std.math performance (SSE vs. real)

[Iain Buclaw via Digitalmars-d]

On 1 July 2014 06:34, dennis luehring via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> Am 01.07.2014 00:18, schrieb Andrei Alexandrescu:
>
>> On 6/30/14, 2:20 AM, Don wrote:
>>>
>>> For me, a stronger argument is that you can get *higher* precision using
>>> doubles, in many cases. The reason is that FMA gives you an intermediate
>>> value with 128 bits of precision; it's available in SIMD but not on x87.
>>>
>>> So, if we want to use the highest precision supported by the hardware,
>>> that does *not* mean we should always use 80 bits.
>>>
>>> I've experienced this in CTFE, where the calculations are currently done
>>> in 80 bits, I've seen cases where the 64-bit runtime results were more
>>> accurate, because of those 128 bit FMA temporaries. 80 bits are not
>>> enough!!
>>
>>
>> Interesting. Maybe we should follow a simple principle - define
>> overloads and intrinsic operations such that real is only used if (a)
>> requested explicitly (b) it brings about an actual advantage.
>
>
> gcc seems to use GMP for (all) its compiletime calculations - is this for
> cross-compile unification of calculation results or just for better result
> at all - or both?
>

More cross-compilation where the host has less precision than the
target.  But at the same time they wouldn't use GMP if it didn't
produce accurate results. :)