simple_v_extension/specification/ld.x.mdwn

   1 [[!tag oldstandards]]
   2
   3 # LD.X and LD.Swizzle
   4
   5 See <http://bugs.libre-riscv.org/show_bug.cgi?id=139>
   6
   7 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.
   8
   9 In addition, Vulkan requires swizzled LD/ST covering:
  10
  11 * identity (no swizzle)
  12 * constant 0
  13 * 1 or 1.0 depending on Op type
  14 * x, y, z, or w.
  15
  16 See <https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkComponentSwizzle.html>
  17
  18 Note that the swizzle has to be on selecting the src that will go into a sequentially indexed dest:
  19
  20     for i in range(4):
  21         srcidx = swizzle[i]
  22         srcaddr = base + srcidx * elwidth
  23         regs[rd+i] = mem[srcaddr]
  24
  25 Swizzling on the dest would not work because xxyz would try to set the same dest twice.