llvm dev - Apr 2011 - [LLVMdev] LLVM Scheduler and Itinieraries: Negative latency?

Hello,

While trying to create back end in LLVM I have stumbled upon a problem  
I have trouble to get past, hopefully someone can give me hints on  
what I am doing wrong. The problem is that the assertion in the file  
ScheduleDAGList.cpp row 187 is triggered: "Negative latency". How does
this happen?

As background:
My target has one issue unit, therefore my Schedule.td file only  
contain one functional unit.
My instruction itineraries are defined to all take 1 machine cycle to  
complete (my target is fully pipelined) but with values 2 and 3  
specifying when the result is ready (not all instructions have  
forwarded results) and 2 as parameter for when the operands are read.

mvh (kind regards)
  -Magnus

Hello Magnus,
> My instruction itineraries are defined to all take 1 machine cycle to
> complete (my target is fully pipelined) but with values 2 and 3
> specifying when the result is ready (not all instructions have
> forwarded results) and 2 as parameter for when the operands are read.
Does this mean that your instruction always have single cycle delay
slot? The latency right now is definitely negative, since you
specified that the instruction takes single cycle, but operands are
read on the second cycle.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

Forwarding to llvm-dev...


---------- Forwarded message ----------
From: Magnus Pettersson <mangepe at gmail.com>
Date: Thu, Apr 14, 2011 at 21:33
Subject: Re: [LLVMdev] LLVM Scheduler and Itinieraries: Negative latency?
To: Anton Korobeynikov <anton at korobeynikov.info>


Hello Anton,
I am trying to model a fairly simple five stage pipelined processor.
The problem is that some instructions need the last stage (write back)
to be finished so the correct operand is selected for a following
instruction in stage 3. So yes, I guess it is a delay slot between two
such instructions.  I choose to model this with just one functional
unit called "pipe". If I set the parameter machine cycles to anything
higher than one, the scheduler think that the instruction occupy the
functional unit for that many cycles but in my case this is not true,
the "pipe" is pipelined. Structural hazards cannot occur in the
pipeline, but data hazards can. I looked at the example in
include/llvm/Target/TargetScheme.td, in that example a value of 1 for
machine cycles and higher values for when the result is ready (3) and
when the operands are referenced (2) are showed. Do you have any hints
on what I am doing wrong, since the example seem to show that this
"negative latency" should work?


kind regards
 -Magnus

14 apr 2011 kl. 17.04 skrev Anton Korobeynikov:
> Hello Magnus,
>
>> My instruction itineraries are defined to all take 1 machine cycle to
>> complete (my target is fully pipelined) but with values 2 and 3
>> specifying when the result is ready (not all instructions have
>> forwarded results) and 2 as parameter for when the operands are read.
>
> Does this mean that your instruction always have single cycle delay
> slot? The latency right now is definitely negative, since you
> specified that the instruction takes single cycle, but operands are
> read on the second cycle.
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University



-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

Hello Magnus,
> I am trying to model a fairly simple five stage pipelined processor.
Ok.
> The problem is that some instructions need the last stage (write back) to
be
> finished so the correct operand is selected for a following instruction in
> stage 3.
Ok, this is pretty typical.
> machine cycles and higher values for when the result is ready (3) and when
> the operands are referenced (2) are showed. Do you have any hints on what I
> am doing wrong, since the example seem to show that this "negative
latency"
> should work?
So, in your case the operands are read in the second cycle, then
several cycles the instruction occupies the functional unit and after
this at some cycle the results are written back. Is this correct?

If yes, then I think you should model stuff with two stages. First
stage will be "issue" with 1 cycle latency Second stage will be the
actual "execution unit", for which you can have, say, 1 cycle duration
of the stage and operands read in the beginning of the stage and
available at, say, 2 cycle from the beginning of the stage.

You might want to look into ARM backend as an example of target with
rather complex scheduling.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University