Merge to XFA: Take adavange of unused bytes at end of CFX strings

core/src/fxcrt/fx_basic_bstring.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "../../include/fxcrt/fx_basic.h"
 #include "../../../third_party/base/numerics/safe_math.h"
 
 static int _Buffer_itoa(char* buf, int i, FX_DWORD flags)
@@ skipping 34 matching lines @@
     char buf[32];
     return CFX_ByteStringC(buf, _Buffer_itoa(buf, i, flags));
 }
 static CFX_StringData* FX_AllocString(int nLen)
 {
     // |nLen| is currently declared as in |int|. TODO(palmer): It should be
     // a |size_t|, or at least unsigned.
     if (nLen == 0 || nLen < 0) {
         return NULL;
     }
+
+    int overhead = sizeof(long) * 3 + 1;  // 3 longs in header plus 1 for NUL.
     pdfium::base::CheckedNumeric<int> nSize = nLen;
-    nSize += sizeof(long) * 3 + 1;
-    CFX_StringData* pData = (CFX_StringData*)FX_Alloc(FX_BYTE, nSize.ValueOrDie());
+    nSize += overhead;
+
+    // Now round to an 8-byte boundary. We'd expect that this is the minimum
+    // granularity of any of the underlying allocators, so there may be cases
+    // where we can save a re-alloc when adding a few characters to a string
+    // by using this otherwise wasted space.
+    nSize += 7;
+    int totalSize = nSize.ValueOrDie() & ~7;
+    int usableSize = totalSize - overhead;
+    FXSYS_assert(usableSize >= nLen);
+
+    CFX_StringData* pData = (CFX_StringData*)FX_Alloc(FX_BYTE, totalSize);
     if (!pData) {
         return NULL;
     }
-    pData->m_nAllocLength = nLen;
+    pData->m_nAllocLength = usableSize;
     pData->m_nDataLength = nLen;
     pData->m_nRefs = 1;
     pData->m_String[nLen] = 0;
     return pData;
 }
 static void FX_ReleaseString(CFX_StringData* pData)
 {
     if (pData == NULL) {
         return;
     }
@@ skipping 1049 matching lines @@
         scale /= 10;
     }
     return buf_size;
 }
 CFX_ByteString CFX_ByteString::FormatFloat(FX_FLOAT d, int precision)
 {
     FX_CHAR buf[32];
     FX_STRSIZE len = FX_ftoa(d, buf);
     return CFX_ByteString(buf, len);
 }