During the compilation? Would you even see the difference if the compiler did 0.1*0.2 by emulating cpu processing, instead of via one instruction? The cost of register allocation for a simple function will be greater than optimising many constant fp expressions. It simply doesn't matter here. If you have a convenient type, you use it, if you don't, you download any libieee754 and interface with its functions...

Seriously - if you're writing a compiler, there are so many more important problems than lack of mapping between the target and local types :( It may not be an elegant solution to handle fp expression evaluation via an external library, but it's only done during the compile time (so only once) and it's easily solvable.