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[LCG] Add the last (and most complex) of the edge insertion mutation
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operations on the call graph. This one forms a cycle, and while not as
complex as removing an internal edge from an SCC, it involves
a reasonable amount of work to find all of the nodes newly connected in
a cycle.

Also somewhat alarming is the worst case complexity here: it might have
to walk roughly the entire SCC inverse DAG to insert a single edge. This
is carefully documented in the API (I hope).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207935 91177308-0d34-0410-b5e6-96231b3b80d8
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@chandlerc
chandlerc committed May 4, 2014
1 parent 57a38b8 commit febf86d
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16 changes: 16 additions & 0 deletions include/llvm/Analysis/LazyCallGraph.h
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Expand Up @@ -275,6 +275,22 @@ class LazyCallGraph {
/// graph.
void insertOutgoingEdge(Node &CallerN, Node &CalleeN);

/// \brief Insert an edge whose tail is in a descendant SCC and head is in
/// this SCC.
///
/// There must be an existing path from the callee to the caller in this
/// case. NB! This is has the potential to be a very expensive function. It
/// inherently forms a cycle in the prior SCC DAG and we have to merge SCCs
/// to resolve that cycle. But finding all of the SCCs which participate in
/// the cycle can in the worst case require traversing every SCC in the
/// graph. Every attempt is made to avoid that, but passes must still
/// exercise caution calling this routine repeatedly.
///
/// FIXME: We could possibly optimize this quite a bit for cases where the
/// caller and callee are very nearby in the graph. See comments in the
/// implementation for details, but that use case might impact users.
SmallVector<SCC *, 1> insertIncomingEdge(Node &CallerN, Node &CalleeN);

/// \brief Remove an edge whose source is in this SCC and target is *not*.
///
/// This removes an inter-SCC edge. All inter-SCC edges originating from
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119 changes: 119 additions & 0 deletions lib/Analysis/LazyCallGraph.cpp
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Expand Up @@ -202,6 +202,125 @@ void LazyCallGraph::SCC::insertOutgoingEdge(Node &CallerN, Node &CalleeN) {
CalleeC.ParentSCCs.insert(this);
}

SmallVector<LazyCallGraph::SCC *, 1>
LazyCallGraph::SCC::insertIncomingEdge(Node &CallerN, Node &CalleeN) {
// First insert it into the caller.
CallerN.insertEdgeInternal(CalleeN);

assert(G->SCCMap.lookup(&CalleeN) == this && "Callee must be in this SCC.");

SCC &CallerC = *G->SCCMap.lookup(&CallerN);
assert(&CallerC != this && "Caller must not be in this SCC.");
assert(CallerC.isDescendantOf(*this) &&
"Caller must be a descendant of the Callee.");

// The algorithm we use for merging SCCs based on the cycle introduced here
// is to walk the SCC inverted DAG formed by the parent SCC sets. The inverse
// graph has the same cycle properties as the actual DAG of the SCCs, and
// when forming SCCs lazily by a DFS, the bottom of the graph won't exist in
// many cases which should prune the search space.
//
// FIXME: We can get this pruning behavior even after the incremental SCC
// formation by leaving behind (conservative) DFS numberings in the nodes,
// and pruning the search with them. These would need to be cleverly updated
// during the removal of intra-SCC edges, but could be preserved
// conservatively.

// The set of SCCs that are connected to the caller, and thus will
// participate in the merged connected component.
SmallPtrSet<SCC *, 8> ConnectedSCCs;
ConnectedSCCs.insert(this);
ConnectedSCCs.insert(&CallerC);

// We build up a DFS stack of the parents chains.
SmallVector<std::pair<SCC *, SCC::parent_iterator>, 8> DFSSCCs;
SmallPtrSet<SCC *, 8> VisitedSCCs;
int ConnectedDepth = -1;
SCC *C = this;
parent_iterator I = parent_begin(), E = parent_end();
for (;;) {
while (I != E) {
SCC &ParentSCC = *I++;

