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

["de_sousa_luis" <de_sousa_luis at netcabo dot pt>]

Hi, Mike.
Thank you very much for your answer.
Since on my system both the flash and sram devices are
external and the data bus is only 16 bit. I thought that
it was important to make clear that in this situation
thumb mode might be a good way to improve performance.
Regards
 
Luís de Sousa
Email: de_sousa_luis@netcabo.pt
 
 
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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