[all-commits] [llvm/llvm-project] 6109e7: [llvm][APFloat] Add NaN-in-negative-zero formats b...

[Krzysztof Drewniak via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 6109e70c72fc5171d25c4467fc3cfe6eb2029f50
      https://github.com/llvm/llvm-project/commit/6109e70c72fc5171d25c4467fc3cfe6eb2029f50
  Author: Krzysztof Drewniak <Krzysztof.Drewniak at amd.com>
  Date:   2023-02-09 (Thu, 09 Feb 2023)

  Changed paths:
    M llvm/include/llvm/ADT/APFloat.h
    M llvm/lib/Support/APFloat.cpp
    M llvm/unittests/ADT/APFloatTest.cpp

  Log Message:
  -----------
  [llvm][APFloat] Add NaN-in-negative-zero formats by AMD and GraphCore

AMD, GraphCore, and Qualcom have published a standard for 8-bit floats that
differs from the 8-bit floats defined by Nvidia, Intel, and ARM. This
commit adds support for these alternate 8-bit floats to APFloat in
order to enable their usage in MLIR. These formats are presented in
the paper at https://arxiv.org/abs/2206.02915 and are implemented in
GRaphCore hardware whose ISA is available at
https://docs.graphcore.ai/projects/isa-mk2-with-fp8/en/latest/_static/TileVertexISA-IPU21-1.3.1.pdf .

In these formats, like the existing Float8E4M3FN, there are no
infinity values and there is only one NaN. Unlike in that format,
however, the NaN values is 0x80, which would be negative 0 in IEEE
formats. This means that these formats also make 0 unsigned.

To allow for these new variant semantics, this commit adds
fltNanEncoding, which can be IEEE (the default), AllOnes (used by
Fleat8E4M3FN), or NegativeZero (used by the new formats,
Float8E5M2FNUZ and Float8E4M3FNUZ). Normalization, arithmetic, and
other such routines have been updated to account for the potential
variant semantics.

The two new formats are Float8E5M2FNUZ (5 bits exponent, 2 bits
mantissa, finite, unsigned zero) and Float8E4M3FNUZ (4 bits exponent,
3 bits mantissa, finite, unsigned zero).

Reviewed By: jakeh-gc, reedwm, lattner

Differential Revision: https://reviews.llvm.org/D141863