "initial" states; computeParameters moves
+  // it into the "computed" state.
+  enum State { InitialWithArgs, InitialWithModeTierDebug, Computed };
+
+  State state_;
+  union {
+    // Value if the state_ == InitialWithArgs.
+    const CompileArgs* args_;
+
+    // Value in the other two states.
+    struct {
+      CompileMode mode_;
+      Tier tier_;
+      DebugEnabled debug_;
+    };
+  };
+
+ public:
+  // Retain a reference to the CompileArgs. A subsequent computeParameters()
+  // will compute all parameters from the CompileArgs and additional values.
+  explicit CompilerEnvironment(const CompileArgs& args);
+
+  // Save the provided values for mode, tier, and debug, and the initial value
+  // for gc/refTypes. A subsequent computeParameters() will compute the
+  // final value of gc/refTypes.
+  CompilerEnvironment(CompileMode mode, Tier tier, DebugEnabled debugEnabled);
+
+  // Compute any remaining compilation parameters.
+  void computeParameters(Decoder& d);
+
+  // Compute any remaining compilation parameters.  Only use this method if
+  // the CompilerEnvironment was created with values for mode, tier, and
+  // debug.
+  void computeParameters();
+
+  bool isComputed() const { return state_ == Computed; }
+  CompileMode mode() const {
+    MOZ_ASSERT(isComputed());
+    return mode_;
+  }
+  Tier tier() const {
+    MOZ_ASSERT(isComputed());
+    return tier_;
+  }
+  DebugEnabled debug() const {
+    MOZ_ASSERT(isComputed());
+    return debug_;
+  }
+  bool debugEnabled() const { return debug() == DebugEnabled::True; }
+};
+
+}  // namespace wasm
+}  // namespace js
+
+#endif  // wasm_compile_args_h