[llvm-dev] DominatorTree, JumpThreading and EarlyCSE non-determinism

Tue Sep 21 08:03:07 PDT 2021

Hi Jakub,

I imagine sorting the children would solve the problem with non-determinism for the test case (just like it helps to invalidate/recalculate the DomTree before EarlyCSE). I haven’t tested it though (I’m not quite sure how to get the basic block order number quickly for a BasicBlock).

Although, that still doesn’t solve the potential problem (fault?) that the nodes aren’t inserted in those vectors with child nodes in the first place.

And neither does it help EarlyCSE to find the optimal order of iterating through child nodes (to eliminate all possible PHI-nodes in my example). I.e. the outcome from EarlyCSE would still depend on the sorting order for the DomTree with such a solution. But maybe that just is how the algorithm in EarlyCSE is supposed to work.

/Björn

From: Jakub (Kuba) Kuderski <kubakuderski at gmail.com>
Sent: den 21 september 2021 16:27
To: Björn Pettersson A <bjorn.a.pettersson at ericsson.com>
Cc: Roman Lebedev <lebedev.ri at gmail.com>; llvm-dev <llvm-dev at lists.llvm.org>; Johan Ringström <johan.ringstrom at ericsson.com>
Subject: Re: [llvm-dev] DominatorTree, JumpThreading and EarlyCSE non-determinism

Hi Björn,

From the perspective of DomTree, node children are generally unordered. I think a pass can be sensitive to DT order by either directly relying on tree children order or on DFS numbers.
If that's the case, one workaround would be to re-order children based on the block order in the parent. If that's a common enough requirement, we could add it to DT.

Would that solve the problem?
-Jakub

On Tue, Sep 21, 2021 at 9:33 AM Björn Pettersson A via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Yes, the IR looks the same out from jump-threading for this specific input (at least when comparing output from -print-after-all).

It is the order of the DominatorTree updates that I suspect differ a bit (at least according to the -debug printouts), giving slightly different node orders in the DominatorTree. And I have no idea if JumpThreading can be blamed for that or if it is the Lazy strategy in the DomTreeUpdater.

I guess I'll file a PR for this. But I'm still not quite sure what the rules are here (if both passes are wrong, or which one to blame), and what to expect more generally in situation like this one.

