Re: RFC: ARM Cortex-A8 and floating point performance

On 16/06/2010 17:09, Andrew Pinski wrote:
>

>> On i?86 we have -mfpmath={sse,x87}, I suppose you could add
>> -mfpmath=neon for arm (properly conflicting with -mfloat-abi=hard
>> and requiring neon support).
>
> Except unlike sse, neon does not fully support IEEE support. So this
> should only be done with -ffast-math :). The point that it is slow is
> not good enough to change it to be something that is wrong and fast.
>

How relevant is full IEEE support?  I would imagine that only a tiny
minority of programs actually need it.  But I would also imagine that
the great majority of programs are compiled without -ffast-math, even
though that would be perfectly good for them, and generate smaller and
faster code on almost all platforms.

So if -ffast-math were the default, would many people actually see wrong
code?  Obviously such a change could not be done arbitrarily - as you
say, slow code is better than wrong code.  It would have to be done
through stages for successive gcc releases of adding a "-fieee-math"
flag (synonymous with -fno-fast-math), and giving warning messages if
neither "-fieee-math" or "-ffast-math" were specified, before it could
be changed.  And it would only apply to the -std=gnu... standards.  But
the end result is that the majority of users would generate faster code.