[all-commits] [llvm/llvm-project] 2b6e9d: [NVPTX] Set default value of nvptx-allow-ftz-atomi...

[Artem Belevich via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2b6e9d261433f5ec8c49502f91d63dd7d07346c3
      https://github.com/llvm/llvm-project/commit/2b6e9d261433f5ec8c49502f91d63dd7d07346c3
  Author: Artem Belevich <tra at google.com>
  Date:   2026-07-17 (Fri, 17 Jul 2026)

  Changed paths:
    M llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp
    M llvm/test/CodeGen/NVPTX/atomicrmw-allow-ftz-atomics.ll
    M llvm/test/CodeGen/NVPTX/atomicrmw-sm60.ll
    M llvm/test/CodeGen/NVPTX/atomicrmw-sm70.ll
    M llvm/test/CodeGen/NVPTX/atomicrmw-sm90.ll
    M llvm/test/CodeGen/NVPTX/atomicrmw.py
    M llvm/test/CodeGen/NVPTX/atomics.ll

  Log Message:
  -----------
  [NVPTX] Set default value of nvptx-allow-ftz-atomics to true (#206154)

This is a follow-up to https://github.com/llvm/llvm-project/pull/200732.

Setting the default value of nvptx-allow-ftz-atomics to true preserves
the status quo. It allows the NVPTX backend to emit native
floating-point atomic add instructions by default, rather than falling
back to the CAS loop required for strict FTZ compliance.

Correct FTZ handling forces a lot of existing code to emit CAS loops.
This introduces thread divergence, breaking code that relies on atomic
operations being non-divergent (for example, warp-wide atomic additions
followed by a load of the reduced value without explicit
synchronization).

While FTZ-correct atomic behavior is desirable as a long-term default,
introducing thread divergence into a common operation that previously
"happened to work" without synchronization is highly disruptive.
Defaulting to true makes the FTZ-correct behavior opt-in for now,
allowing users time to audit and adapt their code.

Test cases are updated to verify both enabled and disabled states of the
option, alongside the new default behavior.



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