[Mesa-dev] Value Range Propagation in NIR (GSoC)

[Jason Ekstrand]

On Tue, Apr 7, 2015 at 4:52 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> Hi Thomas,
>
> Thanks for submitting a proposal! Some comments/answers below.
>
> On Tue, Apr 7, 2015 at 3:34 PM, Thomas Helland
> <thomashelland90 at gmail.com> wrote:
>> Hi,
>>
>> For those that don't know I've submitted a proposal for this years GSoC.
>> I've proposed to implement value range propagation and loop unrolling in
>> NIR.
>> Since I'm no expert on compilers I've read up on some litterature:
>>
>> I started with "Constant propagation with conditional branches"  (thanks
>> Connor).
>> This paper describes an algorithm, "sparse conditional constant
>> propagation",
>> that seems to be the defacto standard in compilers today.
>>
>> I also found the paper;
>> "Accurate static branch prediction by value range propagation " (VRP).
>> This describes a value range propagation implementation based on SCCP.
>> (This also allows one to set heuristics to calculate educated guesses for
>> the
>> probability of a certain branch, but that's probably more than we're
>> interested in.)
>
> Thanks for mentioning that... I had forgotten the name of that paper.
> You're right in that the branch probability stuff isn't too useful for
> us. Also, it raises an important issue about back-edges from phi
> nodes; they present a more sophisticated method to handle it, but I
> think that for now we can just force back edges to have an infinite
> range unless they're constant.
>
>>
>> There is also a GCC paper (with whatever licensing issues that may apply);
>> "A propagation engine for GCC".
>> They have a shared engine for doing all propagation passes.
>> It handles the worklists, and the logic to traverse these.
>> The implementing passes then supply callbacks to define the lattice rules.
>> They reply back if the instruction was interesting or not,
>> and the propagation engine basically handles the rest.
>>
>> Maybe that's an interesting solution? Or it might not be worth the hassle?
>> We already have copy propagation, and with value range propagation
>> we probably don't want separate constant propagation?
>> (I'm hoping to write the pass so that it handles both constants and value
>> ranges.)
>
> Yes, constant propagation probably won't be so useful once we have value
> range propagation; the former is a special case of the latter. Note
> that we have a nifty way of actually doing the constant folding
> (nir_constant_expressions.py and nir_constant_expressions.h), which
> you should still use if all the inputs are constant.

When I started taking a stab at range propagation, I started by trying
to extend the constant folding framework.  I had a patch
(http://cgit.freedesktop.org/~jekstrand/mesa/log/?h=wip/nir-minmax)
but it doesn't do nearly as much as I remembered.  I don't know if
it's practical to try and extend it or if we're better off just
hand-rolling whatever we do for range handling.

>> The GCC guys have used this engine to get copy propagation that propagates
>> copies accross conditionals, maybe this makes such a solution more
>> interesting?
>
> I'm not so sure how useful such a general framework will be. Constant
> propagation that handles back-edges seems interesting, but I'm not
> sure it's worth the time to implement something this general as a
> first pass.

Agreed.  Let's just get it working first.

>>
>> Connor: I just remembered you saying something about your freedesktop
>> git repo, so I poked around some and found that you have already done
>> some work on VRP based on SCCP? How far did you get?
>
> I started on it, but then I realized that the approach I was using was
> too cumbersome/complicated so I don't think what I have is too useful.
> Feel free to work on it yourself, although Jason and I have discussed
> it so we have some ideas of how to do it. I've written a few notes on
> this below that you may find useful.
>
> - I have a branch I created while working on VRP that you'll probably
> find useful: http://cgit.freedesktop.org/~cwabbott0/mesa/log/?h=nir-worklist
> . The first two commits are already in master, but the last two should
> be useful for implementing SCCP/VRP (although they'll need to be
> rebased, obviously).
>
> - There's a comment in the SCCP paper (5.3, Nodes versus Edges) that
> says: "An alternative way of implementing this would be to add nodes
> to the
> graph and then associate an ExecutableFlag with each node. An
> additional node must be inserted between any node that has more than
> one immediate successor and any successor node that has more than one
> immediate predecessor." I think this procedure is what's usually
> called "splitting critical edges"; in NIR, thanks to the structured
> control flow, there are never any critical edges except for one edge
> case you don't really have to care about too much (namely, an infinite
> loop with one basic block) and therefore you can just use the basic
> block worklist that I added in the branch mentioned above, rather than
> a worklist of basic block edges as the paper describes.
>
> - The reason my pass was becoming so cumbersome was because I was
> trying to solve two problems at once. First, there's actually
> propagating the ranges. Then, there's taking into account restrictions
> on range due to branch predicates. For example, if I have something
> like:
>
> if (x > 0) {
>     y = max(x, 0);
> }
>
> then since the use of x is dominated by the then-branch of the if, x
> has to be greater than 0 and we can optimize it. This is a little
> contrived, but we have seen things like:
>
> if (foo)
>     break;
>
> /* ... */
>
> if (foo)
>     break;
>
> in the wild, where we could get rid of the redundant break using this
> analysis by recognizing that since the second condition is dominated
> by the else-branch of the first, foo must be false there. I was trying
> to handle this by storing multiple lattice entries for the same SSA
> value, but it was becoming too messy. Instead, we can solve the first
> problem normally, and then to solve the second problem we can create
> new SSA values, using the standard SSA construction algorithm, any
> time where after a certain point the range of a value is restricted
> (namely, the condition of a branch or the index of an array
> dereference). In the first example, we would create a new value x2:
>
> if (x > 0) {
>     x2 = x;
>     y = max(x2, 0);
> }

This should be easy enough to do.  We already have a
nir_ssa_def_rewrite_uses function.  We would just have to extend it to
nir_ssa_def_rewrite_uses_dominated_by to handle this case but it
shouldn't be hard.

> and the VRP pass will keep track of "special" copies like the one from
> x to x2 that add restrictions on the range. After everything is
> finished, copy propagation and DCE will clean up the extra copies
> created. There's a paper on this somewhere, but I don't quite remember
> the name of it. I'm not sure if you'll be able to get to this over the
> summer, but I thought I'd explain it in case you were interested.

When I was thinking about it, I had a convoluted scheme involving a
stack of "contexts" to handle this situation.  I was also trying to
handle propagating the range information implied by a select to its
arguments.  I'd love to chat about it, but It never ended up being
code so I'll leave it alone unless you really want to discuss my
hair-brained ideas.

>> If we just want to make an SCCP inspired VRP pass then Connor has work in
>> progress.
>> Finishing that, and loop unrolling, may not be enough work for GSoC?
>
> I'm not sure... on the one hand, there's enough here that it may take
> the entire summer for you to do. On the other hand, it's possible that
> you'll be able to finish it with enough time left over, and there are
> plenty of other things you'd be able to do. It depends on several
> factors, and no one has a crystal ball here. I'm not experienced
> enough with GSoC to be able to give you a recommendation, so it would
> be nice for someone more experienced to give their opinion.

Yeah, let's just go with what we have.  I'm ok with calling range
propagation + loop unrolling the entire GSoC project.  If you have
extra time and want to work on something else, there's plenty to do
and no one will stop you.
--Jason