[Allocator] Make the ContentCache object actually carry the 8-byte

alignment constraint rather than using the allocator function's over
alignment "feature". This was the only use of the "feature" which I plan
to remove next. =] Attaching the alignment to the type seems cleaner and
more principled anyways.

llvm-svn: 206324

clang/include/clang/Basic/SourceManager.h

@@ -44,6 +44,7 @@
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/PointerUnion.h"
+#include "llvm/Support/AlignOf.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/MemoryBuffer.h"
@@ -89,6 +90,15 @@ namespace SrcMgr {
       DoNotFreeFlag = 0x02
     };
 
+    // Note that the first member of this class is an aligned character buffer
+    // to ensure that this class has an alignment of 8 bytes. This wastes
+    // 8 bytes for every ContentCache object, but each of these corresponds to
+    // a file loaded into memory, so the 8 bytes doesn't seem terribly
+    // important. It is quite awkward to fit this aligner into any other part
+    // of the class due to the lack of portable ways to combine it with other
+    // members.
+    llvm::AlignedCharArray<8, 1> NonceAligner LLVM_ATTRIBUTE_UNUSED;
+
     /// \brief The actual buffer containing the characters from the input
     /// file.
     ///
@@ -224,6 +234,11 @@ namespace SrcMgr {
     ContentCache &operator=(const ContentCache& RHS) LLVM_DELETED_FUNCTION;
   };
 
+  // Assert that the \c ContentCache objects will always be 8-byte aligned so
+  // that we can pack 3 bits of integer into pointers to such objects.
+  static_assert(llvm::AlignOf<ContentCache>::Alignment >= 8,
+                "ContentCache must be 8-byte aligned.");
+
   /// \brief Information about a FileID, basically just the logical file
   /// that it represents and include stack information.
   ///

clang/lib/Basic/SourceManager.cpp

@@ -436,12 +436,8 @@ SourceManager::getOrCreateContentCache(const FileEntry *FileEnt,
   ContentCache *&Entry = FileInfos[FileEnt];
   if (Entry) return Entry;
 
-  // Nope, create a new Cache entry.  Make sure it is at least 8-byte aligned
-  // so that FileInfo can use the low 3 bits of the pointer for its own
-  // nefarious purposes.
-  unsigned EntryAlign = llvm::AlignOf<ContentCache>::Alignment;
-  EntryAlign = std::max(8U, EntryAlign);
-  Entry = ContentCacheAlloc.Allocate<ContentCache>(1, EntryAlign);
+  // Nope, create a new Cache entry.
+  Entry = ContentCacheAlloc.Allocate<ContentCache>();
 
   if (OverriddenFilesInfo) {
     // If the file contents are overridden with contents from another file,
@@ -468,12 +464,8 @@ SourceManager::getOrCreateContentCache(const FileEntry *FileEnt,
 ///  memory buffer.  This does no caching.
 const ContentCache*
 SourceManager::createMemBufferContentCache(const MemoryBuffer *Buffer) {
-  // Add a new ContentCache to the MemBufferInfos list and return it.  Make sure
-  // it is at least 8-byte aligned so that FileInfo can use the low 3 bits of
-  // the pointer for its own nefarious purposes.
-  unsigned EntryAlign = llvm::AlignOf<ContentCache>::Alignment;
-  EntryAlign = std::max(8U, EntryAlign);
-  ContentCache *Entry = ContentCacheAlloc.Allocate<ContentCache>(1, EntryAlign);
+  // Add a new ContentCache to the MemBufferInfos list and return it.
+  ContentCache *Entry = ContentCacheAlloc.Allocate<ContentCache>();
   new (Entry) ContentCache();
   MemBufferInfos.push_back(Entry);
   Entry->setBuffer(Buffer);