Re: [Qemu-devel] [RFC 10/19] target-alpha: Refactor debug output macros

[Markus Armbruster]

Andreas Färber <address@hidden> writes:

> Am 29.01.2013 11:03, schrieb Markus Armbruster:
>> Andreas Färber <address@hidden> writes:
>> 
>>> b) Working around an issue resulting from hiding C statements inside a
>>> preprocessor macro is totally backwards compared to properly using the C
>>> language in the first place. Its mechanism for reuse are functions, and
>>> for performance considerations static inline functions.
>> 
>> Nobody denies inline functions are to be preferred in many cases.
>> 
>> Valid exceptions include cases where we want genericity, which a macro
>> can easily provide, but an inline function can't.
>> 
>>> Therefore I disagree with you that b) is not an entirely different
>>> disussion as you repeatedly suggest and that according to a) it is not
>>> *generally* necessary to put do { ... } while (0) into any random macro
>>> that one writes. It depends on the contents and on the context.
>> 
>> I don't think Peter suggested to put do ... while (0) into random
>> macros!  He asked to put it exactly into *statement-like* macros, as per
>> standard C practice.  Most macros are expression-like.
>
> Quoting PMM from last night:
> <<<
> "do ... while (0)" is completely standard practice
> for writing robust macros in C. Patches which don't do that
> should fail code review.
>>>>
>
> This is what sparked the most heated part of this debate! I completely
> object to this generalized statement without restriction to
> statement-like macros.

Well, you quite obviously can't use "do ... while (0)" in any but
statement-like macros!

If you use it in an expression-like macro, it's no longer an expression.
Pointlessly breaks uses in non-statement expression context.

If you use it in a declaration-like macro, it's no longer a declaration,
and the declaration part goes into its own scope, which tends to be
counter-productive.

If you use it in a file-level macro, you get syntax errors.

>> I think you're overstating your case.  Statement-like macros should be
>> rare.
>
> This RFC was just a beginning. An average 50%+ of per target-*/ files
> contained macros that were touched here. Even more files are in hw/. Per
> file it was up to ~5 such macro definitions. And my main issue is
> deriving a convention for new files from this discussion.

Stick to established C practice:

1. Avoid statement-like macros when there are superior alternatives,
like inline functions, or expression-like macros.

2. If you have to use a statement-like macro, ensure it expands into a
complete statement regardless of context.  The idiomatic technique for
that is "do ... while (0)".