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

Fri Aug 16 03:41:39 PDT 2019

lenary added a comment.
Herald added subscribers: s.egerton, MaskRay.

Given this is an ABI-compatibility issue, I've been looking at how GCC and Clang differ in how they deal with issues around size and alignment of atomic objects.

All types of size less than or equal to `MaxAtomicPromoteWidth`, when they are turned into their atomic variant, will: a) have their size rounded up to a power of two, and b) have their alignment changed to that same power of two.

The following Gist contains:

- a C program which will print the size and alignment of a variety of strangely sized and aligned objects, and their atomic variants.
- the output of this program when run through gcc and clang, specifying `rv(32,64)ima(,fd)` and `(ilp32,lp64)(,f)` (matching ABIs to architectures). I used `riscv64-unknown-linux-gnu-gcc` for the avoidance of doubt (this compiler allows you to compile for rv32 by specifying a 32-bit -march value).
- the diffs in those outputs for an single ABI, between the two different compilers.

The gist is here: https://gist.github.com/lenary/2e977a8af33876ba8e0605e98c4e1b0d

There are some differences between GCC and Clang, that seem not to be risc-v specific (I saw them when running the C program on my mac)

- Clang ensures zero-sized objects become char-sized when they become atomic. GCC leaves them zero-sized. This is documented in ASTContext.cpp line 2130, to ensure the atomic operation is generated.
- GCC seems not to round up the size and alignment of non-power-of-two-sized structures.

I think this patch needs to be updated to ensure there are no differences in objects of power-of-two size. To do this, both riscv64 and riscv32, should have a `MaxAtomicPromoteWidth` of 128.

@jyknight does this reasoning make sense for ABI compatibility?

Repository:
  rC Clang

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  https://reviews.llvm.org/D57450/new/

https://reviews.llvm.org/D57450