Messages in this thread | | | From | Måns Rullgård <> | Subject | Re: [PATCH v2 2/2] ARM: Replace calls to __aeabi_{u}idiv with udiv/sdiv instructions | Date | Thu, 26 Nov 2015 00:50:08 +0000 |
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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
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