rjmccall 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);
----------------
rsmith wrote:
> 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.
I think all of this is correct except that the frontend does know what the
bit-pattern of the null pointer is in any particular language-level address
space, and it knows what the language-level address space of `this` is. So we
should be able to ask whether the null value in the `this` address space is the
all-zero value and use that to decide whether to apply `nonnull`.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D17993/new/
https://reviews.llvm.org/D17993
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