[PATCH] D112718: Add intrinsics and builtins for PTX atomics with semantic orders

[Tadej Ciglarič via Phabricator via llvm-commits]

t4c1 added inline comments.


================
Comment at: clang/include/clang/Basic/BuiltinsNVPTX.def:1057
+
+BUILTIN(__nvvm_atom_xchg_global_i, "iiD*i", "n")
+TARGET_BUILTIN(__nvvm_atom_cta_xchg_global_i, "iiD*i", "n", SM_60)
----------------
tra wrote:
> t4c1 wrote:
> > tra wrote:
> > > We need to figure out how address-space-specific builtins are supposed to work.
> > > Right now two competing approaches.
> > > 
> > > This patch declares builtins with generic pointer as an argument, but, according to the test, expects to be used with the AS-specific pointer.
> > > It probably does not catch a wrong-AS pointer passed as an argument, either.
> > > It does happen to work, but I think it's mostly due to the fact that LLVM intrinsics are overloaded and we happen to end up addrspacecast'ing  things correctly if the builtin gets the right kind of pointer.
> > > 
> > > The other approach is to declare the pointer with the expected AS. E.g:
> > > > TARGET_BUILTIN(__nvvm_mbarrier_init_shared, "vWi*3i", "", AND(SM_80,PTX70))
> > > 
> > > IMO, this is the correct way to do it, but it is also rather inconvenient to use from CUDA code as it operates on generic pointers.
> > > 
> > > @jdoerfert - WDYT?
> > >TARGET_BUILTIN(__nvvm_mbarrier_init_shared, "vWi*3i", "", AND(SM_80,PTX70))
> > >IMO, this is the correct way to do it, but it is also rather inconvenient to use from CUDA code as it operates on generic pointers.
> > 
> > I tried doing this, however it is also completely unusable from OpenCL code (which is our use case). Trying to use it gives you errors about how casting pointers to different address spaces  - for example from local to AS3 is not allowed.
> Hmm. It should've worked. It would need the same explicit cast to a pointer in AS(3) as in your tests, but it would safeguard against attempts to pass it a generic pointer. E.g. https://godbolt.org/z/qE6oxzheM
> 
> Explicit casting to AS(X) for AS-specific variants is annoying, but it's probably unavoidable at the moment regardless of whether we declare the pointer argument to be AS-specific or not.  LLVM will not always be able to infer that a pointer is in particular AS.
> Using specific AS in the declaration has a minor benefit of safeguarding at compile time against unintentional use of generic pointers.
> 
> Ideally we may want to convert generic variant of the builtin to AS-specific one, if LLVM does know the AS. We currently do this for loads/stores, but not for other instructions.
> 
Well, it does not work. See: https://godbolt.org/z/vM6bKncc4. Declaring the pointer to be in generic AS is the only way I got it to work. If you know a way to call a builtin declared with AS numbers in OpenCL, I am happy to do that instead.


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  https://reviews.llvm.org/D112718/new/

https://reviews.llvm.org/D112718