ahatanak added inline comments.
================
Comment at: clang/include/clang/Basic/AttrDocs.td:4109
- [[clang::not_tail_called]] int foo2() override;
- };
}];
----------------
Quuxplusone wrote:
> (Moving into a thread)
>
> > This patch doesn't prevent the call to method in the code below from being
> > tail called,
> > but I suppose users would expect the attribute to prevent the tail call?
> ```
> struct B {
> virtual void method();
> };
> struct D : B {
> [[clang::not_tail_called]] void method() override;
> };
> ```
>
> The way virtual calls are handled in C++ is, all attributes and properties of
> the call are determined based on the //static// type at the call site; and
> then the //runtime destination// of the call is determined from the pointer
> in the vtable. Attributes and properties have no runtime existence, and so
> they physically cannot affect anything at runtime. Consider
> https://godbolt.org/z/P3799e :
>
> ```
> struct Ba {
> virtual Ba *method(int x = 1);
> };
> struct Da : Ba {
> [[clang::not_tail_called]] [[nodiscard]] Da *method(int x = 2) noexcept
> override;
> };
> auto callera(Da& da) {
> Ba& ba = da;
> ba.method();
> }
> ```
> Here the call that is made is a //virtual// call (because `Ba::method` is
> virtual); with a default argument value of `1` (because `Ba::method`'s `x`
> parameter has a default value of `1`); and it returns something of type `Ba*`
> (because that's what `Ba::method` returns); and it is not considered to be
> noexcept (because `Ba::method` isn't marked noexcept); and it's okay to
> discard the result (because `Ba::method` is not nodiscard) and it is
> tail-called (because `Ba::method` doesn't disallow tail calls). All of these
> attributes and properties are based on the //static// type of variable `ba`,
> despite the fact that //at runtime// we'll end up jumping to the code for
> `Da::method`. According to the source code, statically, `Da::method` has a
> default argument of `2`, returns `Da*`, is noexcept, and is nodiscard, and
> disallows tail-calls. But we're not calling `da.method()`, we're calling
> `ba.method()`; so none of that matters to our call site at `callera`.
>
> I think this patch is a good thing.
OK, I see. I think this patch is fine then.
Should we add an explanation of how virtual functions are handled? The doc
currently just says the attribute prevents tail-call optimization on statically
bound calls.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D96832/new/
https://reviews.llvm.org/D96832
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