Add a macro for prefetching data

[• rafael]

Summary

I want to use this in a binary search. It can make lld up to 1.029X faster.

[dblaikie]

Seems OK - hopefully others will chime in (post commit, etc) if there's a good reason not to do this/some other way we should be encouraging the cache behavior we want.

include/llvm/Support/Compiler.h

 /// On PE/COFF targets, library visibility is the default, so this isn't needed.
 #if (__has_attribute(visibility) || LLVM_GNUC_PREREQ(4, 0, 0)) &&              \
     !defined(__MINGW32__) && !defined(__CYGWIN__) && !defined(LLVM_ON_WIN32)
 #define LLVM_LIBRARY_VISIBILITY __attribute__ ((visibility("hidden")))
 #else
 #define LLVM_LIBRARY_VISIBILITY
 #endif
 
+#if defined(__GNUC__)
+#define LLVM_PREFETCH(addr, rw, locality) __builtin_prefetch(addr, rw, locality)
+#else
+#define LLVM_PREFETCH(addr, rw, locality)
+#endif
+
 #if __has_attribute(sentinel) || LLVM_GNUC_PREREQ(3, 0, 0)
 #define LLVM_END_WITH_NULL __attribute__((sentinel))
 #else
 #define LLVM_END_WITH_NULL
 #endif
 
 #if __has_attribute(used) || LLVM_GNUC_PREREQ(3, 1, 0)
 #define LLVM_ATTRIBUTE_USED __attribute__((__used__))