[SDL] Portable float packing/unpacking.
Ryan C. Gordon
icculus at icculus.org
Mon Feb 13 16:46:25 PST 2006
>>a. compiler option like -fno-strict-aliasing
>>b. use a union
>>c. compiler-specific hack: __attribute__((mayalias))
>>d. use (char*) to access the memory
> Also remember that none of these will help when the variable is
> optimized away into the FPU stack (though volatile may help there,
> I'm not sure). :)
My understanding is that any of the four choices above will squash
aliasing problems and force the variable off the FPU on GCC. Casting to
char was a concession to all the piles of legacy code...of the four, the
GCC developers seem to encourage unions.
In the case of a serialize function, this might just make a copy of the
float onto the stack when shuffling it into the union and leave it on
the FPU too for further processing.
The volatile keyword should, I would think, disable any significant FPU
work on that variable, or force it to store out after each
operation...but I'm really not sure either, now that I think about it. :)
For what it's worth, I've used serialize functions with considerably
less thought put into them across VS.NET 2002/2003/2005, Metrowerks C++,
and GCC on countless CPU architectures and OSes, and not had problems,
so it's probably not worth losing too much sleep over it.
(Until your program is inexplicably crashing, of course!)
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