Treat __sync_synchronize and asm("":::"memory") as stronger fences.

This is a companion patch to:
  https://codereview.chromium.org/22240002/
  https://codereview.chromium.org/22474008/
and deals with the Clang-side of things.

The above patch will handle the fallouts of this Clang patch, including
some changes to un-duplicate work that RewriteAsmDirectives.cpp
does. The goal of this patch is to force some extra ordering on
non-atomics for le32 which LLVM doesn't necessarily provide.

R=eliben@chromium.org
TEST= ninja check-all
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3475
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3611

Review URL: https://codereview.chromium.org/22294002

1 files changed, 22 insertions, 0 deletions

diff --git a/lib/CodeGen/TargetInfo.h b/lib/CodeGen/TargetInfo.h
index a682c183f0..a122e882e2 100644
--- a/lib/CodeGen/TargetInfo.h
+++ b/lib/CodeGen/TargetInfo.h
@@ -171,6 +171,28 @@ namespace clang {
     /// that unprototyped calls to varargs functions still succeed.
     virtual bool isNoProtoCallVariadic(const CodeGen::CallArgList &args,
                                        const FunctionNoProtoType *fnType) const;
+
+    // @LOCALMOD-START
+    /// Determine whether the sequentially consistent fence generated for
+    /// the legacy GCC-style ``__sync_synchronize()`` builtin should be
+    /// surrounded by empty assembly directives which touch all of
+    /// memory. This allows platforms which aim for portability to
+    /// isolate themselves from changes in sequentially consistent
+    /// fence's semantics, since its intent is to represent the
+    /// C11/C++11 memory model which only orders atomic memory accesses.
+    /// This won't guarantee that all accesses (e.g. those to
+    /// non-escaping objects) will not be reordered.
+    virtual bool addAsmMemoryAroundSyncSynchronize() const {
+      return false;
+    }
+
+    /// Determine whether a full sequentially consistent fence should be
+    /// emitted when ``asm("":::"memory")`` is encountered, treating it
+    /// like ``__sync_synchronize()``.
+    virtual bool asmMemoryIsFence() const {
+      return false;
+    }
+    // @LOCALMOD-END
   };
 }