simple_v_extension/specification/ld.x.mdwn

   1 [[!tag oldstandards]]
   2
   3 **THIS PAGE US OBSOLETE, OUT OF DATE, AND MUST BE REPLACED. THE BUGREPORT
   4 IS NOT OUT-OF-DATE**
   5
   6 # LD.X and LD.Swizzle
   7
   8 See <http://bugs.libre-soc.org/show_bug.cgi?id=139>
   9
  10 SV is based on vectorisation of scalar opcodes. LD and ST from scalar RISCV can be extended by either rd or rs1 being vectorised, however a third "immediate element strided" multiples do not fit.
  11
  12 In addition, Vulkan requires swizzled LD/ST covering:
  13
  14 * identity (no swizzle)
  15 * constant 0
  16 * 1 or 1.0 depending on Op type
  17 * x, y, z, or w.
  18
  19 See <https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkComponentSwizzle.html>
  20
  21 Note that the swizzle has to be on selecting the src that will go into a sequentially indexed dest:
  22
  23     for i in range(4):
  24         srcidx = swizzle[i]
  25         srcaddr = base + srcidx * elwidth
  26         regs[rd+i] = mem[srcaddr]
  27
  28 Swizzling on the dest would not work because xxyz would try to set the same dest twice.