updates/007_2018dec25_predication.mdwn

   1 # Out-of-out-of-Order: Unpredictable Predication Predicament
   2
   3 So, early in the development of the Not-So-Simple-V specification it was
   4 identified that it meshed well with a multi-issue microarchitecture.
   5 Recall that the basic principle is that registers are "tagged" through
   6 a CSR table as "vectorised", and, if the Vector Length is greater than 1,
   7 *multiple* (sequential) instructions are issued to the pipeline (where
   8 one would normally be sent), *without* increasing the Program Counter,
   9 the difference between these otherwise identical instructions being that
  10 the source (and/or) destination registers are incremented continuously by
  11 one on each loop.
  12
  13 The nice thing about a multi-issue microarchitecture is that it is very
  14 simple to drop these element-based otherwise-identical instructions directly
  15 into the instruction FIFO.  What is even nicer is: when predication is
  16 introduced, all that needs to be done is that when the relevant element
  17 predicate bit is clear, the associated element-based instruction is
  18 **not** placed into the multi-issue instruction FIFO.
  19
  20 Simple, right?  Couldn't be easier.
  21
  22 The problem is: the predicate and the source and the destination registers
  23 can all come from the *same register file*.  So, one instruction may modify
  24 an integer register that on the *next instruction* is used as a predication
  25 target.  That creates a write-hazard that has to be dealt with, as the
  26 predicated (Vectorised) instruction simply cannot be allowed to proceed
  27 until the instruction that is calculating its predicate has actually
  28 completed.  That means that in this particular out-of-order architecture,
  29 the instruction issue phase **itself has to become a Function Unit**.
  30
  31 Let me repeat that again: the instruction issue phase that deals with
  32 predication **itself** has to have its own scoreboard Dependency Matrix entry.
  33
  34 This brings some quite fascinating (read: scary) challenges and opportunities.
  35 If handled incorrectly, it means that the entire idea of using a multi-issue
  36 instruction FIFO is toast, as there will be guaranteed stalling whenever
  37 a predicated vectorised instruction is encountered.
  38
  39 Normally, a multi-issue engine has a guaranteed regular number of instructions
  40 to process and place in the queue.  Even branches do not stop the flow
  41 of placement into the FIFO, as branch prediction (speculative execution) can
  42 guess with high accuracy where the branch will go.  Predicated vectorised
  43 instruction issue is completely different: we have *no idea* - in advance -
  44 if the issued element-based instruction is actually going to be executed
  45 or not.  We do not have the predicate source register (yet) because it
  46 hasn't been calculated, because the prior instruction (which is being
  47 executed out-of-order, and is **itself** dependent on prior instruction
  48 completion) hasn't even been started yet.
  49
  50 Perhaps - thinking out loud - it would be okay to have a place-holder,
  51 waiting for the predicate bits to arrive.  Perhaps it is as simple as
  52 adding an extra source register (predicate source) to every single Function
  53 Unit.  So instead of each Function Unit having src1 and src2, it has
  54 src1, src2, predicate "bit".  Given that it is just a single bit that each
  55 Function Unit would be waiting for, it does seem somewhat gratuitous,
  56 and a huge complication of an otherwise extremely simple scoreboard
  57 (at present, there are no CAMs and no multi-wire I/Os in any of the
  58 cells of either the FU-to-FU Matrix or the FU-to-Register Dependency Matrix).
  59 Therefore, having **separate** Function Unit(s) which wait for the
  60 predication register to be available, that are themselves plumbed in to
  61 the actual Scoreboard system, decoding and issuing further instructions only
  62 once the predicate register is ready, seems to be a reasonable avenue to
  63 explore.
  64
  65 However, the last thing that we need is to stall execution completely,
  66 so a lot more thought is going to be needed.  The nice thing about having
  67 a predicated vectorisation "Issue" Function Unit is: some of the more
  68 complex decoding (particularly REMAP) can hypothetically be pipelined.
  69 However that is **guaranteed** to result in stalled execution, as the
  70 out-of-order system is going to critically depend on knowing what the
  71 dependencies **are**!  Perhaps it may be possible to put in temporary
  72 "blank" entries that are filled in later?  Issue place-holder instructions
  73 into the Dependency Matrix, where we know that the registers on which
  74 the instruction will depend is known at a later date?
  75
  76 Why that needs to be considered is: remember that the whole basis of
  77 Simple-V is: you issue multiple *sequential* instructions.  Unfortunately,
  78 REMAP jumbles up the word "sequential" using a 1D/2D/3D/offset algorithm,
  79 such that the actual register (or part-register in the case of 8/16/32-bit
  80 element widths) needs a calculation to be performed in order to determine
  81 which register is to be used.  And, secondly, predication can entirely
  82 skip some of those element-based instructions!
  83
  84 Talk about complex!  Simple-V is supposed to be simple!  No wonder
  85 chip designers go for SIMD and let the software sort out the mess...