[PATCH] D57450: [RISCV] Set MaxAtomicInlineWidth and MaxAtomicPromoteWidth for RV32/RV64 targets with atomics

Thu Aug 22 09:32:24 PDT 2019

jyknight added a comment.

In D57450#1641190 <https://reviews.llvm.org/D57450#1641190>, @lenary wrote:

> @jyknight I hear where you're coming from. I'll see what I can do about the psABI document.
>
> In that ticket, it's mentioned that the Darwin ABI explicitly says that non-power-of-two atomic types should be padded and realigned, but I cannot find any documentation explaining this. That would be useful, given presumably GCC does have to pad/align on Darwin.

AFAIK, there is no such documentation, at least publicly. Possibly Apple has some internally, but I suspect it more likely just some in-person conversation or something.

GCC is not really supported on Darwin, so I suspect it just gets it wrong.

> Then the only outstanding question relates to zero-sized atomics, which GCC does not pad, but I think Clang has to pad to get the semantics correct, based on this comment: https://github.com/llvm/llvm-project/blob/master/clang/lib/AST/ASTContext.cpp#L2176

The semantics in GCC are that you can create such an object, but any attempt to load or store it will result in a compile-time error. E.g., "error: argument 1 of ‘__atomic_load’ must be a pointer to a nonzero size object". So I don't think there's really an issue there.

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https://reviews.llvm.org/D57450