[U-Boot] [RFC PATCH v2] ARM: Avoid compiler optimization for usages of readb, writeb and friends.

[Dirk Behme <dirk.behme@googlemail.com>]

On 30.12.2010 00:10, Alessandro Rubini wrote:
> Dirk Behme:
>> Just for the record:
>>
>> The trick is to ensure that the __arch_putx() containing the volatile
>> is not the last statement in the GCC statement-expression. So, using
>> something like
>>
>> #define writeb(v,c)         ({ __iowmb(); __arch_putb(v,c); v;})
>>
>> (note the additional 'v;') should result in correct code, too.
>
> Yes, that's good. Also "0" may work, and may be more readable, (or not,
> according to who reads it).

Yes, indeed,

#define writeb(v,c)         ({ __iowmb(); __arch_putb(v,c); 0;})

seems to work, too :)

Thanks

Dirk

>> The patches sent by Wolfgang and Alexander using
>>
>> #define writeb(v,c)	do { __iowmb(); __arch_putb(v,c); } while (0)
>>
>> do the same with a slightly different syntax, so these patches are
>> fine, too.
>
> It's not just different syntax, it's different semantics.
>
> The ({...})  trick turns statements into expressions, while the "do
> {...} while(0)" does not.  I'd better not forbid statements like
>
> 	while (reg = readb(addr), reg != VALUE) { .... }
>
> or
>
> 	if (readb(addr) == VALUE) { ... }
>
> or
> 	swtich (readb(addr)) { ... }
>
> While I agree they may in general not be clean, I can forsee
> meaningful uses. Moreover, I'd better allow expression-looking macros
> to really behave like expressions -- otherwise error messages are quite
> hard to understand for the unaquainted.
>
> IMHO, the only reason to use "do {} while(0)" over statemente
> expressions is being portable but in u-boot we are gcc-specific
> anyways.
>
> /alessandro
>