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Re: [Qemu-ppc] [PATCH v2 6/7] maint: Fix macros with broken 'do/while(0)

From: Juan Quintela
Subject: Re: [Qemu-ppc] [PATCH v2 6/7] maint: Fix macros with broken 'do/while(0); ' usage
Date: Sat, 02 Dec 2017 12:53:28 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

Eric Blake <address@hidden> wrote:
> The point of writing a macro embedded in a 'do { ... } while (0)'
> loop (particularly if the macro has multiple statements or would
> otherwise end with an 'if' statement) is so that the macro can be
> used as a drop-in statement with the caller supplying the
> trailing ';'.  Although our coding style frowns on brace-less 'if':
>   if (cond)
>     statement;
>   else
>     something else;
> that is the classic case where failure to use do/while(0) wrapping
> would cause the 'else' to pair with any embedded 'if' in the macro
> rather than the intended outer 'if'.  But conversely, if the macro
> includes an embedded ';', then the same brace-less coding style
> would now have two statements, making the 'else' a syntax error
> rather than pairing with the outer 'if'.  Thus, even though our
> coding style with required braces is not impacted, ending a macro
> with ';' makes our code harder to port to projects that use
> brace-less styles.
> The change should have no semantic impact.  I was not able to
> fully compile-test all of the changes (as some of them are
> examples of the ugly bit-rotting debug print statements that are
> completely elided by default, and I didn't want to recompile
> with the necessary -D witnesses - cleaning those up is left as a
> bite-sized task for another day); I did, however, audit that for
> all files touched, all callers of the changed macros DID supply
> a trailing ';' at the callsite, and did not appear to be used
> as part of a brace-less conditional.
> Found mechanically via: $ git grep -B1 'while (0);' | grep -A1 \\\\
> Signed-off-by: Eric Blake <address@hidden>
> Acked-by: Cornelia Huck <address@hidden>
> Reviewed-by: Michael S. Tsirkin <address@hidden>
> Acked-by: Dr. David Alan Gilbert <address@hidden>

Reviewed-by: Juan Quintela <address@hidden>

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