Dan Williams <dan.j.williams@xxxxxxxxx> writes: > Andy Shevchenko wrote: >> On Thu, Oct 24, 2024 at 08:30:39PM +0800, Huang, Ying wrote: >> > 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. >> >> Yep. > > Oh, due to the comment expression, good catch. > >> >> In property.h nobody cares about evaluation which makes the macro as simple as >> >> #define for_each_resource_XXX(_root, _p) \ >> for (_p = next_resource_XXX(__root, NULL); _p; \ >> _p = next_resource_XXX(__root, _p)) >> >> (Dan, >> that's what I called to avoid solving issues we don't have and most likely >> will never have.) > > Ah, my apologies, I thought the objection was to the macro altogether. > >> but if you want to stick with your variant some improvements can be done: >> >> #define for_each_resource_XXX(_root, _p) \ >> for (typeof(_root) __root = (_root), __p = _p = __root->child; \ >> __p && _p; _p = next_resource_XXX(__root, _p)) >> >> >> 1) no need to have local variable in parentheses; >> 2) no need to have iterator in parentheses, otherwise it would be crazy code >> that has put something really wrong there and still expect the thing to work. > > Why not: > > #define for_each_resource_XXX(_root, _p) \ > for (typeof(_root) __root = (_root), __p = _p = __root->child; \ > _p; _p = next_resource_XXX(__root, _p)) > > The __p is only to allow for _p to be initialized in the first statement > without causing a new "_p" shadow to be declared. I have tries this before. Compiler will complain with warning: unused variable ‘__p’ [-Wunused-variable] -- Best Regards, Huang, Ying
