Re: [PATCH] v11 Improves __ieee754_exp() performance by greater than 5x on sparc/x86.

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 02/02/18 15:33, Joseph Myers wrote:
> On Fri, 2 Feb 2018, Szabolcs Nagy wrote:
> 
>> (i also removed the rounding mode settings in both cases
>> as that can be avoided at least on aarch64)
> 
> There is or was certainly code in the existing exp that gives completely
> wrong results in non-default rounding modes (that was bug 3976) - e.g.
> using precision-extension techniques that require round-to-nearest -
> though I don't know if that applies to the fast case code or if it's only
> in code for slow cases (or code for slow cases but using other rounding
> modes results in the slow cases getting used much more).
> 

that's because in the arg reduction rounding is done like

     y = x * log2e.x + three51.x;
     bexp = y - three51.x;
     ...
     y = t + three33.x;
     base = y - three33.x;

but if instead of x+shift-shift a rounding mode independent
instruction is used then there should be no huge error,
this should be possible at least on aarch64, but even on
other targets a rounding mode independent trunc(x+0.5) may
be faster than using fenv.

i think that exp can be implemented without fenv access
and with acceptable error in non-nearest rounding.