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

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

On 29/01/18 21:44, Patrick McGehearty wrote:
> New with this version:
> Adds updates sparc and x86_64 libm-test-ulps files (1 ulp for
> various exp tests). Rewrite of full comment to reflect current
> state of patch.
> 
> Summary of patch rationale
> 
> These changes will be active for all platforms that don't provide
> their own exp() routines. They will also be active for ieee754
> versions of ccos, ccosh, cosh, csin, csinh, sinh, exp10, gamma, and
> erf.
> 
> Typical performance gains are 2x on Sparc s7 and 5x on x86_64.
> The former code included a slow path to assure no 1 ulp errors
> that could be 50-200 times slower than the normal path.
> Informal testing suggests perhaps 1 in 200 values might invoke
> the slow path.
> 
> Using the glibc_perf tests:
>        sparc (nsec)    x86 (nsec)
>        old     new     old     new
> max   18180   936    4863     275
> min     399    96      15      15
> mean   5499   419    1336      24
> 

i tested this patch on aarch64 against the current code
with the slow path removed and the later was about 10%
faster on both my throughput and latency benchmarks.
(i also removed the rounding mode settings in both cases
as that can be avoided at least on aarch64)

so i suggest just removing the slow path first, which
should have good enough error rate and similar performance.

i did some testing and i think it's possible to do the
common case >30% faster with similar table size and around
0.501 ulp error, with a slower path for values close to
overflow/underflow (at least on aarch64, which has
convert-to-nearest-int instruction that does not depend on
rounding mode, i'll see if it can be done in a generic way)