[Libre-soc-bugs] [Bug 713] PartitionedSignal enhancement to add partition-context-aware lengths

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=713

--- Comment #119 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Jacob Lifshay from comment #108)
> (In reply to Luke Kenneth Casson Leighton from comment #107)
> > (In reply to Jacob Lifshay from comment #106)
> > > I read through all the new layout() code, looks mostly fine, though afaict
> > > the computation of bmask is incorrect if the object was to figure out which
> > > partitions are always padding.
> > 
> > yes, i can't quite work it out. if you can fix that, these algorithmic
> > things do my head in.
> 
> yeah, I can do that tomorrow (actually today, but you get what I meant).

Done. I changed bmask to be such that bmask & (1 << i) is set if partition
index i is always padding. Note that there is one more partition than the
number of ppoints, since ppoints separate partitions, not are partitions.

Demo manually worked out in comments:
https://git.libre-soc.org/?p=ieee754fpu.git;a=blob;f=src/ieee754/part/layout_experiment.py;h=6c32022ec750433ff5adca21f4345cfca51181e2;hb=HEAD#l324

# combined with vec_el_counts {0:1, 1:1, 2:2, 3:4} we have:
# elwidth=0b00 1x 24-bit
# elwidth=0b01 1x 12-bit
# elwidth=0b10 2x 5-bit
# elwidth=0b11 4x 6-bit
#
# bmask<--------1<----0<---------10<---0<-------1<0<----0<---0<----00<---0
# always unused:|     |     |    ||    |    |   | |     |    |     ||    |
#      1111111111000000 1111111111000000 1111111100000000 0000000000000000
#               |     |     |    ||    |    |   | |     |    |     ||    |
# 0b00 xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx xxxxxxxx........ ..............24|
# 0b01 xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx xxxx..........12|
# 0b10 xxxxxxxxxxxxxxxx xxxxxxxxxxx....5|xxxxxxxxxxxxxxxx xxxxxxxxxxx....5|
# 0b11 xxxxxxxxxx.....6|xxxxxxxxxx.....6|xxxxxxxxxx.....6|xxxxxxxxxx.....6|
#               ^     ^          ^^    ^        ^ ^     ^    ^     ^^
#     ppoints:  |     |          ||    |        | |     |    |     ||
#               |  bit-48        /\    | bit-24-/ |     | bit-12   /\-bit-5
#            bit-54      bit-38-/  \ bit-32       |   bit-16      /
#                                 bit-37       bit-22          bit-6

So, in that example bmask = 0b101001000000

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