add note.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95445 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86MCCodeEmitter.cpp b/lib/Target/X86/X86MCCodeEmitter.cpp
index ec0e444..4d82d2b 100644
--- a/lib/Target/X86/X86MCCodeEmitter.cpp
+++ b/lib/Target/X86/X86MCCodeEmitter.cpp
@@ -293,6 +293,8 @@
     if (!MO.isReg()) continue;
     unsigned Reg = MO.getReg();
     if (!X86InstrInfo::isX86_64NonExtLowByteReg(Reg)) continue;
+    // FIXME: The caller of DetermineREXPrefix slaps this prefix onto anything
+    // that returns non-zero.
     REX |= 0x40;
     break;
   }