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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