Re: [Qemu-arm] [PATCH 3/3] maint: Fix macros with broken 'do/while(0); ' usage

[Eric Blake]

On 11/30/2017 08:43 AM, Michael S. Tsirkin wrote:
> On Thu, Nov 30, 2017 at 07:41:59AM -0600, Eric Blake wrote:
> > The point of writing a macro embedded in a 'do { ... } while (0)'
> > loop 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;
> > the use of do/while (0) in a macro is absolutely essential for the
> > purpose of avoiding a syntax error on the 'else' - but it only works
> > if there is no trailing ';' in the macro (as the ';' in the code
> > calling the macro would then be a second statement and cause the
> > 'else' to not pair to the 'if').
>
> Shouldn't matter if everyone puts the statements in {}, right?

Correct - where we follow our style, spurious ';' don't make a 
difference (other than they might trigger a warning in a very particular 
compiler).  But it also makes our code harder to copy-and-paste into 
other projects with a different style.

> > Many of the places touched in this code are examples of the ugly
> > bit-rotting debug print statements; cleaning those up is left as
> > a bite-sized task for another day.
> >
> > Found mechanically via: $ git grep -B1 'while (0);' | grep -A1 \\\\
> >
> > Signed-off-by: Eric Blake <address@hidden>
>
> We can't really rely on code still building for this to do the right
> thing, can we?

Some of the uses that were changed are in dead #ifdefs for debugging 
purposes - the only way to still compile is to turn on the debugging, 
but that may fail to compile for other reasons (if the format strings 
have bit-rotted, for example).

The only sure way to know that this did not break anything is to audit 
that for every macro where I eliminated the ';', all callers of that 
macro call 'foo();'.  I did not perform that audit (the patch was 
mechanical) - but am willing to do so and reply back with results if you 
need the extra confidence.

>  I did my best to look for uses and I think it's OK, so
>
> Reviewed-by: Michael S. Tsirkin <address@hidden>
>
> but I'm not merging this.

What tree should it go through instead? Does it need to be split along 
maintainer boundaries?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org