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assemblyarmneon

ARM NEON: Which pairs of instructions have to wait for write back?

In the ARM NEON documentation, it says:

[...] some pairs of instructions might have to wait until the value is written back to the register file.

I haven't come across a list that defines the instruction pairs that can use forwarded results and the instruction pairs that have to wait for write back.

Does anyone know of a table or documentation that lists these pairs?

Solution

  • Broadly speaking, what you would reasonably expect to forward, forwards. vmul.f32 forwards to vadd.f32 and the like.

    I don't believe that the exact forwarding paths are precisely documented anywhere in the manner you're looking for. I haven't found them, anyway. If you do find them, be sure to let us know where. It is, of course, not too hard to determine for any given pair of instructions whether or not forwarding occurs, but that's not a general solution. Sorry.