[Libre-soc-bugs] [Bug 458] New: PartitionedSignal needs nmigen constructs "m.If", Switch etc

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=458

            Bug ID: 458
           Summary: PartitionedSignal needs nmigen constructs "m.If",
                    Switch etc
           Product: Libre-SOC's first SoC
           Version: unspecified
          Hardware: Other
                OS: Linux
            Status: CONFIRMED
          Severity: enhancement
          Priority: ---
         Component: Source Code
          Assignee: lkcl at lkcl.net
          Reporter: lkcl at lkcl.net
                CC: libre-soc-bugs at lists.libre-soc.org
   NLnet milestone: ---

PartitionedSignal is a dynamic SIMD version of Signal.  it needs to work with
the following constructs:

* m.If / Elif / Else
* m.Switch / Case
* Arrays

without PartitionedSignal, SIMD has to be done as follows:

with m.If(partition == 64):
    operations treating Signals at full 64 bit
with m.Elif(partition == 2x32bit):
    for i in range(2):
         exactly the same code as 64 bit
         except now it is repeated twice,
         first on the low 32 bits and then
         on the hi 32

and that repetition continues right the way down to 8 bit, complicating design
massively.

PartitionedSignal hides that entirely, as far as arithmetic and logic
operations are concerned (__lt__, __or__, see bug #132)

where things break down is this:

ppts = PartitionPoints(64, 8)
x = PartitionedSignal(64, partition=ppts)

with m.If(x == 5):
    ... do something.

the reason it breaks down is because PartitionedSignal.__eq__ does *not* return
a single bit object, it returns a *dynamic multi-bit* object that dynamically
reflects the current state of the partitioning.

example:

* partition points is set to 8 breaks
* this subdivides the 64 bit signal into 8 separate 8 bit values
* comparison against a constant (5) creates EIGHT separate 8-bit comparisons
* those 8 comparisons are bundled together

when the partitions are cleared to indicate that the 64 bits are to be treated
as a single 64 bit value, the comparison "5" is done against the entire 64 bit,
however the answer still goes into the same 8 bit object used when the
partition was set to 8x8bit.

the m.If construct cannot cope with this.

the current workaround is to completely avoid using m.If entirely and to use a
special PartitionedMux construct, PMux, instead.

this however is less than ideal.

similar logic applies to Switch/Case. Arrays need some discussion.

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