Add BUILTIN_EXPECT Support/Compiler macro.

- Use for exceptional buffer conditions in raw_ostream:write to shave
   off a cycle or two.

 - Please rename if you have a better one.

llvm-svn: 67103

include/llvm/Support/Compiler.h

@@ -29,6 +29,12 @@
 #define ATTRIBUTE_USED
 #endif
 
+#if (__GNUC__ >= 4)
+#define BUILTIN_EXPECT(EXPR, VALUE) __builtin_expect((EXPR), (VALUE))
+#else
+#define BUILTIN_EXPECT(EXPR, VALUE) (EXPR)
+#endif
+
 // C++ doesn't support 'extern template' of template specializations.  GCC does,
 // but requires __extension__ before it.  In the header, use this:
 //   EXTERN_TEMPLATE_INSTANTIATION(class foo<bar>);

lib/Support/raw_ostream.cpp

@@ -16,6 +16,7 @@
 #include "llvm/System/Program.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Config/config.h"
+#include "llvm/Support/Compiler.h"
 #include <ostream>
 
 #if defined(HAVE_UNISTD_H)
@@ -142,7 +143,7 @@ raw_ostream &raw_ostream::write(unsigned char C) {
 
 raw_ostream &raw_ostream::write(const char *Ptr, unsigned Size) {
   // Group exceptional cases into a single branch.
-  if (OutBufCur+Size > OutBufEnd) {
+  if (BUILTIN_EXPECT(OutBufCur+Size > OutBufEnd, false)) {
     if (Unbuffered) {
       write_impl(Ptr, Size);
       return *this;