Gcc46 and 128 Bit Floating Point

[Bruce Evans]

On Wed, 29 Feb 2012, Thomas D. Dean wrote:

> On 02/28/12 22:03, Bruce Evans wrote:
>> 
>> But why would you want it? It is essentially unusable on sparc64,
>> since it is several thousand times slower than 80-bit floating point
>> on i386. At equal CPU clock speeds, it is only about 1000 times slower.
>> ...
>
> I have an application that takes 10 days to run on a 4.16GHz Core-i7 3930K. 
> No output until it finishes.

Look elsewhere :-).  1000 times slower than that would be bad :-).

> When I first started looking at this, I naively thought the 80-bit FPU floats 
> were scaled to 128-bits.  Would be nice...
>
> The application uses libgmp, but, about 1/2 to 2/3 of the work will fit in a 
> 128-bit float.

libgmp is probably slow for the same reasons that multi-word software FP
is slow -- it has to do similar things to combine the words.  But it is
much older so it it likely to be smarter about combining the words, and
might already know how to use vectors on AVX.  Combining the words seems
to be much harder with floating point words, so you probably don't want
floating point if you only need large integers.

Bruce