Intended behavior of CGSCC pass manager.

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From: “Sean Silva via llvm-dev” <llvm-dev@lists.llvm.org>
To: “llvm-dev” <llvm-dev@lists.llvm.org>
Sent: Wednesday, June 8, 2016 6:19:03 AM
Subject: [llvm-dev] Intended behavior of CGSCC pass manager.

Hi Chandler, Philip, Mehdi, (and llvm-dev,)

(this is partially a summary of some discussions that happened at the last LLVM bay area social, and partially a discussion about the direction of the CGSCC pass manager)

A the last LLVM social we discussed the progress on the CGSCC pass manager. It seems like Chandler has a CGSCC pass manager working, but it is still unresolved exactly which semantics we want (more about this below) that are reasonably implementable.

AFAICT, there has been no public discussion about what exact semantics we ultimately want to have. We should figure that out.

The main difficulty which Chandler described is the apparently quite complex logic surrounding needing to run function passes nested within an SCC pass manager, while providing some guarantees about exactly what order the function passes are run. The existing CGSCC pass manager just punts on some of the problems that arise (look in CGPassManager::runOnModule, CGPassManager::RunAllPassesOnSCC, and CGPassManager::RunPassOnSCC in llvm/lib/Analysis/CallGraphSCCPass.cpp), and these are the problems that Chandler has been trying to solve.

(
Why is this “function passes inside CGSCC passes” stuff interesting? Because LLVM can do inlining on an SCC (often just a single function) and then run function passes to simplify the function(s) in the SCC before it tries to inline into a parent SCC. (the SCC visitation order is post-order)
For example, we may inline a bunch of code, but after inlining we can tremendously simplify the function, and we want to do so before considering this function for inlining into its callers so that we get an accurate evaluation of the inline cost.
Based on what Chandler said, it seems that LLVM is fairly unique in this regard and other compilers don’t do this (which is why we can’t just look at how other compilers solve this problem; they don’t have this problem (maybe they should? or maybe we shouldn’t?)). For example, he described that GCC uses different inlining “phases”; e.g. it does early inlining on the entire module, then does simplifications on the entire module, then does late inlining on the entire module; so it is not able to incrementally simplify as it inlines like LLVM does.

This incremental simplification is an important feature of our inliner, and one we should endeavor to keep. We might also want different phases at some point (e.g. a top-down and a bottom-up phase), but that’s another story.

)

As background for what is below, the LazyCallGraph tracks two graphs: the “call graph” and the “ref graph”.
Conceptually, the call graph is the graph of direct calls, where indirect calls and calls to external functions do not appear (or are connected to dummy nodes). The ref graph is basically the graph of all functions transitively accessible based on the globals/constants/etc. referenced by a function (e.g. if a function foo references a vtable that is defined in the module, there is an edge in the ref graph from foo to every function in the vtable).
The call graph is a strict subset of the ref graph.

Chandler described that he had a major breakthrough in that the CGSCC pass manager only had to deal with 3 classes of modifications that can occur:

  • a pass may e.g. propagate a load of a function pointer into an indirect call, turning it into an direct call. This requires adding an edge in the CG but not in the ref graph.
  • a pass may take a direct call and turn it into an indirect call. This requires removing an edge from the CG, but not in the ref graph.
  • a pass may delete a direct call. This removes an edge in the CG and also in the ref graph.

From the perspective of the CGSCC pass manager, these operations can affect the SCC structure. Adding an edge might merge SCC’s and deleting an edge might split SCC’s. Chandler mentioned that apparently the issues of splitting and merging SCC’s within the current infrastructure are actually quite challenging and lead to e.g. iterator invalidation issues, and that is what he is working on.

(
The ref graph is important to guide the overall SCC visitation order because it basically represents “the largest graph that the CG may turn into due to our static analysis of this module”. I.e. no transformation we can statically make in the CGSCC passes can ever cause us to need to merge SCC’s in the ref graph.
)

I have a couple overall questions/concerns:

  1. The ref graph can easily go quadratic. E.g.

typedef void (*fp)();
fp funcs = {
&foo1,
&foo2,
…
&fooN
}
void foo1() { funcssomething; }
void foo2() { funcssomething; }
…
void fooN() { funcssomething; }

One real-world case where this might come about is in the presence of vtables.

The existing CGSCC pass manager does not have this issue AFAIK because it does not consider the ref graph.

