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> Sadly, at some point, new hardware makes software tricks obsolete. On x86, said new hardware was the F16C instruction set, which (amongst other things) adds a

> Sadly, at some point, new hardware makes software tricks obsolete. On x86, said new hardware was the F16C instruction set, which (amongst other things) adds a VCVTPS2PH instruction for converting (a vector of) FP32 to FP16. "Sadly"? That's subjective, I guess: I personally feel that the less "softfloat" code there is in use around, the better. All this twisted bitshifting/juggling with conditional propagation is pretty well suited for baking into sillicon.

kzrdude 4y ago
The converse is also deserving of a 'sadly': the new hardware features can be widely available but still hard to use since they are not going to be used by platform defaults when you compile your program.
- stephencanon 4y ago
  F16C has been available for well over a decade (it was introduced in the Ivybridge uArch), and the equivalent conversions are supported on all ARMv7 CPUs since around the same time (and all ARMv8 CPUs unconditionally); most software should be able to safely assume their availability by now. There exist some niches where this doesn’t hold, but that’s why we have software fallbacks.
  - corsix 4y ago
    The first niche that came to mind was x86 code running under Rosetta 2; despite ARM having an equivalent to F16C, Rosetta 2 doesn’t translate AVX, and F16C doesn’t have a non-AVX encoding.
    - stephencanon 4y ago
      Indeed. Worth noting that Accelerate.framework provides fast and correct bulk f16 <-> f32 conversions as `vImageConvert_Planar16FtoPlanarF` and `vImageConvert_PlanarFtoPlanar16F`, and that the arm conversion instructions are unconditionally available for apps that compile for arm64 (they're part of the base ARMv8 ISA), so any _new_ code shouldn't need to worry about this.
diydsp 4y ago
Im solving this problem by writing fp16 routines for 6502 :)
- fuzzfactor 4y ago
  In my case it wasn't exactly conversion but I had pages of FORTRAN algorithm based on precision numerical constants in FP32, and the outcome needed to be mimicked in 8-bit BASIC using 0.5K of memory for program plus data. Not enough memory to even store the text of the FORTRAN code. Integer math was required :/

kzrdude 4y ago

The converse is also deserving of a 'sadly': the new hardware features can be widely available but still hard to use since they are not going to be used by platform defaults when you compile your program.

stephencanon 4y ago
F16C has been available for well over a decade (it was introduced in the Ivybridge uArch), and the equivalent conversions are supported on all ARMv7 CPUs since around the same time (and all ARMv8 CPUs unconditionally); most software should be able to safely assume their availability by now. There exist some niches where this doesn’t hold, but that’s why we have software fallbacks.
- corsix 4y ago
  The first niche that came to mind was x86 code running under Rosetta 2; despite ARM having an equivalent to F16C, Rosetta 2 doesn’t translate AVX, and F16C doesn’t have a non-AVX encoding.
  - stephencanon 4y ago
    Indeed. Worth noting that Accelerate.framework provides fast and correct bulk f16 <-> f32 conversions as `vImageConvert_Planar16FtoPlanarF` and `vImageConvert_PlanarFtoPlanar16F`, and that the arm conversion instructions are unconditionally available for apps that compile for arm64 (they're part of the base ARMv8 ISA), so any _new_ code shouldn't need to worry about this.

diydsp 4y ago

Im solving this problem by writing fp16 routines for 6502 :)

fuzzfactor 4y ago
In my case it wasn't exactly conversion but I had pages of FORTRAN algorithm based on precision numerical constants in FP32, and the outcome needed to be mimicked in 8-bit BASIC using 0.5K of memory for program plus data. Not enough memory to even store the text of the FORTRAN code. Integer math was required :/