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From: henno on 7 Mar 2010 14:45
Tom St Denis wrote:
> On Mar 7, 6:40 am, Nomen Nescio <nob...(a)dizum.com> wrote:
>> http://www.das-labor.org/wiki/AVR-Crypto-Lib/en
>>
>> I was stunned to see that the ASM (assembler) implementation of AES is
>> more than ten times (!) as fast as the C implementation. I'd have
>> thought that compiler technology had narrowed the difference to maybe
>> 10-20%. The compiler is probably GCC-AVR, which isn't as advanced as
>> say, the Microsoft compilers, but the difference is remarkable.
>
> Say what? GCC on x86 *is* more advanced than MSVC by a long shot.
> Most lilkely though GCC for AVR isn't 100%. Just like GCC for ARM is
> not as good as ARM's own compiler.
>
> It's not uncommon to see assembler based AES 2-3 times faster than C.
> It shouldn't be 10x though...

It could be if you used the new AES instructions in x64 chipset introduced in
the i5 and i7 proecessors.

Henno

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From: Paulo Marques on 8 Mar 2010 07:45
Nomen Nescio wrote:
> http://www.das-labor.org/wiki/AVR-Crypto-Lib/en
>
> I was stunned to see that the ASM (assembler) implementation of AES is
> more than ten times (!) as fast as the C implementation. I'd have
> thought that compiler technology had narrowed the difference to maybe
> 10-20%. The compiler is probably GCC-AVR, which isn't as advanced as
> say, the Microsoft compilers, but the difference is remarkable.

The AVR is an 8-bit microcontroller that struggles to handle the 32 bit
values that most AES implementations use.

As an educated guess, the assembler implementation is probably able to
use the 32 8-bit registers available to hold the internal state of the
AES and even do some of the movement operations by simple register
"renaming". In an AVR, a memory load or store operation takes 2 cycles.
Avoiding them by using only registers is a major speed up.

--
Paulo Marques - www.grupopie.com

"To be, or not to be? That is ..... liable to be removed at -O2 and above."
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