Re: Floating point performance issue

[Vincent Lefevre <vincent+gcc@xxxxxxxxxx>]

On 2011-12-20 12:48:53 +0100, Dario Saccavino wrote:
> In the second program, if 0.5 < f < 1 the values of a and b eventually
> become the smallest representable denormal value and never change
> afterwards, resulting in a large number of operations involving
> denormal numbers.

Yes, I agree (I forgot about that)... except that if f is close enough
to 1, you won't have subnormals and the program will be fast (like in
the case f <= 0.5).

> gcc enables FTZ when using SSE and ffast-math (I think the specific
> compiler flag is -funsafe-math-optimizations).

Thanks, good to know...

> Therefore the flags needed are -msse2 -mfpmath=sse -ffast-math

I would discourage the use of -ffast-math, which can affect generic
code very badly (due to -funsafe-math-optimizations). Isn't there
an option to enable FTZ?

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