[libre-riscv-dev] div/mod algorithm written in python

Luke Kenneth Casson Leighton lkcl at lkcl.net
Thu Jul 25 15:44:56 BST 2019

On Thu, Jul 25, 2019 at 2:13 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
> I got everything working!


> I disabled the frsqrt test for fp64 since we
> don't have a proper reference implementation yet.

well with the others working, how about a test that at least gets within 2 bits?

> fp32 and fp16 seem
> to work well enough for now by emulating using fp64 arithmetic, but

 yehyeh, i got it - i thought about doing that, and how if there was
fp128 it could be used for all of them.  hmm...

> it was a single giant messy commit, so I pushed to the
> messy-div-pipe-works branch instead of master.
> I'll split it up properly tomorrow.


             # create (in each pipe) a StageChain n_comb_stages in length
             pipechain.append(kls(self.pspec, n_comb_stages, stage_idx))
-            stage_idx += n_comb_stages  # increment so that each CalcStage
-            # gets a (correct) unique index
+            stage_idx += n_comb_stages # increment so that each CalcStage
+                                       # gets a (correct) unique index

you ran autopep8, didn't you? :)  i like lining up comments, however
it's not pep8-compliant.

couple of spaces have been added (on the end of lines), like the
addition of the names (pain in the neck having z$0 z$1 z$2),

> Would you consider this good enough to declare the frsqrt, fsqrt, and
> fdiv bugs resolved for now?

 once the unit tests have been re-run (minimum 20,000 random runs,
increase n_vals=1000 on the "coverage" tests), yes.  i can run some of
them, if you do others.

> I still want to build in SIMD capabilities

 absolutely counting on that - as separate bugreports.

> like the multiplier and
> support fp64 operations by running them through the pipeline twice.

 yehyeh that'd be really good - have to think how it can be done in a
way that does *not* require 2 separate micro-ops.  a hardware
feedback-er.  that way the ReservationStation stays "reserved".


> Also, integer division is not implemented yet.

 (to go under http://bugs.libre-riscv.org/show_bug.cgi?id=116)

 yes.  this one will be fun - being able to set the op to FP or INT
and bypass the FP normalisation and post-processing.  that's this one:


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