On Mon, 9 Jul 2018 18:25:09 +0200 Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> To avoid compilers complainig about ambigious else blocks when putting
> an if condition into a for_each macro one needs to invert the
> condition and add a dummy else. We have a nice little convenience
> macro for that in drm headers, let's move it out. Subsequent patches
> will roll it out to other places.
>
> The issue the compilers complain about are nested if with an else
> block and no {} to disambiguate which if the else belongs to. The C
> standard is clear, but in practice people forget:
>
> if (foo)
> if (bar)
> /* something */
> else
> /* something else
um, yeah, don't do that. Kernel coding style is very much to do
if (foo) {
if (bar)
/* something */
else
/* something else
}
And if not doing that generates a warning then, well, do that.
> The same can happen in a for_each macro when it also contains an if
> condition at the end, except the compiler message is now really
> confusing since there's only 1 if:
>
> for_each_something()
> if (bar)
> /* something */
> else
> /* something else
>
> The for_each_if() macro, by inverting the condition and adding an
> else, avoids the compiler warning.
Ditto.
> Motivated by a discussion with Andy and Yisheng, who want to add
> another for_each_macro which would benefit from for_each_if() instead
> of hand-rolling it.
Ditto.
> v2: Explain a bit better what this is good for, after the discussion
> with Peter Z.
Presumably the above was discussed in whatever-thread-that-was.
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