1 files changed, 90 insertions, 30 deletions

diff --git a/js/src/vm/Scope.h b/js/src/vm/Scope.h
index 5304d6713..1d04fd9f6 100644
--- a/js/src/vm/Scope.h
+++ b/js/src/vm/Scope.h
@@ -12,6 +12,7 @@
 
 #include "jsobj.h"
 #include "jsopcode.h"
+#include "jsutil.h"
 
 #include "gc/Heap.h"
 #include "gc/Policy.h"
@@ -111,6 +112,47 @@ class BindingName
     void trace(JSTracer* trc);
 };
 
+/**
+ * The various {Global,Module,...}Scope::Data classes consist of always-present
+ * bits, then a trailing array of BindingNames.  The various Data classes all
+ * end in a TrailingNamesArray that contains sized/aligned space for *one*
+ * BindingName.  Data instances that contain N BindingNames, are then allocated
+ * in sizeof(Data) + (space for (N - 1) BindingNames).  Because this class's
+ * |data_| field is properly sized/aligned, the N-BindingName array can start
+ * at |data_|.
+ *
+ * This is concededly a very low-level representation, but we want to only
+ * allocate once for data+bindings both, and this does so approximately as
+ * elegantly as C++ allows.
+ */
+class TrailingNamesArray
+{
+  private:
+    alignas(BindingName) unsigned char data_[sizeof(BindingName)];
+
+  private:
+    // Some versions of GCC treat it as a -Wstrict-aliasing violation (ergo a
+    // -Werror compile error) to reinterpret_cast<> |data_| to |T*|, even
+    // through |void*|.  Placing the latter cast in these separate functions
+    // breaks the chain such that affected GCC versions no longer warn/error.
+    void* ptr() {
+        return data_;
+    }
+
+  public:
+    // Explicitly ensure no one accidentally allocates scope data without
+    // poisoning its trailing names.
+    TrailingNamesArray() = delete;
+
+    explicit TrailingNamesArray(size_t nameCount) {
+        if (nameCount)
+            JS_POISON(&data_, 0xCC, sizeof(BindingName) * nameCount);
+    }
+    BindingName* start() { return reinterpret_cast<BindingName*>(ptr()); }
+
+    BindingName& operator[](size_t i) { return start()[i]; }
+};
+
 class BindingLocation
 {
   public:
@@ -337,16 +379,19 @@ class LexicalScope : public Scope
         //
         //   lets - [0, constStart)
         // consts - [constStart, length)
-        uint32_t constStart;
-        uint32_t length;
+        uint32_t constStart = 0;
+        uint32_t length = 0;
 
         // Frame slots [0, nextFrameSlot) are live when this is the innermost
         // scope.
-        uint32_t nextFrameSlot;
+        uint32_t nextFrameSlot = 0;
 
         // The array of tagged JSAtom* names, allocated beyond the end of the
         // struct.
-        BindingName names[1];
+        TrailingNamesArray trailingNames;
+
+        explicit Data(size_t nameCount) : trailingNames(nameCount) {}
+        Data() = delete;
 
         void trace(JSTracer* trc);
     };
@@ -433,11 +478,11 @@ class FunctionScope : public Scope
         // The canonical function of the scope, as during a scope walk we
         // often query properties of the JSFunction (e.g., is the function an
         // arrow).
-        GCPtrFunction canonicalFunction;
+        GCPtrFunction canonicalFunction = {};
 
         // If parameter expressions are present, parameters act like lexical
         // bindings.
-        bool hasParameterExprs;
+        bool hasParameterExprs = false;
 
         // Bindings are sorted by kind in both frames and environments.
         //
@@ -452,17 +497,20 @@ class FunctionScope : public Scope
         // positional formals - [0, nonPositionalFormalStart)
         //      other formals - [nonPositionalParamStart, varStart)
         //               vars - [varStart, length)
-        uint16_t nonPositionalFormalStart;
-        uint16_t varStart;
-        uint32_t length;
+        uint16_t nonPositionalFormalStart = 0;
+        uint16_t varStart = 0;
+        uint32_t length = 0;
 
