Stephen Boyd <sb...@codeaurora.org> writes:
> On 11/09/13 21:03, Nicolas Pitre wrote:
>> Bah..... NAK. We are doing runtime patching of the kernel for many
>> many things already. So why not do the same here?
>
> static keys are a form of runtime patching, albeit not as extreme as
> you're suggesting.
>
>>
>> The obvious strategy is to simply overwrite the start of the existing
>> __aeabi_idiv code with the "sdiv r0, r0, r1" and "bx lr" opcodes.
>>
>> Similarly for the unsigned case.
>
> I was thinking the same thing when I wrote this, but I didn't know how
> to tell the compiler to either inline this function or to let me inilne
> an assembly stub with some section magic.
>
>>
>> That let you test the hardware capability only once during boot instead
>> of everytime a divide operation is performed.
>
> The test for hardware capability really isn't done more than once during
> boot. The assembly is like so at compile time
>
> 00000000 <__aeabi_idiv>:
> 0: nop {0}
> 4: b 0 <___aeabi_idiv>
> 8: sdiv r0, r0, r1
> c: bx lr
>
> and after we test and find support for the instruction it will be
> replaced with
>
> 00000000 <__aeabi_idiv>:
> 0: b 8
> 4: b 0 <___aeabi_idiv>
> 8: sdiv r0, r0, r1
> c: bx lr
>
> Unfortunately we still have to jump to this function. It would be great
> if we could inline this function at the call site but as I already said
> I don't know how to do that.
Ideally the bl instruction at the call site would be patched over with
sdiv/udiv when supported. This would leave things exactly as they are
for hardware without div capability and incur only the call setup cost
(but no actual call) on div-capable hardware. No, I don't know how to
achieve this.
--
Måns Rullgård
m...@mansr.com
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