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

From: bugzilla-daemon@libre-soc.org
To: libre-riscv-dev@lists.libre-riscv.org
Subject: [libre-riscv-dev] [Bug 321] BUG: Running ltp on a pipeline gives warnings about loops
Date: Mon, 18 May 2020 15:57:37 +0000
Message-ID: <bug-321-13-mTt2vw6rhO@https.bugs.libre-soc.org/> (raw)
In-Reply-To: <bug-321-13@https.bugs.libre-soc.org/>

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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