Add nsStyleContext::Alloc for allocations scoped to the lifetime of the style context. (Bug 363249) r=bzbarsky

layout/style/nsStyleContext.cpp

@@ -70,6 +70,7 @@ nsStyleContext::nsStyleContext(nsStyleContext* aParent,
     mEmptyChild(nsnull),
     mPseudoTag(aPseudoTag),
     mRuleNode(aRuleNode),
+    mAllocations(nsnull),
     mCachedResetData(nsnull),
     mBits(((PRUint32)aPseudoType) << NS_STYLE_CONTEXT_TYPE_SHIFT),
     mRefCnt(0)
@@ -123,6 +124,8 @@ nsStyleContext::~nsStyleContext()
   if (mCachedResetData) {
     mCachedResetData->Destroy(mBits, presContext);
   }
+
+  FreeAllocations(presContext);
 }
 
 void nsStyleContext::AddChild(nsStyleContext* aChild)
@@ -768,3 +771,31 @@ nsStyleContext::CombineVisitedColors(nscolor *aColors, PRBool aLinkIsVisited)
   return NS_RGBA(NS_GET_R(colorColor), NS_GET_G(colorColor),
                  NS_GET_B(colorColor), NS_GET_A(alphaColor));
 }
+
+void*
+nsStyleContext::Alloc(size_t aSize)
+{
+  nsIPresShell *shell = PresContext()->PresShell();
+
+  aSize += offsetof(AllocationHeader, mStorageStart);
+  AllocationHeader *alloc =
+    static_cast<AllocationHeader*>(shell->AllocateMisc(aSize));
+
+  alloc->mSize = aSize; // NOTE: inflated by header
+
+  alloc->mNext = mAllocations;
+  mAllocations = alloc;
+
+  return static_cast<void*>(&alloc->mStorageStart);
+}
+
+void
+nsStyleContext::FreeAllocations(nsPresContext *aPresContext)
+{
+  nsIPresShell *shell = aPresContext->PresShell();
+
+  for (AllocationHeader *alloc = mAllocations, *next; alloc; alloc = next) {
+    next = alloc->mNext;
+    shell->FreeMisc(alloc->mSize, alloc);
+  }
+}

layout/style/nsStyleContext.h

@@ -301,6 +301,27 @@ public:
   static nscolor CombineVisitedColors(nscolor *aColors,
                                       PRBool aLinkIsVisited);
 
+  /**
+   * Allocate a chunk of memory that is scoped to the lifetime of this
+   * style context, i.e., memory that will automatically be freed when
+   * this style context is destroyed.  This is intended for allocations
+   * that are stored on this style context or its style structs.  (Use
+   * on style structs is fine since any style context to which this
+   * context's style structs are shared will be a descendant of this
+   * style context and thus keep it alive.)
+   *
+   * This currently allocates the memory out of the pres shell arena.
+   *
+   * It would be relatively straightforward to write a Free method
+   * for the underlying implementation, but we don't need it (or the
+   * overhead of making a doubly-linked list or other structure to
+   * support it).
+   *
+   * WARNING: Memory allocated using this method cannot be stored in the
+   * rule tree, since rule nodes may outlive the style context.
+   */
+  void* Alloc(size_t aSize);
+
 #ifdef DEBUG
   void List(FILE* out, PRInt32 aIndent);
 #endif
@@ -311,6 +332,8 @@ protected:
 
   void ApplyStyleFixups(nsPresContext* aPresContext);
 
+  void FreeAllocations(nsPresContext* aPresContext);
+
   // Helper function that GetStyleData and GetUniqueStyleData use.  Only
   // returns the structs we cache ourselves; never consults the ruletree.
   inline const void* GetCachedStyleData(nsStyleStructID aSID);
@@ -368,6 +391,15 @@ protected:
   // |mRule| member of |mRuleNode|.
   nsRuleNode* const       mRuleNode;
 
+  // Private to nsStyleContext::Alloc and FreeAllocations.
+  struct AllocationHeader {
+    AllocationHeader* mNext;
+    size_t mSize;
+
+    void* mStorageStart; // ensure the storage is at least pointer-aligned
+  };
+  AllocationHeader*       mAllocations;
+
   // mCachedInheritedData and mCachedResetData point to both structs that
   // are owned by this style context and structs that are owned by one of
   // this style context's ancestors (which are indirectly owned since this