// If we have already processed this parent SCC, skip it, and remember
// whether it was connected so we don't have to check the rest of the
// stack. This also handles when we reach a child of the 'this' SCC (the
// callee) which terminates the search.
if (ConnectedSCCs.count(&ParentSCC)) {
ConnectedDepth = std::max<int>(ConnectedDepth, DFSSCCs.size());
continue;
}
if (VisitedSCCs.count(&ParentSCC))
continue;

// We fully explore the depth-first space, adding nodes to the connected
// set only as we pop them off, so "recurse" by rotating to the parent.
DFSSCCs.push_back(std::make_pair(C, I));
C = &ParentSCC;
I = ParentSCC.parent_begin();
E = ParentSCC.parent_end();
}

// If we've found a connection anywhere below this point on the stack (and
// thus up the parent graph from the caller), the current node needs to be
// added to the connected set now that we've processed all of its parents.
if ((int)DFSSCCs.size() == ConnectedDepth) {
--ConnectedDepth; // We're finished with this connection.
ConnectedSCCs.insert(C);
} else {
// Otherwise remember that its parents don't ever connect.
assert(ConnectedDepth < (int)DFSSCCs.size() &&
"Cannot have a connected depth greater than the DFS depth!");
VisitedSCCs.insert(C);
}

if (DFSSCCs.empty())
break; // We've walked all the parents of the caller transitively.

// Pop off the prior node and position to unwind the depth first recursion.
std::tie(C, I) = DFSSCCs.pop_back_val();
E = C->parent_end();
}

// Now that we have identified all of the SCCs which need to be merged into
// a connected set with the inserted edge, merge all of them into this SCC.
// FIXME: This operation currently creates ordering stability problems
// because we don't use stably ordered containers for the parent SCCs or the
// connected SCCs.
unsigned NewNodeBeginIdx = Nodes.size();
for (SCC *C : ConnectedSCCs) {
if (C == this)
continue;
for (SCC *ParentC : C->ParentSCCs)
if (!ConnectedSCCs.count(ParentC))
ParentSCCs.insert(ParentC);
C->ParentSCCs.clear();

for (Node *N : *C) {
for (Node &ChildN : *N) {
SCC &ChildC = *G->SCCMap.lookup(&ChildN);
if (&ChildC != C)
ChildC.ParentSCCs.erase(C);
}
G->SCCMap[N] = this;
Nodes.push_back(N);
}
C->Nodes.clear();
}
for (auto I = Nodes.begin() + NewNodeBeginIdx, E = Nodes.end(); I != E; ++I)
for (Node &ChildN : **I) {
SCC &ChildC = *G->SCCMap.lookup(&ChildN);
if (&ChildC != this)
ChildC.ParentSCCs.insert(this);
}

// We return the list of SCCs which were merged so that callers can
// invalidate any data they have associated with those SCCs. Note that these
// SCCs are no longer in an interesting state (they are totally empty) but
// the pointers will remain stable for the life of the graph itself.
return SmallVector<SCC *, 1>(ConnectedSCCs.begin(), ConnectedSCCs.end());
}

void LazyCallGraph::SCC::removeInterSCCEdge(Node &CallerN, Node &CalleeN) {
// First remove it from the node.
CallerN.removeEdgeInternal(CalleeN.getFunction());
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155 changes: 155 additions & 0 deletions unittests/Analysis/LazyCallGraphTest.cpp
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Expand Up @@ -426,6 +426,161 @@ TEST(LazyCallGraphTest, OutgoingSCCEdgeInsertion) {
EXPECT_EQ(&DC, CG.lookupSCC(D));
}

TEST(LazyCallGraphTest, IncomingSCCEdgeInsertion) {
// We want to ensure we can add edges even across complex diamond graphs, so
// we use the diamond of triangles graph defined above. The ascii diagram is
// repeated here for easy reference.
//
// d1 |
// / \ |
// d3--d2 |
// / \ |
// b1 c1 |
// / \ / \ |
// b3--b2 c3--c2 |
// \ / |
// a1 |
// / \ |
// a3--a2 |
//
std::unique_ptr<Module> M = parseAssembly(DiamondOfTriangles);
LazyCallGraph CG(*M);

// Force the graph to be fully expanded.
for (LazyCallGraph::SCC &C : CG.postorder_sccs())
(void)C;

