[libre-riscv-dev] number of stages just increased to 6 per pipe-stage

[Jacob Lifshay]

nm, was interpreting n_comb_stages as the total number of pipeline stages.

On Mon, Jul 29, 2019, 06:59 Jacob Lifshay <programmerjake at gmail.com> wrote:

> I think you're looking at the diff backwards by mistake. when i
> implemented frsqrt, I switched it to cram everything into 2 pipeline stages
> as a debugging aid. the commit I just made switches it to what we had
> agreed on in the gflops thread: 2 calc stages per pipeline stage.
>
> https://git.libre-riscv.org/?p=ieee754fpu.git;a=blobdiff;f=src/ieee754/fpdiv/pipeline.py;h=bcd99e26dddcffbf35aa6cfe75511312149022d8;hp=68d07eefdb4651cc2b961700f295eac727fbd6ed;hb=30a3d1e975cd3df7ea701fdf0fe3bd0ba0757d3e;hpb=7c44b247e63bdbe1121819b114efeb9b67e7f4a6
>
> On Mon, Jul 29, 2019, 06:52 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> wrote:
>
>> hi jacob,
>>
>> +        # TODO (depends on how many RS's we want)
>> +        #n_comb_stages = width // (2 * log2_radix)  # 2 compute steps
>> per stage
>> +        n_comb_stages = 2  # FIXME: switch back
>> +
>>          fraction_width = fmt.fraction_width
>>
>>          # extra bits needed: guard + round
>> @@ -172,8 +176,6 @@ class FPDIVMuxInOut(ReservationStations):
>>          # the last stage
>>          cfg = DivPipeCoreConfig(fmt.width, fraction_width, log2_radix)
>>
>> -        n_comb_stages = (cfg.n_stages + 1) // 2  # 2 compute steps per
>> stage
>> -
>>          self.pspec.fpformat = fmt
>>          self.pspec.n_comb_stages = n_comb_stages
>>          self.pspec.core_config = cfg
>>
>>
>> what that's done is, well, you can see in the FPDIV32 pipeline, from
>> the attached: it's now *seven* combinatorial stages long (!!)
>>
>> the number of combinatorial stages sets the "safe limit" for the gate
>> count.  if it's not a fixed quantity, then as the bitwidth increases,
>> so will the number of combinatorial stages chained together into a
>> single pipeline stage.
>>
>> so, for FPDIV32, because n_comb_stages is set to the total number of
>> *required* stages divided by two, that will be *FOURTEEN*
>> combinatorial blocks jammed into each pipeline stage.
>>
>> the change that you made specifies, by mistake, that the *entire
>> pipeline* must be no bigger than 2 deep.
>>
>> l.
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>