Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> writes: > On Wed, Oct 23, 2024 at 02:07:52PM -0700, Dan Williams wrote: >> Andy Shevchenko wrote: >> > On Fri, Oct 11, 2024 at 09:06:37AM +0800, Huang, Ying wrote: >> > > David Hildenbrand <david@xxxxxxxxxx> writes: >> > > > On 10.10.24 08:55, Huang Ying wrote: > > ... > >> > > > for ((_p) = (_root)->child; (_p); (_p) = next_resource_XXX(_root, _p)) >> > > >> > > Yes. This can improve code readability. >> > > >> > > A possible issue is that "_root" will be evaluated twice in above macro >> > > definition. IMO, this should be avoided. >> > >> > Ideally, yes. But how many for_each type of macros you see that really try hard >> > to achieve that? I believe we shouldn't worry right now about this and rely on >> > the fact that root is the given variable. Or do you have an example of what you >> > suggested in the other reply, i.e. where it's an evaluation of the heavy call? >> > >> > > Do you have some idea about >> > > how to do that? Something like below? >> > > >> > > #define for_each_resource_XXX(_root, _p) \ >> > > for (typeof(_root) __root = (_root), __p = (_p) = (__root)->child; \ >> > > __p && (_p); (_p) = next_resource_XXX(__root, _p)) >> > >> > This is a bit ugly :-( I would avoid ugliness as long as we have no problem to >> > solve (see above). >> >> Using a local defined variable to avoid double evaluation is standard >> practice. I do not understand "avoid ugliness as long as we have no problem to >> solve", the problem to solve will be if someone accidentally does >> something like "for_each_resource_descendant(root++, res)". *That* will >> be a problem when someone finally realizes that the macro is hiding a >> double evaluation. > > Can you explain, why do we need __p and how can we get rid of that? > I understand the part of the local variable for root. If don't use '__p', the macro becomes #define for_each_resource_XXX(_root, _p) \ for (typeof(_root) __root = (_root), (_p) = (__root)->child; \ (_p); (_p) = next_resource_XXX(__root, _p)) Where, '_p' must be a variable name, and it will be a new variable inside for loop and mask the variable with same name outside of macro. IIUC, this breaks the macro convention in kernel and has subtle variable masking semantics. >> So no, this proposal is not "ugly", it is a best practice. See the >> definition of min_not_zero() for example. > > I know that there are a lot of macros that look uglier that this one. -- Best Regards, Huang, Ying