LazyCallGraph::Node &A1 = *CG.lookup(lookupFunction(*M, "a1"));
LazyCallGraph::Node &A2 = *CG.lookup(lookupFunction(*M, "a2"));
LazyCallGraph::Node &A3 = *CG.lookup(lookupFunction(*M, "a3"));
LazyCallGraph::Node &B1 = *CG.lookup(lookupFunction(*M, "b1"));
LazyCallGraph::Node &B2 = *CG.lookup(lookupFunction(*M, "b2"));
LazyCallGraph::Node &B3 = *CG.lookup(lookupFunction(*M, "b3"));
LazyCallGraph::Node &C1 = *CG.lookup(lookupFunction(*M, "c1"));
LazyCallGraph::Node &C2 = *CG.lookup(lookupFunction(*M, "c2"));
LazyCallGraph::Node &C3 = *CG.lookup(lookupFunction(*M, "c3"));
LazyCallGraph::Node &D1 = *CG.lookup(lookupFunction(*M, "d1"));
LazyCallGraph::Node &D2 = *CG.lookup(lookupFunction(*M, "d2"));
LazyCallGraph::Node &D3 = *CG.lookup(lookupFunction(*M, "d3"));
LazyCallGraph::SCC &AC = *CG.lookupSCC(A1);
LazyCallGraph::SCC &BC = *CG.lookupSCC(B1);
LazyCallGraph::SCC &CC = *CG.lookupSCC(C1);
LazyCallGraph::SCC &DC = *CG.lookupSCC(D1);
ASSERT_EQ(&AC, CG.lookupSCC(A2));
ASSERT_EQ(&AC, CG.lookupSCC(A3));
ASSERT_EQ(&BC, CG.lookupSCC(B2));
ASSERT_EQ(&BC, CG.lookupSCC(B3));
ASSERT_EQ(&CC, CG.lookupSCC(C2));
ASSERT_EQ(&CC, CG.lookupSCC(C3));
ASSERT_EQ(&DC, CG.lookupSCC(D2));
ASSERT_EQ(&DC, CG.lookupSCC(D3));
ASSERT_EQ(1, std::distance(D2.begin(), D2.end()));

// Add an edge to make the graph:
//
// d1 |
// / \ |
// d3--d2---. |
// / \ | |
// b1 c1 | |
// / \ / \ / |
// b3--b2 c3--c2 |
// \ / |
// a1 |
// / \ |
// a3--a2 |
CC.insertIncomingEdge(D2, C2);
// Make sure we connected the nodes.
EXPECT_EQ(2, std::distance(D2.begin(), D2.end()));

// Make sure we have the correct nodes in the SCC sets.
EXPECT_EQ(&AC, CG.lookupSCC(A1));
EXPECT_EQ(&AC, CG.lookupSCC(A2));
EXPECT_EQ(&AC, CG.lookupSCC(A3));
EXPECT_EQ(&BC, CG.lookupSCC(B1));
EXPECT_EQ(&BC, CG.lookupSCC(B2));
EXPECT_EQ(&BC, CG.lookupSCC(B3));
EXPECT_EQ(&CC, CG.lookupSCC(C1));
EXPECT_EQ(&CC, CG.lookupSCC(C2));
EXPECT_EQ(&CC, CG.lookupSCC(C3));
EXPECT_EQ(&CC, CG.lookupSCC(D1));
EXPECT_EQ(&CC, CG.lookupSCC(D2));
EXPECT_EQ(&CC, CG.lookupSCC(D3));

// And that ancestry tests have been updated.
EXPECT_TRUE(AC.isParentOf(BC));
EXPECT_TRUE(AC.isParentOf(CC));
EXPECT_FALSE(AC.isAncestorOf(DC));
EXPECT_FALSE(BC.isAncestorOf(DC));
EXPECT_FALSE(CC.isAncestorOf(DC));
}

TEST(LazyCallGraphTest, IncomingSCCEdgeInsertionMidTraversal) {
// This is the same fundamental test as the previous, but we perform it
// having only partially walked the SCCs of the graph.
std::unique_ptr<Module> M = parseAssembly(DiamondOfTriangles);
LazyCallGraph CG(*M);

