Re: [PATCH v2 2/2] ARM: Replace calls to __aeabi_{u}idiv with udiv/sdiv instructions

Måns Rullgård
2015-11-26T00:50:15Z

Nicolas Pitre <nico@fluxnic.net> writes:

> On Thu, 26 Nov 2015, Måns Rullgård wrote:
>
>> Nicolas Pitre <nico@fluxnic.net> writes:
>>
>> > 3) In fact I was wondering if the overhead of the branch and back is
>> > really significant compared to the non trivial cost of a idiv
>> > instruction and all the complex infrastructure required to patch
>> > those branches directly, and consequently if the performance
>> > difference is actually worth it versus simply doing (2) alone.
>>
>> Depending on the operands, the div instruction can take as few as 3
>> cycles on a Cortex-A7.
>
> Even the current software based implementation can produce a result with
> about 5 simple ALU instructions depending on the operands.
>
> The average cycle count is more important than the easy-way-out case.
> And then how significant the two branches around it are compared to idiv
> alone from direct patching of every call to it.

If not calling the function saves an I-cache miss, the benefit can be
substantial. No, I have no proof of this being a problem, but it's
something that could happen.

Of course, none of this is going to be as good as letting the compiler
generate div instructions directly.

--
Måns Rullgård
mans@mansr.com