[PATCH] D75811: [CUDA] Choose default architecture based on CUDA installation

[Justin Lebar via cfe-commits]

>From the peanut gallery: Perhaps something like
--cuda_arch=min_supported would solve your problem while still meeting
tra's request not to change behavior of the compiler based on
something external.

On Mon, Mar 9, 2020 at 12:58 PM Raul Tambre via Phabricator
<reviews at reviews.llvm.org> wrote:
>
> tambre added a comment.
>
> > Magically changing compiler target based on something external to compiler is a bad idea IMO. I would expect a compilation with exactly the same compiler options to do exactly the same thing. If we magically change default target, that will not be the case.
>
> It'd be the same behaviour as NVCC, which compiles for the lowest architecture it supports.
>
> I'm currently implementing Clang CUDA support for CMake and lack of this behaviour compared to other languages and compilers complicates matters.
> During compiler detection CMake compiles a simple program, which includes preprocessor stuff for embedding compiler info in the output. Then it parses that and determines the compiler vendor, version, etc.
>
> The general assumption is that a compiler can compile a simple program for its language without us having to do compiler-specific options, flags, etc. If the compiler fails on this simple program, it's considered broken.
> A limited list of flags is usually cycled through to support exotic compilers and I could do the same here, but it'd require us invoking the compiler multiple times and increasingly more as old architectures are deprecated.
> We could detect the CUDA installation ourselves and specify a list of arches for each. This seems quite unnecessary when Clang already knows the version and could select a default that at least compiles.
> Note that this detection happens before any user CMake files are ran, so we can't pass the user's preferred arch (which could also differ per file).
>
>
> Repository:
>   rG LLVM Github Monorepo
>
> CHANGES SINCE LAST ACTION
>   https://reviews.llvm.org/D75811/new/
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> https://reviews.llvm.org/D75811
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