// Walk the SCCs until we find the one containing 'c1'.
auto SCCI = CG.postorder_scc_begin(), SCCE = CG.postorder_scc_end();
ASSERT_NE(SCCI, SCCE);
LazyCallGraph::SCC &DC = *SCCI;
ASSERT_NE(&DC, nullptr);
++SCCI;
ASSERT_NE(SCCI, SCCE);
LazyCallGraph::SCC &CC = *SCCI;
ASSERT_NE(&CC, nullptr);

ASSERT_EQ(nullptr, CG.lookup(lookupFunction(*M, "a1")));
ASSERT_EQ(nullptr, CG.lookup(lookupFunction(*M, "a2")));
ASSERT_EQ(nullptr, CG.lookup(lookupFunction(*M, "a3")));
ASSERT_EQ(nullptr, CG.lookup(lookupFunction(*M, "b1")));
ASSERT_EQ(nullptr, CG.lookup(lookupFunction(*M, "b2")));
ASSERT_EQ(nullptr, CG.lookup(lookupFunction(*M, "b3")));
LazyCallGraph::Node &C1 = *CG.lookup(lookupFunction(*M, "c1"));
LazyCallGraph::Node &C2 = *CG.lookup(lookupFunction(*M, "c2"));
LazyCallGraph::Node &C3 = *CG.lookup(lookupFunction(*M, "c3"));
LazyCallGraph::Node &D1 = *CG.lookup(lookupFunction(*M, "d1"));
LazyCallGraph::Node &D2 = *CG.lookup(lookupFunction(*M, "d2"));
LazyCallGraph::Node &D3 = *CG.lookup(lookupFunction(*M, "d3"));
ASSERT_EQ(&CC, CG.lookupSCC(C1));
ASSERT_EQ(&CC, CG.lookupSCC(C2));
ASSERT_EQ(&CC, CG.lookupSCC(C3));
ASSERT_EQ(&DC, CG.lookupSCC(D1));
ASSERT_EQ(&DC, CG.lookupSCC(D2));
ASSERT_EQ(&DC, CG.lookupSCC(D3));
ASSERT_EQ(1, std::distance(D2.begin(), D2.end()));

CC.insertIncomingEdge(D2, C2);
EXPECT_EQ(2, std::distance(D2.begin(), D2.end()));

// Make sure we have the correct nodes in the SCC sets.
EXPECT_EQ(&CC, CG.lookupSCC(C1));
EXPECT_EQ(&CC, CG.lookupSCC(C2));
EXPECT_EQ(&CC, CG.lookupSCC(C3));
EXPECT_EQ(&CC, CG.lookupSCC(D1));
EXPECT_EQ(&CC, CG.lookupSCC(D2));
EXPECT_EQ(&CC, CG.lookupSCC(D3));

// Check that we can form the last two SCCs now in a coherent way.
++SCCI;
EXPECT_NE(SCCI, SCCE);
LazyCallGraph::SCC &BC = *SCCI;
EXPECT_NE(&BC, nullptr);
EXPECT_EQ(&BC, CG.lookupSCC(*CG.lookup(lookupFunction(*M, "b1"))));
EXPECT_EQ(&BC, CG.lookupSCC(*CG.lookup(lookupFunction(*M, "b2"))));
EXPECT_EQ(&BC, CG.lookupSCC(*CG.lookup(lookupFunction(*M, "b3"))));
++SCCI;
EXPECT_NE(SCCI, SCCE);
LazyCallGraph::SCC &AC = *SCCI;
EXPECT_NE(&AC, nullptr);
EXPECT_EQ(&AC, CG.lookupSCC(*CG.lookup(lookupFunction(*M, "a1"))));
EXPECT_EQ(&AC, CG.lookupSCC(*CG.lookup(lookupFunction(*M, "a2"))));
EXPECT_EQ(&AC, CG.lookupSCC(*CG.lookup(lookupFunction(*M, "a3"))));
++SCCI;
EXPECT_EQ(SCCI, SCCE);
}

TEST(LazyCallGraphTest, InterSCCEdgeRemoval) {
std::unique_ptr<Module> M = parseAssembly(
"define void @a() {\n"
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