From 45b803677012d50d93404bf866e0eac44b0e64c5 Mon Sep 17 00:00:00 2001
From: Simon Giesecke <sgiesecke@mozilla.com>
Date: Mon, 3 Aug 2020 14:47:54 +0000
Subject: [PATCH] Bug 1654991 - Provide and use special case of
 ShrinkCapacityToZero. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D84805
---
 xpcom/ds/nsTArray-inl.h | 25 +++++++++++++++++++++++--
 xpcom/ds/nsTArray.h     |  8 +++++++-
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/xpcom/ds/nsTArray-inl.h b/xpcom/ds/nsTArray-inl.h
index 7103ead37ac4..9b8c093ca77d 100644
--- a/xpcom/ds/nsTArray-inl.h
+++ b/xpcom/ds/nsTArray-inl.h
@@ -267,6 +267,27 @@ void nsTArray_base<Alloc, RelocationStrategy>::ShrinkCapacity(
   mHdr->mCapacity = length;
 }
 
+template <class Alloc, class RelocationStrategy>
+void nsTArray_base<Alloc, RelocationStrategy>::ShrinkCapacityToZero(
+    size_type aElemSize, size_t aElemAlign) {
+  MOZ_ASSERT(mHdr->mLength == 0);
+
+  if (mHdr == EmptyHdr() || UsesAutoArrayBuffer()) {
+    return;
+  }
+
+  const bool isAutoArray = IsAutoArray();
+
+  nsTArrayFallibleAllocator::Free(mHdr);
+
+  if (isAutoArray) {
+    mHdr = GetAutoArrayBufferUnsafe(aElemAlign);
+    mHdr->mLength = 0;
+  } else {
+    mHdr = EmptyHdr();
+  }
+}
+
 template <class Alloc, class RelocationStrategy>
 template <typename ActualAlloc>
 void nsTArray_base<Alloc, RelocationStrategy>::ShiftData(index_type aStart,
@@ -284,7 +305,7 @@ void nsTArray_base<Alloc, RelocationStrategy>::ShiftData(index_type aStart,
   // Compute the resulting length of the array
   mHdr->mLength += aNewLen - aOldLen;
   if (mHdr->mLength == 0) {
-    ShrinkCapacity(aElemSize, aElemAlign);
+    ShrinkCapacityToZero(aElemSize, aElemAlign);
   } else {
     // Maybe nothing needs to be shifted
     if (num == 0) {
@@ -320,7 +341,7 @@ void nsTArray_base<Alloc, RelocationStrategy>::SwapFromEnd(index_type aStart,
 
   if (mHdr->mLength == 0) {
     // If we have no elements remaining in the array, we can free our buffer.
-    ShrinkCapacity(aElemSize, aElemAlign);
+    ShrinkCapacityToZero(aElemSize, aElemAlign);
     return;
   }
 
diff --git a/xpcom/ds/nsTArray.h b/xpcom/ds/nsTArray.h
index 2a640b61a23c..acd1865935d5 100644
--- a/xpcom/ds/nsTArray.h
+++ b/xpcom/ds/nsTArray.h
@@ -457,6 +457,12 @@ class nsTArray_base {
   // @param aElemAlign The alignment in bytes of an array element.
   void ShrinkCapacity(size_type aElemSize, size_t aElemAlign);
 
+  // Resizes the storage to 0. This may only be called when Length() is already
+  // 0.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  void ShrinkCapacityToZero(size_type aElemSize, size_t aElemAlign);
+
   // This method may be called to resize a "gap" in the array by shifting
   // elements around.  It updates mLength appropriately.  If the resulting
   // array has zero elements, then the array's memory is free'd.
@@ -1891,7 +1897,7 @@ class nsTArray_Impl
 
   void Clear() {
     ClearAndRetainStorage();
-    Compact();
+    base_type::ShrinkCapacityToZero(sizeof(elem_type), MOZ_ALIGNOF(elem_type));
   }
 
   // This method removes elements based on the return value of the