When to use __fastcall

The __fastcall keyword should be used when a function is expecting parameters to be passed in registers. It must be used for all TForm methods. (see http://www.codepedia.com/1/CppBuilderFastcall)

This guy talks about it in a Win32 sense. He mentions that I buys little in a x86 world because of the limited register set. However, under the x64 calling convention this is not the case. (see http://www.nynaeve.net/?p=63). This guy, Ken Johnson, really seems to know what he is talking about.