Does anybody have any info/experience about how densely connected the ref graph can get in programs that might reasonably be fed to the compiler?
I just did a quick sanity check with LLD/ELF using --lto-newpm-passes=cgscc(no-op-cgscc) and it at least seemed to terminate / not run out of memory. Based on some rough calculations looking at the profile, it seem like the entire run of the inliner in the old LTO pipeline (which is about 5% of total LTO time on this particular example I looked at) is only 2-3x as expensive as just --lto-newpm-passes=cgscc(no-op-cgscc), so the LazyCallGraph construction is certainly not free.

  1. What is the intended behavior of CGSCC passes when SCC’s are split or merged? E.g. a CGSCC pass runs on an SCC (e.g. the inliner). Now we run some function passes nested inside the CGSCC pass manager (e.g. to simplify things after inlining). Consider:

This is not how I thought the current scheme worked :wink: – I was under the impression that we had a call graph with conservatively-connected dummy nodes for external/indirect functions.

The fact that we have a separate nodes for calling into an external function and “being called” from an external function, these don’t form SCC. So it means we can end up merging SCC IIUC.

Yes, although I thought there was only one dummy node for those things.

-Hal

2

Hi,

Does it make sense to change RefSCCs to hold a list of
RefSCC-DAG-Roots that were split out of it because of edge deletion?
Then one way to phrase the inliner/function pass iteration would be
(assuming I understand the issues):

  Stack.push(RefSCC_Leaves);
  while (!Stack.empty()) {
    RefSCC = Stack.pop();
    InlineCallSites(RefSCC);
    if (!RefSCC.splitOutSCCs.empty())
      goto repush;
    for each func in RefSCC:
      FPM.run(func);
      if (!RefSCC.splitOutSCCs.empty())
        goto repush;
    continue;
repush:
     for (refscc_dag_root in RefSCCs.splitOutSCCs)
       // here we don't want to push every leaf, but leafs that
       // have functions that haven't had the FPM run on (maybe we can
do this by maintaining a set?)
       // if we don't push a leaf, we push its parent (which we want
to push even if we've run FPM on it
       // since we'd like to re-run the inliner on it).
       refscc_dag_root.push_leaves_to(Stack)
   }

(I know this isn't ideal, since now RefSCC is no longer "just a data
structure", but is actually has incidental information.)

3

I'm not sure what you mean above, but IIUC I think it is a bit more complex since the inliner runs at the same place you put FPM.run().

4

Hi,

Does it make sense to change RefSCCs to hold a list of
RefSCC-DAG-Roots that were split out of it because of edge deletion?
Then one way to phrase the inliner/function pass iteration would be
(assuming I understand the issues):

  Stack.push(RefSCC_Leaves);
  while (!Stack.empty()) {
    RefSCC = Stack.pop();
    InlineCallSites(RefSCC);
    if (!RefSCC.splitOutSCCs.empty())
      goto repush;
    for each func in RefSCC:
      FPM.run(func);
      if (!RefSCC.splitOutSCCs.empty())
        goto repush;
    continue;
repush:
     for (refscc_dag_root in RefSCCs.splitOutSCCs)
       // here we don't want to push every leaf, but leafs that
       // have functions that haven't had the FPM run on (maybe we can
do this by maintaining a set?)
       // if we don't push a leaf, we push its parent (which we want
to push even if we've run FPM on it
       // since we'd like to re-run the inliner on it).
       refscc_dag_root.push_leaves_to(Stack)
   }

(I know this isn't ideal, since now RefSCC is no longer "just a data
structure", but is actually has incidental information.)

Is it in the category of invalidating the iterator while iterating' which
feels very wrong to me. We should avoid going there and find better ways
to solve the motivating problems (perhaps defining them more clearly first
?).

thanks,

David

5

Hi David,

Is it in the category of invalidating the iterator while iterating' which
feels very wrong to me. We should avoid going there and find better ways to
solve the motivating problems (perhaps defining them more clearly first ?).

I'm not a 100% sure of what you meant by that, so I'll try to give a
general answer, and hope that it covers the points you wanted to
raise. :slight_smile:

In the scheme above I'm not trying to solve iterator invalidation --
I'm trying to solve the following problem: the CGSCC pass manager ran
the inliner and a set of function passes on a function, and they did
something to invalidate the RefSCC we were iterating over[0]. How do
we _continue_ our iteration over this now non-existent RefSCC?

