[fpc-devel] Patch, font rendering on Arm-Linux devices.

[Christian Iversen]

Daniël Mantione wrote:
> 
> 
> Op Tue, 26 Feb 2008, schreef Luiz Americo Pereira Camara:
> 
>> Yury Sidorov wrote:
>>> The patch removes packed record for some platforms.
>>> IMO packed can be removed for all platforms. It will gain some speed.
>>
>> I'd like to understand more this issue.
>> Why are non packed records faster?
> 
> Cache trashing. One of the most underestimated performance killers in 
> modern software.
> 
>> The difference occurs at memory allocation or at memory access?
> 
> Memory access. What happens is that the non-packed version causes more 
> cache misses. A cache miss costs many cycles on a modern cpu, a 
> misaligned read just costs an extra memory access (which is fast if 
> cached) on x86, and extra load instruction on ARM. This much cheaper 
> than a chache miss.

It's much worse than that. Some architectures simply _can't_ do 
unaligned access, and they will trigger an exception.

This exception will in many configurations be caught by the OS, that 
then might simulate the read by doing 2 reads, putting the result 
together, writing into the application memory, and doing a task switch.

This, in total, is several _orders of magnitude_ worse than unaligned 
access on a supported platform.

Of course, unaligned access in itself is pretty bad.

-- 
Med venlig hilsen
Christian Iversen