/Björn

> -----Original Message-----
> From: Roman Lebedev <lebedev.ri at gmail.com<mailto:lebedev.ri at gmail.com>>
> Sent: den 21 september 2021 15:15
> To: Björn Pettersson A <bjorn.a.pettersson at ericsson.com<mailto:bjorn.a.pettersson at ericsson.com>>
> Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Johan Ringström
> <johan.ringstrom at ericsson.com<mailto:johan.ringstrom at ericsson.com>>
> Subject: Re: [llvm-dev] DominatorTree, JumpThreading and EarlyCSE non-
> determinism
>
> Does JumpThreading produce exactly the same IR in all of the situations?
> But even if it does, this does seem like a bug.
>
> Roman
>
> On Tue, Sep 21, 2021 at 4:10 PM Björn Pettersson A via llvm-dev
> <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
> >
> > Hello llvm-dev,
> >
> > Recently I've been debugging a non-determinism problem.
> >
> > I've managed to narrow it down to running:
> >
> >  opt -passes='function(jump-threading,print<domtree>,early-cse)'
> >
> >
> > From debug printouts (also adding -debug to the cmd line) it looks like
> jump-threading is doing doing dominator tree updates in a non-deterministic
> order (when comparing different runs).
> >
> > I randomly get either of these two DominatorTrees in the print<domtree>
> printouts:
> >
> > DominatorTree for function: g
> > =============================--------------------------------
> > Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
> >   [1] %entry {4294967295,4294967295} [0]
> >     [2] %for.cond1 {4294967295,4294967295} [1]
> >       [3] %if.then {4294967295,4294967295} [2]
> >         [4] %for.cond5.preheader {4294967295,4294967295} [3]
> >           [5] %cleanup {4294967295,4294967295} [4]
> >             [6] %cleanup16 {4294967295,4294967295} [5]
> >               [7] %unreachable {4294967295,4294967295} [6]
> >               [7] %for.end21 {4294967295,4294967295} [6]
> >           [5] %for.body7 {4294967295,4294967295} [4]
> >             [6] %for.inc {4294967295,4294967295} [5]
> >           [5] %return {4294967295,4294967295} [4]
> >       [3] %cleanup16.thread {4294967295,4294967295} [2]
> >       [3] %for.inc19 {4294967295,4294967295} [2]
> >     [2] %for.cond {4294967295,4294967295} [1]
> > Roots: %entry
> >
> > DominatorTree for function: g
> > =============================--------------------------------
> > Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
> >   [1] %entry {4294967295,4294967295} [0]
> >     [2] %for.cond1 {4294967295,4294967295} [1]
> >       [3] %for.inc19 {4294967295,4294967295} [2]
> >       [3] %if.then {4294967295,4294967295} [2]
> >         [4] %for.cond5.preheader {4294967295,4294967295} [3]
> >           [5] %cleanup {4294967295,4294967295} [4]
> >             [6] %cleanup16 {4294967295,4294967295} [5]
> >               [7] %unreachable {4294967295,4294967295} [6]
> >               [7] %for.end21 {4294967295,4294967295} [6]
> >           [5] %for.body7 {4294967295,4294967295} [4]
> >             [6] %for.inc {4294967295,4294967295} [5]
> >           [5] %return {4294967295,4294967295} [4]
> >       [3] %cleanup16.thread {4294967295,4294967295} [2]
> >     [2] %for.cond {4294967295,4294967295} [1]
> > Roots: %entry
> >
> >
> > I think both trees are correct, the nodes are just in a different order.
> This can also be seen if I add invalidate<domtree> before print<domtree> to
> force a recalculation. Then it will look like this instead (same content,
> but the level 3 nodes printed in yet another order):
> >
> > DominatorTree for function: g
> > =============================--------------------------------
> > Inorder Dominator Tree: DFSNumbers invalid: 0 slow queries.
> >   [1] %entry {4294967295,4294967295} [0]
> >     [2] %for.cond1 {4294967295,4294967295} [1]
> >       [3] %cleanup16.thread {4294967295,4294967295} [2]
> >       [3] %for.inc19 {4294967295,4294967295} [2]
> >       [3] %if.then {4294967295,4294967295} [2]
> >         [4] %for.cond5.preheader {4294967295,4294967295} [3]
> >           [5] %cleanup {4294967295,4294967295} [4]
> >             [6] %cleanup16 {4294967295,4294967295} [5]
> >               [7] %unreachable {4294967295,4294967295} [6]
> >               [7] %for.end21 {4294967295,4294967295} [6]
> >           [5] %return {4294967295,4294967295} [4]
> >           [5] %for.body7 {4294967295,4294967295} [4]
> >             [6] %for.inc {4294967295,4294967295} [5]
> >     [2] %for.cond {4294967295,4294967295} [1]
> > Roots: %entry
> >
> >
> > *** Question one ***
> > Maybe it is OK from a correctness point-of-view that JumpThreading isn't
> producing the same node order every time (given same input), even though it
> might be a bit confusing when debugging. Or should this be seen as a bug?
> >
> >
> > Next problem/question is related to EarlyCSE. It looks like the output
> from EarlyCSE depends on the node order in the dominator tree. So depending
> on if JumpThreading has produced the first or second of the DominatorTree
> structures above we might end up eliminating one more/less PHI node during
> EarlyCSE.
> >
> > *** Question two ***
> > Should be seen as a bug? Or are passes in general sensitive to how
> analysis information is produced (such as the node order in a dominator
> tree) and thus being allowed to produce better/worse code depending on such
> things?
> >
> >
> > To summarize:
> > JumpThreading is non-deterministic but produces semantically "equivalent"
> results.
> > EarlyCSE is deterministic, but the result depends on order of nodes in
> the DominatorTree.
> > Those two things together gives a non-deterministic IR result. And I
> figure that should be seen as a bug. Just not sure if the fault is in
> DominatorTreeUpdater, JumpThreading or EarlyCSE (or if it should be seen as
> multiple faults).
> >
> > Regards,
> > Björn
> > _______________________________________________
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--
Jakub Kuderski
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