cjdb added inline comments.
================ Comment at: clang/include/clang/AST/Stmt.h:796-802 + /// If this expression is not value-dependent, this stores the value. + unsigned Value : 8; /// The number of arguments to this type trait. According to [implimits] /// 8 bits would be enough, but we require (and test for) at least 16 bits /// to mirror FunctionType. + unsigned NumArgs : 16; ---------------- erichkeane wrote: > dblaikie wrote: > > cjdb wrote: > > > These numbers feel very low for how this modification is using them. > > > Perhaps they should both be bumped to 32? > > clang does, I think, have specific implementation limits for this sort of > > thing - > > > > Well, maybe we don't actually enforce a limit on number of template > > arguments... https://godbolt.org/z/accncYson compiles successfully, and has > > 2^18 parameters, beyond the 2^16 suggested here. > > > > But maybe 2^16 is just what we test for/somewhat guarantee. > > > > But if the `Value` is going to be the index into the args, shouldn't it be > > the same size as `NumArgs`, at least? (& the comment says 16, so 16 for > > both Value and NumArgs would seem suitably symmetric, and no larger than > > the current situation - since it'd just be splitting NumArgs in half, while > > still meeting the comment's suggestion/requirements?) > > > > An assert, at least, when populating NumArgs to ensure it's no larger might > > not hurt... (which might be reachable from code/not checked prior - so it > > could be a hard compilation error, I guess, but I wouldn't feel super > > strongly about it) > Can you explain the math here of how you chose to change this to 16? I see > that you removed 7 bits, but took away 16. I'm not sure what NumExprBits is > doing though, I got lost a little in that, so if you can calculate that to > make sure this needs to be done, it would be appreciated. > > Additionally, a static_assert after this to ensure the size isn't changing > would also be appreciated. > > But if the Value is going to be the index into the args, shouldn't it be the > same size as NumArgs, at least? (& the comment says 16, so 16 for both Value > and NumArgs would seem suitably symmetric, and no larger than the current > situation - since it'd just be splitting NumArgs in half, while still meeting > the comment's suggestion/requirements?) Someone independently confirmed that you can have 200k types in a template, so we should probably be able to count at least that high (or alternatively, we should possibly consider not allowing more than 2^16 template parameters). > Can you explain the math here of how you chose to change this to 16? I see > that you removed 7 bits, but took away 16. I'm not sure what NumExprBits is > doing though, I got lost a little in that, so if you can calculate that to > make sure this needs to be done, it would be appreciated. The value of `NumArgs` hasn't changed: I've just codified it. I can't work out why we need `NumExprBits`, but it's apparently required padding (when I removed it, a bunch of tests failed). Its value is 18, courtesy of clangd in VS Code. > Additionally, a static_assert after this to ensure the size isn't changing > would also be appreciated. There's a static_assert in one of the `Stmt` constructors, which doesn't want more than eight bytes (we apparently need 16 if we're to change this). Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D151952/new/ https://reviews.llvm.org/D151952 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits