After D17993, with -fno-delete-null-pointer-checks we add the dereferenceable attribute to the `this` pointer.
We have observed that one internal target which worked before fails even with -fno-delete-null-pointer-checks.
Switching to dereferenceable_or_null fixes the problem.
dereferenceable currently does not always respect NullPointerIsValid and may
imply nonnull and lead to aggressive optimization. The optimization may be
related to `CallBase::isReturnNonNull`, `Argument::hasNonNullAttr`, or
`Value::getPointerDereferenceableBytes`. See D66664 and D66618 for some discussions.
Reviewed By: bkramer, rsmith
Differential Revision: https://reviews.llvm.org/D92297
arguments.
* Adds 'nonnull' and 'dereferenceable(N)' to 'this' pointer arguments
* Gates 'nonnull' on -f(no-)delete-null-pointer-checks
* Introduces this-nonnull.cpp and microsoft-abi-this-nullable.cpp tests to
explicitly test the behavior of this change
* Refactors hundreds of over-constrained clang tests to permit these
attributes, where needed
* Updates Clang12 patch notes mentioning this change
Reviewed-by: rsmith, jdoerfert
Differential Revision: https://reviews.llvm.org/D17993
Fangrui Song