[PM/LCG] Teach LCG to support spurious reference edges.

[chandlerc]

Summary

Somewhat amazingly, this only requires teaching it to clean them up when
deleting a dead function from the graph. And we already have exactly the
necessary data structures to do that in the parent RefSCCs.

This allows ArgPromote to work in a much simpler way be merely letting
reference edges linger in the graph after the causing IR is deleted. We
will clean up these edges when we run any function pass over the IR, but
don't remove them eagerly.

This avoids all of the quadratic update issues both in the current pass
manager and in my previous attempt with the new pass manager.

Depends on D29577.

[davide]

LGTM conditional to D29577 going in.

lib/Analysis/LazyCallGraph.cpp

void LazyCallGraph::removeDeadFunction(Function &F) {
   SCCMap.erase(CI);
   RefSCC &RC = C.getOuterRefSCC();
 
   // This node must be the only member of its SCC as it has no callers, and
   // that SCC must be the only member of a RefSCC as it has no references.
   // Validate these properties first.
   assert(C.size() == 1 && "Dead functions must be in a singular SCC");
   assert(RC.size() == 1 && "Dead functions must be in a singular RefSCC");
-  assert(RC.Parents.empty() && "Cannot have parents of a dead RefSCC!");
+
+  // Clean up any remaining reference edges. Note that we walk an unordered set
+  // here but are just removing and so the order doesn't matter.
+  for (RefSCC &ParentRC : RC.parents())
+    for (SCC &ParentC : ParentRC)
+      for (Node &ParentN : ParentC)
+        if (ParentN)
+          ParentN->removeEdgeInternal(N);
 
   // Now remove this RefSCC from any parents sets and the leaf list.
   for (Edge &E : *N)
     if (RefSCC *TargetRC = lookupRefSCC(E.getNode()))
       TargetRC->Parents.erase(&RC);
   // FIXME: This is a linear operation which could become hot and benefit from
   // an index map.
   auto LRI = find(LeafRefSCCs, &RC);

unittests/Analysis/LazyCallGraphTest.cpp

TEST(LazyCallGraphTest, ReplaceNodeFunction) {
 
   // Now replace the function in the graph.
   RC1.replaceNodeFunction(DN, E);
 
   EXPECT_EQ(&E, &DN.getFunction());
   EXPECT_EQ(&DN, &(*CN)[DN].getNode());
   EXPECT_EQ(&DN, &(*BN)[DN].getNode());
 }
+
+TEST(LazyCallGraphTest, RemoveFunctionWithSpurriousRef) {
+  LLVMContext Context;
+  // A graph with a couple of RefSCCs.
+  std::unique_ptr<Module> M =
+      parseAssembly(Context,
+                    "define void @a(i8** %ptr) {\n"
+                    "entry:\n"
+                    "  store i8* bitcast (void(i8**)* @d to i8*), i8** %ptr\n"
+                    "  ret void\n"
+                    "}\n"
+                    "define void @b(i8** %ptr) {\n"
+                    "entry:\n"
+                    "  store i8* bitcast (void(i8**)* @c to i8*), i8** %ptr\n"
+                    "  ret void\n"
+                    "}\n"
+                    "define void @c(i8** %ptr) {\n"
+                    "entry:\n"
+                    "  call void @d(i8** %ptr)"
+                    "  ret void\n"
+                    "}\n"
+                    "define void @d(i8** %ptr) {\n"
+                    "entry:\n"
+                    "  call void @c(i8** %ptr)"
+                    "  store i8* bitcast (void(i8**)* @b to i8*), i8** %ptr\n"
+                    "  ret void\n"
+                    "}\n"
+                    "define void @dead() {\n"
+                    "entry:\n"
+                    "  ret void\n"
+                    "}\n");
+  LazyCallGraph CG(*M);
+
+  // Insert spurious ref edges.
+  LazyCallGraph::Node &AN = CG.get(lookupFunction(*M, "a"));
+  LazyCallGraph::Node &BN = CG.get(lookupFunction(*M, "b"));
+  LazyCallGraph::Node &CN = CG.get(lookupFunction(*M, "c"));
+  LazyCallGraph::Node &DN = CG.get(lookupFunction(*M, "d"));
+  LazyCallGraph::Node &DeadN = CG.get(lookupFunction(*M, "dead"));
+  AN.populate();
+  BN.populate();
+  CN.populate();
+  DN.populate();
+  DeadN.populate();
+  CG.insertEdge(AN, DeadN, LazyCallGraph::Edge::Ref);
+  CG.insertEdge(BN, DeadN, LazyCallGraph::Edge::Ref);
+  CG.insertEdge(CN, DeadN, LazyCallGraph::Edge::Ref);
+  CG.insertEdge(DN, DeadN, LazyCallGraph::Edge::Ref);
+
+  // Force the graph to be fully expanded.
+  CG.buildRefSCCs();
+  auto I = CG.postorder_ref_scc_begin();
+  LazyCallGraph::RefSCC &DeadRC = *I++;
+  LazyCallGraph::RefSCC &RC1 = *I++;
+  LazyCallGraph::RefSCC &RC2 = *I++;
+  EXPECT_EQ(CG.postorder_ref_scc_end(), I);
+
+  ASSERT_EQ(2u, RC1.size());
+  LazyCallGraph::SCC &C1 = RC1[0];
+  LazyCallGraph::SCC &C2 = RC1[1];
+
+  EXPECT_EQ(&DeadRC, CG.lookupRefSCC(DeadN));
+  EXPECT_EQ(&C1, CG.lookupSCC(DN));
+  EXPECT_EQ(&C1, CG.lookupSCC(CN));
+  EXPECT_EQ(&C2, CG.lookupSCC(BN));
+  EXPECT_EQ(&RC1, CG.lookupRefSCC(DN));
+  EXPECT_EQ(&RC1, CG.lookupRefSCC(CN));
+  EXPECT_EQ(&RC1, CG.lookupRefSCC(BN));
+  EXPECT_EQ(&RC2, CG.lookupRefSCC(AN));
+
+  // Now delete 'dead'. There are no uses of this function but there are
+  // spurious references.
+  CG.removeDeadFunction(DeadN.getFunction());
+
+  // The only observable change should be that the RefSCC is gone from the
+  // postorder sequence.
+  I = CG.postorder_ref_scc_begin();
+  EXPECT_EQ(&RC1, &*I++);
+  EXPECT_EQ(&RC2, &*I++);
+  EXPECT_EQ(CG.postorder_ref_scc_end(), I);
+}
 }