ahatanak added a comment. In D96832#2570009 <https://reviews.llvm.org/D96832#2570009>, @zequanwu wrote:
> In D96832#2569987 <https://reviews.llvm.org/D96832#2569987>, @ahatanak wrote: > >> In D96832#2569481 <https://reviews.llvm.org/D96832#2569481>, @zequanwu wrote: >> >>> In D96832#2568257 <https://reviews.llvm.org/D96832#2568257>, @aaron.ballman >>> wrote: >>> >>>> It was explicitly disallowed in the initial patch: >>>> https://reviews.llvm.org/D12922 and the original author said "I'm still >>>> trying to figure out the best way to handle c++ virtual functions: this >>>> attribute is not very useful for someone who is looking for a way to >>>> reliably prevent tail-call to a virtual function." and "I made this change >>>> because this attribute isn't useful when the compiler cannot resolve the >>>> function call statically at compile time and it isn't important for the >>>> use case I have." Has this situation changed in the backend? >>> >>> Oh, I didn't see that. But when I tested `not_tail_called` on normal >>> functions, it seems like not working(https://godbolt.org/z/znr5b5, `f1` is >>> marked as `not_tail_called`, it still get inlined). Or, I misunderstand how >>> to use it properly? >> >> I'm still trying to remember the discussions we had, but I think we banned >> the attribute on virtual functions because you can't in general promise a >> call to an annotated function won't be tail called if the function is >> virtual. >> >> This patch doesn't prevent the call to `method` in the code below from being >> tail called, but I suppose users would except the attribute to prevent the >> tail call? >> >> struct B { >> virtual void method(); >> }; >> >> struct D : B { >> [[clang::not_tail_called]] void method() override; >> }; >> >> void test(D *d) { >> B *b = D; >> b->method(); >> } > > I got the point about why preventing it on virtual functions. But, on the > above godbolt link, I provided a example where `not_tail_called` doesn't > prevent a non-virtual function from tail call. I wonder if I use it correctly. `not_tail_called` adds `notail` to the call `c->f1` in the IR, but doesn't do anything to prevent the calls inside `f1` from being tail called. You can use `disable-tail-calls` instead for that purpose. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D96832/new/ https://reviews.llvm.org/D96832 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
