Lewis Crawford ea3fdc5972
Avoid maxnum(sNaN, x) optimizations / folds (#170181)
The behaviour of constant-folding `maxnum(sNaN, x)` and `minnum(sNaN,
x)` has become controversial, and there are ongoing discussions about
which behaviour we want to specify in the LLVM IR LangRef.

See:
  - https://github.com/llvm/llvm-project/issues/170082
  - https://github.com/llvm/llvm-project/pull/168838
  - https://github.com/llvm/llvm-project/pull/138451
  - https://github.com/llvm/llvm-project/pull/170067
-
https://discourse.llvm.org/t/rfc-a-consistent-set-of-semantics-for-the-floating-point-minimum-and-maximum-operations/89006

This patch removes optimizations and constant-folding support for
`maxnum(sNaN, x)` but keeps it folded/optimized for `qNaN`. This should
allow for some more flexibility so the implementation can conform to
either the old or new version of the semantics specified without any
changes.

As far as I am aware, optimizations involving constant `sNaN` should
generally be edge-cases that rarely occur, so here should hopefully be
very little real-world performance impact from disabling these
optimizations.
2025-12-02 12:43:03 +00:00
..

+==============================================================================+
| How to organize the lit tests                                                |
+==============================================================================+

- If you write a test for matching a single DAG opcode or intrinsic, it should
  go in a file called {opcode_name,intrinsic_name}.ll (e.g. fadd.ll)

- If you write a test that matches several DAG opcodes and checks for a single
  ISA instruction, then that test should go in a file called {ISA_name}.ll (e.g.
  bfi_int.ll

- For all other tests, use your best judgement for organizing tests and naming
  the files.

+==============================================================================+
| Naming conventions                                                           |
+==============================================================================+

- Use dash '-' and not underscore '_' to separate words in file names, unless
  the file is named after a DAG opcode or ISA instruction that has an
  underscore '_' in its name.