[PATCH] D61458: [hip] Relax CUDA call restriction within `decltype` context.

[Finkel, Hal J. via cfe-commits]

Thanks, Justin. It sees like we have the standard set of options: We can disallow the mismatch. We can allow it with a warning. We can allow it without a warning. We can say that if the mismatch contributes to the type of a kernel function, that's illformed (NDR).

In any case, it seems like your examples argue for disallowing a return-type mismatch between host and device overloads, not disallowing observing the type? Or maybe disallowing observing the type only when there's a mismatch?

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: Justin Lebar <jlebar at google.com>
Sent: Thursday, May 2, 2019 9:16 PM
To: reviews+D61458+public+f6ea501465ad52d2 at reviews.llvm.org
Cc: michael.hliao at gmail.com; Artem Belevich; John McCall; Liu, Yaxun (Sam); Finkel, Hal J.; Richard Smith; Clang Commits; mlekena at skidmore.edu; blitzrakete at gmail.com; Han Shen
Subject: Re: [PATCH] D61458: [hip] Relax CUDA call restriction within `decltype` context.

> So, actually, I wonder if that's not the right answer. We generally allow different overloads to have different return types. What if, for example, the return type on the host is __float128 and on the device it's `MyLongFloatTy`?

The problem is that conceptually compiling for host/device does not create a new set of overloads.

When we compile for (say) host, we build a full AST for all functions, including device functions, and that AST must pass sema checks.  This is significant for example because when compiling for device we need to know which kernel templates were instantiated on the host side, so we know which kernels to emit.

Here's a contrived example.

```
 __host__ int8 bar();
__device__ int16 bar();
__host__ __device__ auto foo() -> decltype(bar()) {}

template <int N> __global__ kernel();

void launch_kernel() {
  kernel<sizeof(decltype(foo()))><<<...>>>();
}
```

This template instantiation had better be the same when compiling for host and device.

That's contrived, but consider this much simpler case:

```
void host_fn() {
  static_assert(sizeof(decltype(foo())) == sizeof(int8));
}
```

If we let foo return int16 in device mode, this static_assert will fail when compiling in *device* mode even though host_fn is never called on the device.  https://gcc.godbolt.org/z/gYq901

Why are we doing sema checks on the host code when compiling for device?  See contrived example above, we need quite a bit of info about the host code to infer those templates.

On Thu, May 2, 2019 at 7:05 PM Hal Finkel via Phabricator <reviews at reviews.llvm.org<mailto:reviews at reviews.llvm.org>> wrote:
hfinkel added a comment.

In D61458#1488970 <https://reviews.llvm.org/D61458#1488970>, @jlebar wrote:

> Here's one for you:
>
>   __host__ float bar();
>   __device__ int bar();
>   __host__ __device__ auto foo() -> decltype(bar()) {}
>
>
> What is the return type of `foo`?  :)
>
> I don't believe the right answer is, "float when compiling for host, int when compiling for device."


So, actually, I wonder if that's not the right answer. We generally allow different overloads to have different return types. What if, for example, the return type on the host is __float128 and on the device it's `MyLongFloatTy`?

> I'd be happy if we said this was an error, so long as it's well-defined what exactly we're disallowing.  But I bet @rsmith can come up with substantially more evil testcases than this.




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