RE: [ECOS] Re: Silly question about ARM thumb

["Michael Anburaj" <embeddedeng at hotmail dot com>]

Hi,

It is possible to see some improvement in performance with Thumb mode when 
the core does not have cache (fetching each instruction off the external 
memory) & the bus size is 16 bits. Obviously, for Thumb mode each 
instruction fetch needs a single external memory read, where as ARM mode 
requires 2 reads.

Personally I have not verified such a setup. I have included performance & 
code size numbers measured on LPC (ARM7) board (32 bit internal memory).

http://geocities.com/michaelanburaj/downloads/dhry_lpc.gif
http://geocities.com/michaelanburaj/downloads/code_lpc.txt

Cheers,
-Mike.



> From: "de_sousa_luis" <de_sousa_luis@netcabo.pt>
> To: "Grant Edwards" <grante@visi.com>,<ecos-discuss@sources.redhat.com>
> Subject: RE: [ECOS] Re: Silly question about ARM thumb
> Date: Tue, 22 Feb 2005 16:10:28 -0000
>
> Hi,
> I've been reading the AT91 manual from Atmel, and they claim a
> performance increase when using thumb mode, but only if the code
> is located on external memory, that have a 16 bit data bus.
> For example on the AT91EB55 we have 8KB of internal SRAM (32 bits data bus)
> and on hte default configuration 256KB of SRAM (16 bit data bus).
> If my code is located on the external SRAM (16 bit data bus), and I'm using
> thumb mode should I expect the so claimed speed increase ?
> When switching between ARM and Thumb mode some performance penalty might 
> occur or not ?
> Can anyone coment on this ?
> Thanks in advance.
> Best regards
>
> Luís de Sousa
> -----Mensagem original-----
> De: ecos-discuss-owner@ecos.sourceware.org em nome de Grant Edwards
> Enviada: ter 22-02-2005 15:48
> Para: Øyvind Harboe
> Cc: ecos-discuss@sources.redhat.com
> Assunto: [ECOS] Re: Silly question about ARM thumb
>
>
>
> 	On Tue, Feb 22, 2005 at 02:25:58PM +0100, Øyvind Harboe wrote:
>
> 	> If the code is smaller and faster,
>
> 	It's not.  It's slower.
>
> 	> why aren't all ARM programs compiled w/ARM thumb?
>
> 	> This posting quotes 20% code size decrease and 50% speed increase.
> 	>
> 	> http://sourceware.org/ml/ecos-discuss/2002-11/msg00036.html
>
> 	I don't think the speed increase is something that is seen in
> 	the general case.  Perhaps the 20% size decrease allowed the
> 	entire inner loop to stay resident in cache or something like
> 	that?
>
> 	--
> 	Grant Edwards
> 	grante@visi.com
>
> 	--
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