[U-Boot] [RFC PATCH 1/2] armv7: enable Thumb build for armv7

[Albert ARIBAUD]

Le 17/03/2011 08:30, Aneesh V a écrit :
> Albert,
>
> On Wednesday 16 March 2011 10:55 PM, Albert ARIBAUD wrote:
>> Le 16/03/2011 09:39, Aneesh V a écrit :
> [snip ..]
>>
>> Some toolchains in current use (notably the one provided with ELDK 4.2,
>> and possibly others) do not support -march=armv7[-a]. Do we really need
>> armv7 instructions?
>
> 'cc-option' will make sure that build won't break for old compilers. We
> can do something like this:
>
> PLATFORM_CPPFLAGS += $(call cc-option,-march=armv7-a, -march=armv5)
>
> We are not planning to use armv7-a instructions in the code. In fact,
> recently I removed even an armv5 instruction from armv7 generic code in
> the interest of not breaking Tegra2.
>
> However, keeping -march=armv7-a (armv6 and above) helps in having
> Thumb2, which I believe is a good compromise between code density and
> performance.
>
> So, OMAP4 U-Boot when built with ELDK will give you Thumb1 code where
> as when it is built with newer compilers it will give you Thumb2 code.
> As long we don't use non-compliant assembly instructions, both should
> build and work fine.
>
> br,
> Aneesh

Understood. As for the question about config options (assuming I got it 
right) I'd rather have the SoC define which -march it wants, and only 
have a CONFIG for saying if we want thumb or not.

Amicalement,
-- 
Albert.