Messages in this thread | | | Date | Mon, 11 Nov 2013 18:34:08 -0800 | From | Stephen Boyd <> | Subject | Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions |
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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.
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