It seems pure ain't so pure after all

[Marco Leise]

Am Tue, 02 Oct 2012 09:38:56 +0200
schrieb Don Clugston <dac at nospam.com>:

> Any code that behaves differently when compiled with -O, will do this as 
> well. Constant folding of floating point numbers does the same thing, if 
> the numbers are represented in the compiler in a different precision to 
> how the machine calculates them. I believe that GCC, for example, uses 
> very much higher precision (hundreds of bits) at compile time.

I'm not an expert, but I would have thought compilers
strive to be IEEE compliant - whatever that means in detail.
I've seen a compression algorithm that relies on exact
floating-point semantics and accuracy. It would just fail,
if compilers were creative or lax at certain optimization
levels. (excluding the "I know what I am doing -ffast-math.)

-- 
Marco