         // Frame slots [0, nextFrameSlot) are live when this is the innermost
         // scope.
-        uint32_t nextFrameSlot;
+        uint32_t nextFrameSlot = 0;
 
         // The array of tagged JSAtom* names, allocated beyond the end of the
         // struct.
-        BindingName names[1];
+        TrailingNamesArray trailingNames;
+
+        explicit Data(size_t nameCount) : trailingNames(nameCount) {}
+        Data() = delete;
 
         void trace(JSTracer* trc);
     };
@@ -548,15 +596,18 @@ class VarScope : public Scope
     struct Data
     {
         // All bindings are vars.
-        uint32_t length;
+        uint32_t length = 0;
 
         // Frame slots [firstFrameSlot(), nextFrameSlot) are live when this is
         // the innermost scope.
-        uint32_t nextFrameSlot;
+        uint32_t nextFrameSlot = 0;
 
         // The array of tagged JSAtom* names, allocated beyond the end of the
         // struct.
-        BindingName names[1];
+        TrailingNamesArray trailingNames;
+
+        explicit Data(size_t nameCount) : trailingNames(nameCount) {}
+        Data() = delete;
 
         void trace(JSTracer* trc);
     };
@@ -638,14 +689,17 @@ class GlobalScope : public Scope
         //            vars - [varStart, letStart)
         //            lets - [letStart, constStart)
         //          consts - [constStart, length)
-        uint32_t varStart;
-        uint32_t letStart;
-        uint32_t constStart;
-        uint32_t length;
+        uint32_t varStart = 0;
+        uint32_t letStart = 0;
+        uint32_t constStart = 0;
+        uint32_t length = 0;
 
         // The array of tagged JSAtom* names, allocated beyond the end of the
         // struct.
-        BindingName names[1];
+        TrailingNamesArray trailingNames;
+
+        explicit Data(size_t nameCount) : trailingNames(nameCount) {}
+        Data() = delete;
 
         void trace(JSTracer* trc);
     };
@@ -736,16 +790,19 @@ class EvalScope : public Scope
         //
         // top-level funcs - [0, varStart)
         //            vars - [varStart, length)
-        uint32_t varStart;
-        uint32_t length;
+        uint32_t varStart = 0;
+        uint32_t length = 0;
 
         // Frame slots [0, nextFrameSlot) are live when this is the innermost
         // scope.
-        uint32_t nextFrameSlot;
+        uint32_t nextFrameSlot = 0;
 
         // The array of tagged JSAtom* names, allocated beyond the end of the
         // struct.
-        BindingName names[1];
+        TrailingNamesArray trailingNames;
+
+        explicit Data(size_t nameCount) : trailingNames(nameCount) {}
+        Data() = delete;
 
         void trace(JSTracer* trc);
     };
@@ -827,7 +884,7 @@ class ModuleScope : public Scope
     struct Data
     {
         // The module of the scope.
-        GCPtr<ModuleObject*> module;
+        GCPtr<ModuleObject*> module = {};
 
         // Bindings are sorted by kind.
         //
@@ -835,18 +892,21 @@ class ModuleScope : public Scope
         //    vars - [varStart, letStart)
         //    lets - [letStart, constStart)
         //  consts - [constStart, length)
-        uint32_t varStart;
-        uint32_t letStart;
-        uint32_t constStart;
-        uint32_t length;
+        uint32_t varStart = 0;
+        uint32_t letStart = 0;
+        uint32_t constStart = 0;
+        uint32_t length = 0;
 
         // Frame slots [0, nextFrameSlot) are live when this is the innermost
         // scope.
-        uint32_t nextFrameSlot;
+        uint32_t nextFrameSlot = 0;
 
         // The array of tagged JSAtom* names, allocated beyond the end of the
         // struct.
-        BindingName names[1];
+        TrailingNamesArray trailingNames;
+
+        explicit Data(size_t nameCount) : trailingNames(nameCount) {}
+        Data() = delete;
 
         void trace(JSTracer* trc);
     };