Re: RISC-V: Add divmod instruction support

[Jeff Law <jeffreyalaw@gmail.com>]

On 2/18/23 12:31, Maciej W. Rozycki wrote:
> On Sat, 18 Feb 2023, Andrew Pinski via Gcc-patches wrote:
> 
>>>> If we have division and remainder calculations with the same operands:
>>>>
>>>>    a = b / c;
>>>>    d = b % c;
>>>>
>>>> We can replace the calculation of remainder with multiplication +
>>>> subtraction, using the result from the previous division:
>>>>
>>>>    a = b / c;
>>>>    d = a * c;
>>>>    d = b - d;
>>>>
>>>> Which will be faster.
>>>
>>> Do you have any benchmarks that show that performance increase?  The ISA
>>> manual specifically says the suggested sequence is div+mod, and while
>>> those suggestions don't always pan out for real hardware it's likely
>>> that at least some implementations will end up with the ISA-suggested
>>> fusions.
>>
>> I suspect I will be needing this kind of patch for the core that I am
>> going to be using.
>> If anything this should be under a tuning option.
> 
>   Barring the fusion case, which indeed asks for a dedicated `divmod'
> pattern (and then I suppose a post-reload splitter or a peephole so that
> where one of the two results produced has eventually turned out unused, we
> have means to discard the unneeded machine instruction), isn't the generic
> transformation something for the middle end to do based on RTX costs?
I originally though the same way you did Maciej.

The problem is you don't see it as a divmod in expand_divmod unless you 
expose a divmod optab.  See tree-ssa-mathopts.cc's divmod handling.

jeff