[[!tag oldstandards]]

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# LD.X and LD.Swizzle

See <http://bugs.libre-soc.org/show_bug.cgi?id=139>

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.

In addition, Vulkan requires swizzled LD/ST covering:

* identity (no swizzle)
* constant 0
* 1 or 1.0 depending on Op type
* x, y, z, or w.

See <https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkComponentSwizzle.html>

Note that the swizzle has to be on selecting the src that will go into a sequentially indexed dest:

    for i in range(4):
        srcidx = swizzle[i]
        srcaddr = base + srcidx * elwidth
        regs[rd+i] = mem[srcaddr]

Swizzling on the dest would not work because xxyz would try to set the same dest twice.