3d_gpu/architecture/dynamic_simd/shape.mdwn

   1 # SimdShape
   2
   3 A logical extension of the nmigen `ast.Shape` concept, `SimdShape`
   4 provides sufficient context to both define overrides for individual lengths
   5 on a per-mask basis as well as sufficient information to "upcast"
   6 back to a SimdSignal, in exactly the same way that c++ virtual base
   7 class upcasting works when RTTI (Run Time Type Information) works.
   8
   9 By deriving from `ast.Shape` both `width` and `signed` are provided
  10 already leaving the `SimdShape` class with the responsibility to
  11 additionally define lengths for each mask basis. This is best illustrated
  12 with an example.
  13
  14 The Libre-SOC IEEE754 ALUs need to be converted to SIMD Partitioning
  15 but without massive disruptive code-duplication or intrusive explicit
  16 coding as outlined in the worst of the techniques documented in
  17 [[dynamic_simd]].  This in turn implies that Signals need to be declared
  18 for both mantissa and exponent that **change width to non-power-of-two
  19 sizes** depending on Partition Mask Context.
  20
  21 Mantissa:
  22
  23 * when the context is 1xFP64 the mantissa is 54 bits (excluding guard
  24   rounding and sticky)
  25 * when the context is 2xFP32 there are **two** mantissas of 23 bits
  26 * when the context is 4xFP16 there are **four** mantissas of 10 bits
  27 * when the context is 4xBF16 there are four mantissas of 5 bits.
  28