Twiddling with 64-bit values as 2 ints;

[Adhemerval Zanella]

On 23/08/2021 14:32, Stefan Kanthak wrote:
> Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> 
>> On 23/08/2021 12:37, Stefan Kanthak wrote:
>>> Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
>>>
>>>> On 23/08/2021 10:18, Stefan Kanthak wrote:
>>>>> Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> 
>>>>> The simple implementation I showed in my initial post improved the
>>>>> throughput in my benchmark (on AMD64) by an order of magnitude.
>>>>> In Szabolcs Nagy benchmark measuring latency it took 0.04ns/call
>>>>> longer (5.72ns vs. 5.68ns) -- despite the POOR job GCC does on FP.
>>>>
>>>> Your implementation triggered a lot of regression,
>>>
>>> The initial, FP-preferring code was a demonstration, not a patch.
>>
>> Right, but it does do not much sense comparing performance numbers with
>> an implementation that adds a lot of regressions. 
> 
> This argument also holds for a correct FP-preferring implementation due to
> the POOR code GCC currently generates: the 4 superfluous FP-comparisions
> plus conditional branches GCC generates have worse runtime than the missing
> code to handle fenv/underflow/overflow/errno.

In any case, please come up with number only *after* you fix any regression
on testcases.

> 
> [...]
> 
>>> Having dedicated implementations for different architectures is even more
>>> costly!
>>> My intention/proposal is to have at most two different generic implementations,
>>> one using integer bit-twiddling wherever possible, thus supporting soft-fp well,
>>> the second using floating-point wherever possible, thus supporting modern
>>> hardware well.
>>
>> The only reservation I have for such approach it it would add some more maintenance
>> and testing.
> 
> Insert "wherever needed" before/after "wherever possible".
> 
> Stefan
>