Ok, I'll make Params typedefs and params() usage targets of opportunity. Gabe
On Thu, Oct 22, 2020 at 8:26 AM Jason Lowe-Power via gem5-dev < gem5-dev@gem5.org> wrote: > Hey Gabe, > > Thanks for bringing this up. I have also been bothered by the lack of > consistency with how params are used. I can't think of an example of when > you need to store the params object. I would be all for getting rid of the > params() function and updating the documentation to say that it's best > practice to *not* save the params struct after the constructor. If some > object had a good reason to go against this best practice, that would be > OK, and we wouldn't need to enforce any specific design or pattern on these > exceptions. I would prefer to remove the params() function than add more > template magic. > > On Thu, Oct 22, 2020 at 1:18 AM Gabe Black via gem5-dev <gem5-dev@gem5.org> > wrote: > >> Hi folks. I'm looking at the SimObject header, and I see a few things in >> there which are marked as part of the API and maybe shouldn't be. >> Specifically I'm talking about the Params typedef, and the params() method. >> There is also the _params member variable which I can see being a part of >> the API since it can be used by other classes to make their own params() >> function (more on that below), but the Params typedef is more or less an >> implementation detail, and the params() method is essentially worthless >> because it returns a SimObjectParams which doesn't have anything except the >> object's name in it, and you can already get that with the name() method. >> > > I agree. I think the typedef is useless and shouldn't be in the API. It's > unfortunate that the API proposals didn't get more reviews. I think it's > probably OK to just drop that from the API, but the params() function > itself is going to need to be deprecated. > > >> >> *Params pattern* >> >> This gets to the whole Params params() pattern which is sporadically >> implemented in some SimObjects in gem5. This pattern is that a given >> SimObject subclass will define a Params typedef which corresponds to its >> Params struct type, and then also define a params() method which returns >> the _params from SimObject cast up to that type. >> >> The Params typedef itself primarily makes the definition of the >> constructor a little more legible, since the FullFooTypeForTheArmISAParams >> can be really verbose. >> > > I think verbose is fine. I would vote to abolish all params typedefs. > > >> >> Storing the params struct itself could theoretically serve the purpose of >> having a bunch of parameters and not having to create a member variable for >> each one, spend a bunch of text copying values over in the constructor, >> etc. I think most of the time this is unnecessary, but if an object has >> tons of values in it for some reason this could make sense. >> > > I don't think there are any examples of this in the codebase. I think in > all cases the params data is copied into member variables. If there are > cases where data isn't copied, I doubt it was with a strong reason in mind. > The one exception to this might be Ruby, but it's an exception for all the > wrong reasons ;). > > >> >> The params() method then makes the _params member available with the >> appropriate type, so that all the FooParams members are accessible. It also >> makes the params struct accessible outside the object which is used in a >> place or two to read parameters without there needing to be a member in the >> object, and probably some sort of accessor to read it. >> >> There are two main problems with this system. First, when used, it adds a >> small but not trivial amount of boilerplate to any given class. Second, >> it's very sporadically implemented. I think in a lot of places it's there >> just because people aren't sure what it's for or if they need it, so they >> just put one there regardless. I think I've done that in the past. >> >> *Alternative* >> >> The existence of the Params type and the params() method could be >> partially eliminated by defining a templated params() method which took a >> SimObject reference and/or pointer as its first parameter. It could then >> figure out what Params struct went with that SimObject type using >> typedef/template magic set up by the Params building code, and then perform >> the appropriate cast. >> >> This has three downsides, two minor and one more serious. The minor one >> is that when a class uses this method internally, it would have to do >> something like params(this) instead of just params(). That's a fairly minor >> difference and not that big a deal. For external consumers that would be >> less of a problem since it would change from foo->params() to params(foo). >> >> The second minor issue is that the name params() is very short, and >> likely to collide with other names. We could define that with SimObject as >> a static method, but then that would make foo->params() turn into the more >> verbose SimObject::params(foo), or (and I haven't checked if this is legal >> syntax) the more odd looking foo->params(foo). The params() class can't be >> a non-static method, because then the type of "this" would be fixed by >> where it was defined, meaning it would not cast _params to the right type. >> I was not able to find any mechanism in c++ that would let you treat "this" >> as an argument for template type resolution. >> >> The third more serious problem is that this implicitly breaks the ability >> to use two different SimObject types in python to represent the same >> SimObject type in C++. I don't know if this happens in practice, and it's >> also broken by the original Params pattern, since there can be only one >> typedef in a given class. Since Params is applied adhoc manually, something >> that is generally not good, it actually avoids this problem by just not >> existing anywhere that would break that assumption. >> >> *Other option* >> >> Another option would be to have a templated class which would define a >> Params type and a params() method, and inherit that into each SimObject >> which wants to have those members. It would itself inherit from its >> parameter which would keep the inheritance hierarchy intact, and make it >> possible to override Params and params from super classes: >> >> FooObject : public ParamsMembers<SimObject> >> >> This has a similar problem to the above if exactly what Params type to >> use is automatic, although here it could be an additional template >> argument. This also trades some boilerplate for less boilerplate, has to be >> applied manually to any classes that want to take advantage of it, and >> obfuscates the definition of those classes. >> >> Gabe >> _______________________________________________ >> gem5-dev mailing list -- gem5-dev@gem5.org >> To unsubscribe send an email to gem5-dev-le...@gem5.org >> %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s > > _______________________________________________ > gem5-dev mailing list -- gem5-dev@gem5.org > To unsubscribe send an email to gem5-dev-le...@gem5.org > %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s
_______________________________________________ gem5-dev mailing list -- gem5-dev@gem5.org To unsubscribe send an email to gem5-dev-le...@gem5.org %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s
