[llvm-branch-commits] [llvm][RISCV] Enable trailing fences for seq-cst stores by default (PR #87376)

[Alex Bradbury via llvm-branch-commits]

asb wrote:

Thanks for splitting this out. The changes look good to me, but could you please add a brief release note to llvm/docs/ReleaseNotes? Perhaps something like "The default [atomics mapping](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-atomic.adoc) was changed to emit an additional trailing fence for sequentially consistent stores, offering compatibility with a future mapping using load-acquire and store-release instructions while remaining fully compatible with objects produced prior to this change. The mapping (ABI) used is recorded [as an ELF attribute](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#tag_riscv_atomic_abi-14-uleb128version). "

https://github.com/llvm/llvm-project/pull/87376