The solution I'm trying to propose is this: The only possibility for
invalidation is that the RefSCC was broken up into a forest of
RefSCC-DAGs[1]. This means if we had a way to get to the leaves of
this forest of RefSCCs, we could restart our iteration from there
(I've tacitly assumed we're interested in a bottom-up SCC order).
This may be difficult in general, but my idea was to "cheat" and
explicitly remember the Ref-SCC-forest a RefSCC was broken down into
when we do that invalidation. Then once an RefSCC is split, we can
pick up the forest from the original RefSCC* (which is otherwise
useless now), gather the leaves, and re-start our iteration from those
leaves.

This leaves the question of what to do with the SCC DAG nested inside
the RefSCC. I'm not sure what Chandler has in mind in how much
influence these should have over the iteration order, but if we wanted
to iterate over the SCC-DAG in bottom up order as well (as we iterated
over a single RefSCC), we could have the same scheme to handle
SCC-splits, and a similar scheme to handle SCC-merges (when you merge
an SCC, the SCC that gets cleaned out gets a pointer to the SCC where
all the functions went, and if the SCC you were iterating over gets
such a pointer after running the inliner/FPM you chase that pointer
(possibly multiple times, if more than one SCC was merged) and
re-start iteration over that SCC).

By "incident data structure" I meant that with these additions the
RefSCC or SCC is no longer a "pure" function of the structure of the
module, but has state that is a function of what the pass manager did
which is not ideal. That is, in theory this isn't significantly
cleaner than the passes reaching out into and changing the CGSCC pass
manager's state, but perhaps we are okay with this kind of design for
practicality's sake?

[0]: One question I don't know the answer to -- how will we detect
that something has removed a call or ref edge? Will we rescan
functions to verify that edges that we though existed still exist? Or
will we have a ValueHandles-like scheme?

[1]: As Sean mentioned, by design nothing in the function pass
manaager pipeline could have invalidated the RefSCC by merging it with
other RefSCCs.

-- Sanjoy

6

Hi David,

> Is it in the category of invalidating the iterator while iterating' which
> feels very wrong to me. We should avoid going there and find better
ways to
> solve the motivating problems (perhaps defining them more clearly first
?).

I'm not a 100% sure of what you meant by that, so I'll try to give a
general answer, and hope that it covers the points you wanted to
raise. :slight_smile:

In the scheme above I'm not trying to solve iterator invalidation --
I'm trying to solve the following problem: the CGSCC pass manager ran
the inliner and a set of function passes on a function, and they did
something to invalidate the RefSCC we were iterating over[0]. How do
we _continue_ our iteration over this now non-existent RefSCC?

What you described is: "the iterator pointing to the current SCC gets
invalidated because the pointee disappears, so we need to find a way to let
continue working" - - so it does sound like it is trying to solve the
iterator invalidation problem?

What I suggested is that we should try to avoid getting into that situation
in the first place -- not trying to find a solution for the problem we
introduce in the design.

The solution I'm trying to propose is this: The only possibility for
invalidation is that the RefSCC was broken up into a forest of
RefSCC-DAGs[1]. This means if we had a way to get to the leaves of
this forest of RefSCCs, we could restart our iteration from there
(I've tacitly assumed we're interested in a bottom-up SCC order).
This may be difficult in general, but my idea was to "cheat" and
explicitly remember the Ref-SCC-forest a RefSCC was broken down into
when we do that invalidation. Then once an RefSCC is split, we can
pick up the forest from the original RefSCC* (which is otherwise
useless now), gather the leaves, and re-start our iteration from those
leaves.

This leaves the question of what to do with the SCC DAG nested inside
the RefSCC. I'm not sure what Chandler has in mind in how much
influence these should have over the iteration order, but if we wanted
to iterate over the SCC-DAG in bottom up order as well (as we iterated
over a single RefSCC), we could have the same scheme to handle
SCC-splits, and a similar scheme to handle SCC-merges (when you merge
an SCC, the SCC that gets cleaned out gets a pointer to the SCC where
all the functions went, and if the SCC you were iterating over gets
such a pointer after running the inliner/FPM you chase that pointer
(possibly multiple times, if more than one SCC was merged) and
re-start iteration over that SCC).

By "incident data structure" I meant that with these additions the
RefSCC or SCC is no longer a "pure" function of the structure of the
module, but has state that is a function of what the pass manager did
which is not ideal. That is, in theory this isn't significantly
cleaner than the passes reaching out into and changing the CGSCC pass
manager's state, but perhaps we are okay with this kind of design for
practicality's sake?

The complexity you described above is my main source of concerns.
Complicated algorithms are nice, but simplicity is better :slight_smile:

thanks,

David

[0]: One question I don't know the answer to -- how will we detect