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

Alessandro Rubini
2010-12-29T23:10:04Z

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