[all-commits] [llvm/llvm-project] 363b05: LangRef: Clarify llvm.minnum and llvm.maxnum about...

[YunQiang Su via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 363b05944f9212511ee6811d0eb1af841c177226
      https://github.com/llvm/llvm-project/commit/363b05944f9212511ee6811d0eb1af841c177226
  Author: YunQiang Su <yunqiang at isrc.iscas.ac.cn>
  Date:   2025-02-27 (Thu, 27 Feb 2025)

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

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

The documents claims that it ignores sNaN, while in the current code it
may be different.

- as the finally callback, it use libc call fmin(3)/fmax(3). while C23
clarifies that fmin(3)/fmax(3) should return NaN for sNaN vs NUM.
- on some architectures, such as aarch64, it converts to `fmaxnm`, which
returns qNaN for sNaN vs NUM.
- on RISC-V (SPEC 2019+), it converts to `fmax`, which returns NUM for
sNaN vs NUM.

Since we have introduced llvm.minimumnum and llvm.maximumnum, which
follow IEEE 754-2019's minimumNumber/maximumNumber.

So, it's time for us to clarify llvm.minnum and llvm.maxnum. Since the
final fallback of llvm.minnum and llvm.maxnum is
fmin(3)/fmax(3), so that it is reasonable to follow the behaviors of
fmin(3)/fmax(3).

Although C23 clarified the behavior about sNaN and +0.0/-0.0:
     (NUM or NaN) vs sNaN -> qNaN
     +0.0 vs -0.0 -> either one of +0.0/-0.0
It is the same the IEEE754-2008's maxNUM and minNUM.
Not all implementation work as expected.
     
Since some architectures such as aarch64/MIPSr6/LoongArch, have
instructions that implements +0.0>-0.0.
So Let's define llvm.minnum and llvm.maxnum to IEEE754-2008 with
+0.0>-0.0.

The architectures without such instructions can implements `NSZ` flavor
to speed up,
and the frontend, such as clang, can call them with `nsz` attribute.



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