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

[Alessandro Rubini <rubini-list@gnudd.com>]

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).

> 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