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Pipeline Stage Read and Write

Etienne SOBOLE
Note: This was originally posted on 5th March 2011 at http://forums.arm.com

I'm still trying to understand the cycle table of the cortex A8.

Most of the test I've made suppose this:
- source register are needed at the beginning of the stage
- destination register are released at the end of the stage.

With those rules it's seem's that result are quite good.

For example, this code take 3 cycles

    add   r5, r5, #1
    mov   r6, r5

because ADD release r5 on the end of stage 2 while MOV need it at the beginning of stage 1

That 's work and that's real cycle execution timing.

But I've a problem with the MLA shortcuts


    mul   r4, r5, r4
    mla   r0, r6, r7, r4


the MUL should release R4 at the end of stage 5 (of the second cycle of the MUL)
the MLA need r4 at the beginning of the stage 4 (due to MLA shortcut).

So the code should take 5 cycles, but in fact It takes only 4 cycles.

Is it possible that r4 is only needed at the beginning of the stage 4 of the second cycle of the MLA ???
Or may be the forwarding is done at the end of the stage 4. So I could suppose this is the same thing as the beginning of the stage 5 !

That could explain the missing cycle.
Parents
  • Note: This was originally posted on 23rd March 2011 at http://forums.arm.com

    A multiply that is followed by a MAC with a dependency on the accumulator, Rn register, triggers a special accumulator
    forwarding. This enables both instructions to issue back-to-back because Rn is required as a source in E4. If this accumulator
    forwarding is not used, Rn is required in E2.
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  • Note: This was originally posted on 23rd March 2011 at http://forums.arm.com

    A multiply that is followed by a MAC with a dependency on the accumulator, Rn register, triggers a special accumulator
    forwarding. This enables both instructions to issue back-to-back because Rn is required as a source in E4. If this accumulator
    forwarding is not used, Rn is required in E2.
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