Philippe Gerum wrote:
> Gilles Chanteperdrix wrote:
> > Philippe Gerum wrote:
> > > static inline unsigned long long ns_2_cycles(unsigned long long ns)
> > > {
> > > return ns * ns2cyc_scale >> NS2CYC_SCALE_FACTOR;
> >
> > This multiplication is 64 bits * 32 bits, the intermediate result may
> > need more than 64 bits, so you should compute it the same way as the
> > beginning of ullimd. Something like:
>
> Sure, but the point is that if we were to use such code, we should bound
> the 64bit operand and would not use it beyond the tolerable loss of
> accuracy on output (e.g. 2ms). This would require to break longer shots
> in several smaller ones, relying on the internal timer management logic
> to redo the shot until it has actually elapsed (which should be a rare
> case for oneshot timing), a bit like we are currently doing in bounding
> the values to 2^32-1 right now. Going for ullimd alike implementation
> somehow impedes the overall effort in reducing the CPU footprint, I
> guess. This said, I have still no clue if the gain in computation cycles
> is worth the additional overhead of dealing with possibly early shots -
> I tend to think it would be better on average though.
Ok, we could then write:
static inline unsigned long long ns_2_cycles(unsigned ns)
{
return (unsigned long long) ns * ns2cyc_scale >> NS2CYC_SCALE_FACTOR;
}
--
Gilles Chanteperdrix.
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