[PATCH] D17993: [CodeGen] Apply 'nonnull' and 'dereferenceable(N)' to 'this' pointer arguments.

[John McCall via Phabricator via cfe-commits]

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`.


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  https://reviews.llvm.org/D17993/new/

https://reviews.llvm.org/D17993