[Clang] __builtin_assume_dereferenceable should be constexpr

[philnik777]

Currently __builtin_assume_dereferenceable isn't constexpr. I don't think there is a fundamental problem with constant evaluating it though. It should be possible to check whether, from a certain pointer, there are indeed N bytes that can be dereferenced. The compiler has to do that anyways AFAICT, since out of bounds accesses need to be diagnosed during constant evaluation.

[tbaederr]

from a certain pointer, there are indeed N bytes that can be dereferenced.

That's basically never done, you'd have to create a buffer for the N bytes and copy the current values of all the pointer fields into that and check if all the values are initialized. That is, unless reading past the end of a field is not allowed. Check if the pointed-to field or array has enough bytes left is easier.

[chaitanyav]

@philnik777 @tbaederr can i work on this?.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nikolas Klauser (philnik777)

Currently `__builtin_assume_dereferenceable` isn't constexpr. I don't think there is a fundamental problem with constant evaluating it though. It should be possible to check whether, from a certain pointer, there are indeed N bytes that can be dereferenced. The compiler has to do that anyways AFAICT, since out of bounds accesses need to be diagnosed during constant evaluation.