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Don't optimize by hand (Was Re: Lack of fabsf on Solaris (patch included))


Cedric Berger writes:
 > Ok, this is becoming very, very interresting.
 > 
 > I've tried compiling 3 variants of this code, ABS1() 
 > and ABS3() being legal C, ABS2() giving a warning.
 > 
 > All of them compile properly ang give me valid code
 > on my ARM platform (Green Hills compiler, optimized
 > for space, thumb mode). 
 > 
 > But the surprising thing is that ABS1 is the variant
 > that generates the better, optimum code!!! (only 3
 > instruction, 6 byte of code, no data).

It's not all that surprising: the Green Hills compiler has a pattern
for fabs that it's used in this case. 

It would be wrong to generalize too much from this, except perhaps
that the compiler ought to be able to do a better job with unions.

Andrew.

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