[libre-riscv-dev] [Bug 321] New: BUG: Running ltp on a pipeline gives warnings about loops

[libre-riscv-dev] [Bug 321] New: BUG: Running ltp on a pipeline gives warnings about loops

[bugzilla-daemon]

https://bugs.libre-soc.org/show_bug.cgi?id=321

            Bug ID: 321
           Summary: BUG: Running ltp on a pipeline gives warnings about
                    loops
           Product: Libre-SOC's first SoC
           Version: unspecified
          Hardware: PC
                OS: Windows
            Status: CONFIRMED
          Severity: normal
          Priority: ---
         Component: Source Code
          Assignee: lkcl@lkcl.net
          Reporter: mtnolan2640@gmail.com
                CC: libre-riscv-dev@lists.libre-riscv.org
   NLnet milestone: ---

Created attachment 55
  --> https://bugs.libre-soc.org/attachment.cgi?id=55&action=edit
Photo of loop in yosys graph

Running `ltp` in yosys on a pipeline gives many warnings about loops being
detected. For instance, running yosys -p `read_ilang logical_pipeline.il;
synth; ltp` in soc/fu/logical/test gives the following output:

yosys> ltp

3. Executing LTP pass (find longest path).

Longest topological path in input (length=4):
    0: \op__input_carry [1]
    1: $abc$6362$new_n198_ (via
$abc$6362$auto$blifparse.cc:377:parse_blif$6427)
    2: $abc$6362$new_n199_ (via
$abc$6362$auto$blifparse.cc:377:parse_blif$6428)
    3: $abc$6362$new_n201_ (via
$abc$6362$auto$blifparse.cc:377:parse_blif$6430)
    4: \carry_in$25 (via $abc$6362$auto$blifparse.cc:377:parse_blif$6432)

Longest topological path in output (length=9):
    0: \stage_o [63]
<snip>
    9: \cr0$24 [3] (via $abc$8116$auto$blifparse.cc:377:parse_blif$8261)

Longest topological path in main (length=34):
    0: \op__insn_type [5]
   <snip>
   34: \stage_o [5] (via $abc$6433$auto$blifparse.cc:377:parse_blif$7943)

Longest topological path in countz (length=27):
    0: \rs_i [47]
<snip>
   27: \result_o [0] (via $abc$6123$auto$blifparse.cc:377:parse_blif$6352)

Warning: Detected loop at \stage_o$next [63] in pipe
Warning: Detected loop at \stage_o$next [62] in pipe
Warning: Detected loop at \stage_o$next [61] in pipe
Warning: Detected loop at \stage_o$next [60] in pipe
<snip>
Warning: Detected loop at \stage_o$next [0] in pipe
Warning: Detected loop at \so$23$next in pipe
Warning: Detected loop at \r_busy$next in pipe
Warning: Detected loop at \op__insn$20$next [31] in pipe
Warning: Detected loop at \op__insn$20$next [30] in pipe
Warning: Detected loop at \op__insn$20$next [29] in pipe
Warning: Detected loop at \op__insn$20$next [28] in pipe
...


Graphing the module (show pipe) shows a couple of loops in the top level pipe
object, outside of the stages

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[libre-riscv-dev] [Bug 321] BUG: Running ltp on a pipeline gives warnings about loops

[bugzilla-daemon]

https://bugs.libre-soc.org/show_bug.cgi?id=321

--- Comment #1 from Luke Kenneth Casson Leighton <lkcl@lkcl.net> ---
(In reply to Michael Nolan from comment #0)
> Created attachment 55 [details]
> Photo of loop in yosys graph

> Warning: Detected loop at \stage_o$next [63] in pipe

ok yes this is fine.

look closely at the MUXes.  note that when the front mux is set, the register
is outputted (from the DFF, which is on a clock latch).

however if the MUX is not set, the input is sent through combinatorially.

so the "loop" is not a combinatoriak looo, it is a time synced loop that traps
the value of the input, endlessly replaying it until such time as soneone sets
the MUX to low.

there is probably a better way to do this  however it is not doing any harm.

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