update

diff --git a/updates/007_2018dec25_predication.mdwn b/updates/007_2018dec25_predication.mdwn
index ca0eb391262d4689e5dff7195eedb7fada937abe..45a30479846912892d28927497f7e9bebe63afa5 100644 (file)
--- a/updates/007_2018dec25_predication.mdwn
+++ b/updates/007_2018dec25_predication.mdwn
@@ -29,4 +29,55 @@ issue phase itself has to become a Function Unit.
 Let me repeat that again: the instruction issue phase *itself* has to
 have its own scoreboard Dependency Matrix entry.
 
-This brings some quite fascinating (read: scary) 
+This brings some quite fascinating (read: scary) challenges and opportunities.
+If handled incorrectly, it means that the entire idea of using a multi-issue
+instruction FIFO is toast, as there will be guaranteed stalling whenever
+a predicated vectorised instruction is encountered.
+
+Normally, a multi-issue engine has a guaranteed regular number of instructions
+to process and place in the queue.  Even branches do not stop the flow
+of placement into the FIFO, as branch prediction (speculative execution) can
+guess with high accuracy where the branch will go.  Predicated vectorised
+instruction issue is completely different: we have *no idea* - in advance -
+if the issued element-based instruction is actually going to be executed
+or not.  We do not have the predicate source register (yet) because it
+hasn't been calculated, because the prior instruction (which is being
+executed out-of-order, and is **itself** dependent on prior instruction
+completion) hasn't even been started yet.
+
+Perhaps - thinking out loud - it would be okay to have a place-holder,
+waiting for the predicate bits to arrive.  Perhaps it is as simple as
+adding an extra source register (predicate source) to every single Function
+Unit.  So instead of each Function Unit having src1 and src2, it has 
+src1, src2, predicate "bit".  Given that it is just a single bit that each
+Function Unit would be waiting for, it does seem somewhat gratuitous,
+and a huge complication of an otherwise extremely simple scoreboard
+(at present, there are no CAMs and no multi-wire I/Os in any of the
+cells of either the FU-to-FU Matrix or the FU-to-Register Dependency Matrix).
+Therefore, having **separate** Function Unit(s) which wait for the
+predication register to be available, that are themselves plumbed in to
+the actual Scoreboard system, decoding and issuing further instructions only
+once the predicate register is ready, seems to be a reasonable avenue to
+explore.
+
+However, the last thing that we need is to stall execution completely,
+so a lot more thought is going to be needed.  The nice thing about having
+a predicated vectorisation "Issue" Function Unit is: some of the more
+complex decoding (particularly REMAP) can hypothetically be pipelined.
+However that is **guaranteed** to result in stalled execution, as the
+out-of-order system is going to critically depend on knowing what the
+dependencies **are**!  Perhaps it may be possible to put in temporary
+"blank" entries that are filled in later?  Issue place-holder instructions
+into the Dependency Matrix, where we know that the registers on which
+the instruction will depend is known at a later date?
+
+Why that needs to be considered is: remember that the whole basis of
+Simple-V is: you issue multiple *sequential* instructions.  Unfortunately,
+REMAP jumbles up the word "sequential" using a 1D/2D/3D/offset algorithm,
+such that the actual register (or part-register in the case of 8/16/32-bit
+element widths) needs a calculation to be performed in order to determine
+which register is to be used.  And, secondly, predication can entirely
+skip some of those element-based instructions!
+
+Talk about complex!  Simple-V is supposed to be simple!  No wonder
+chip designers go for SIMD and let the software sort out the mess...