Re: Fwd: Information regarding scheduler description

[Vladimir Makarov <vmakarov@xxxxxxxxxx>]

On 01/05/2012 08:01 AM, ganesh gopalasubramanian wrote:
> Hi Makarov,
>
> I posted a mail couple of days back.
> Could you throw some light on this?
>
> Thanks
> Ganesh
>
>
> ---------- Forwarded message ----------
> From: ganesh gopalasubramanian<gganeshgcc@xxxxxxxxx>
> Date: Mon, Jan 2, 2012 at 12:22 PM
> Subject: Information regarding scheduler description
> To: gcc-help@xxxxxxxxxxx
>
>
> Hi,
>
> I am trying to describe my processor pipeline.
>
> Consider that I am having 5 instructions (instruction1,
> instruction2... instruction5).
> For four of those instructions, I have described the instruction
> reservations along with the functional units and their alternatives
> available for each cycle.
> For the fifth instruction, I haven't described the instruction reservation.
> What will happen now? Will the instruction be stalling my pipeline.
>
> Consider that five instruction instruction are generated as below.
>
> instruction1,
> instruction2,
> instruction3,
> instruction4,
> instruction5
>
> My scheduler description supports the following
>
> 1. instruction1&  instruction2 can be issued as a single bundle.
> 2. instruction3&  instruction4 can be issued as a single bundle.
>
> Now, I don't have description about instruction5, will it stall my
> pipeline and get issued as a separation instruction bundle (assuming
> issue rate as 1)?
As I remember that absence of description means no reservation at all 
and if all input data for such insn are ready and there is no constraint 
from the issue rate, the scheduler will consider that there is no 
pipeline stall at all and will issue the insn at any time.

The issue rate is upper bound for number of real insns (which excludes 
USE/CLOBBER and debug insns) can be issued on the cycle.  So if you set 
it to 1, the scheduler believes that each insn starts on a new simulated 
cycle.  In this case, insns #1, #2, #3, and #4 will be considered to be 
issued on new cycle too.  I guess that is not what you want as I 
understand the bundle means issuing insns in the bundle on the same cycle.

You should always describe each insn reservation unless setting the 
issue rate is not enough for describing issue rules for specific insn.
> Or should I describe the instruction reservation (for instruction5)
> separately along with stall information?
>
> Regards
> Ganesh