[PATCH RFC] ARM: unwind: optimize to not convert each table value but the address

[Catalin Marinas]

On Sun, Nov 20, 2011 at 11:12:42PM +0000, Uwe Kleine-König wrote:
> The offsets in the unwind index section are signed 31 bit numbers and
> the structs are sorted by this offset. So it first has offsets between
> 0x40000000 and 0x7fffffff (i.e. the negative offsets) and then offsets
> between 0x00000000 and 0x3fffffff. When seperating these two blocks the
> numbers are sorted even when interpreting the offsets as unsigned longs.
> 
> So instead of converting each offset hit during bisection to an absolute
> address, first determine which of the blocks needs to be searched and
> then adapt the key to find for the offset while bisecting using a simple
> unsigned long comparison.
> 
> In my tests this is faster than the original implementation modifying
> the unwind index section by 4.5%.

If you don't care about the 7% unwinding performance drop on XIP
kernels, we could just have a prel31_lt() macro (or whatever other name)
which is a standard '<' comparison on !XIP kernels and does the prel31
conversion with XIP. It's only two places in search_index() where it is
needed and the code would still be more readable than the changes in
this patch :).

If you still want such improvement, I'll do a proper review of the
patch.

Thanks.

-- 
Catalin