[all-commits] [llvm/llvm-project] 75aa01: Revert "LangRef: Clarify llvm.minnum and llvm.maxn...

[Phoebe Wang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 75aa01b89553bf4213a3b0e83829b6d0689941b9
      https://github.com/llvm/llvm-project/commit/75aa01b89553bf4213a3b0e83829b6d0689941b9
  Author: Phoebe Wang <phoebe.wang at intel.com>
  Date:   2025-12-01 (Mon, 01 Dec 2025)

  Changed paths:
    M llvm/docs/LangRef.rst
    M llvm/include/llvm/CodeGen/ISDOpcodes.h

  Log Message:
  -----------
  Revert "LangRef: Clarify llvm.minnum and llvm.maxnum about sNaN and signed zero (#112852)" (#168838)

This reverts commit 363b05944f9212511ee6811d0eb1af841c177226.

This is a follow up of #166912. Sorry for not noticing the change at the
beginning, but I disagree with both sNaN and signed zero semantics
change.

I have 3 justifications:

- llvm.minnum and llvm.maxnum are common intrinsics, we cannot change
the definition just because "some architectures" support the changed
semantic. For example, X86 min/max instructions neither distinguish sNaN
nor signed zero. We have to add couples of extra instructions to match
with the new definition, which makes the intrinsics less efficient. But
efficient is not the reason for the objection. I object because such
cost is unnecessary;
- As the example ``minnum(fadd(sNaN, -0.0), 1.0)`` shows, minnum/maxnum
themself cannot guarantee consistent result if multiple FP arithmetic
operations involved. It makes the sacrifice of performance totally
unnecessary. `Behavior of Floating-Point NaN values` notes all NaNs can
be treated as quiet NaNs unless using Constrained Floating-Point
Intrinsics. So the cost is only worth for constrained minnum/maxnum ones
if we want to define them;
- Signed zero handling is unnecessary either, because even the C
functions don't require it. If any other front ends require, they can
use the existing fminnum_ieee/fmaxnum_ieee or define new intrinsics;

Fixes: https://github.com/llvm/llvm-project/issues/138303 and
https://github.com/llvm/llvm-project/issues/169122



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