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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Cost of handling misaligned access
Date: Tue, 25 Feb 2025 18:07:10 +0000
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On Tue, 25 Feb 2025 14:20:45 +0000, EricP wrote:

> Michael S wrote:
--------------------
>>
>> No, I mean florplanning, as well as most other manual physical-level
>> optimization are not used at all in 99% percents of FPGA designs that
>> started after year 2005.
>
> Is that because the auto place and route got good enough that it is
> unnecessary? Or maybe the fpga resources grew enough that autoroute
> didn't have to struggle to find optimal positions and paths
> (being an optimal packing problem and a traveling salesman problem).

Athlon (1998) used hand place auto-route. So, auto-route has been
good enough since 2000 at latest.

> Also BGB mentioned in another thread a while back that he was getting
> what sounded like random variation of critical paths from run to run.
> That suggests to me the automatic tools may not be properly recognizing
> the different modules and produce some non-optimal positions or paths.
> So giving it a hint that "this stuff goes together" might help.

Consider the optimizer/place/route thingamabob; and a signal that
crosses from one module to another. The optimizer changes from
a 2-LUT delay to a 1 LUT delay, but now the fan-out of that LUT
doubles, so instead of speeding up, the signal path slows down.

> Anyway, it should be testable. Inspect the auto placements module wiring
> and if there are any obviously crazy decision then try the placement
> tool
> an see if the speed improves or critical path variation goes away.