rsmith added inline comments.
================
Comment at: clang/lib/CodeGen/CGCall.cpp:2169
+ if (!CodeGenOpts.NullPointerIsValid &&
+ getContext().getTargetAddressSpace(FI.arg_begin()->type) == 0) {
+ Attrs.addAttribute(llvm::Attribute::NonNull);
----------------
jdoerfert wrote:
> arichardson wrote:
> > Isn't the `this` pointer also nonnull in other address spaces?
> >
> > In our CHERI fork we use AS200 for the this pointer and would quite like to
> > have the nonnull attribute.
> > I can obviously change this line locally when I next merge from upstream,
> > but I would like to avoid diffs and it seems to me like this restriction is
> > unnecessary.
> I also think `NullPointerIsValid` is sufficient.
It's my understanding that:
* The LLVM `null` value in any address space is the all-zero-bits value.
* In address space zero, the `null` value does not correspond to addressable
memory, but this is not assumed to hold in other address spaces.
* An address-space-zero `null` value that is addressspacecast to a different
address space might not be the `null` in the target address space.
* The `nonnull` attribute implies that the pointer value is not the `null`
value.
* A null pointer in the frontend in a non-zero address space corresponds to the
value produced by an addressspacecast of an address-space-zero `null` value to
the target address space.
That being the case, there is simply no connection between the C and C++ notion
of a null pointer and a `null` LLVM pointer value in a non-zero address space
in general, so it is not correct to use the `nonnull` attribute in a non-zero
address space in general. Only if we know that a C++ null pointer is actually
represented by the LLVM `null` value in the corresponding address space can we
use the `nonnull` attribute to expose that fact to LLVM. And we do not know
that in general.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D17993/new/
https://reviews.llvm.org/D17993
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