Flang name mangling for enabling OpenMP NVPTX backend

Subprojects Flang
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1

Flang only supports OpenMP offload for AMDGPU. I’m interested in adding support for NVPTX -the patch is less than 20 lines; however, I found that the current name mangling scheme is incompatible with PTX, as it uses dots . to separate some identifiers, which results in invalid identifiers in PTX.

There are two possible approaches:

  1. Replacing the dot . mangling scheme with underscores _.
  2. Adding a pass that re-mangles the names to PTX-compatible names.

I prefer the first option as it results in a more uniform offloading path. However, it means breaking the ABI.

Thoughts and comments?

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which name do you see mangled with a . Do you have an example?

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Compiling basic_target_region.f90 produces a couple of mangled names with dots, for example:

@_QQcl.78203D20

Which corresponds to the string "x = ". The fact that the string ends up in the device module when it shouldn’t is a different can of worms.

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Thanks in advance for this.

Which ABI are you referring to here?

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The name mangling scheme, so that no . appear in names. My reasoning is that the impact is not going to be that big as flang is not in production yet.

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We had the same problem with OpenMP. _ is not a good alternative to . because the former is often allowed in base language identifiers. We opted to use “$”, IIRC, for compiler generated names. That said, if we are sure the . in names will not clash with user provided names we can go for it.

@Artem-B What do you think about option 2? I’m not a fan but it might be worth having as a fallback anyway.

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Quick question, are you using $ in identifiers regardless of the target or just for NVPTX?

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Quick question, are you using $ in identifiers regardless of the target or just for NVPTX?

We use it for all GPU targets but only in Clangs OpenMP offloading name gen. IIRC. Consistency is generally worth a lot.

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Currently there are only few places in flang using dots, it might be worth considering changing the separator in all cases for the sake of consistency.

Here’s a PR on changing the separator [flang] Change `uniqueCGIdent` separator from `.` to `_` by fabianmcg · Pull Request #71338 · llvm/llvm-project · GitHub