[LCG] Add aliased functions as LCG roots

[Carrot]

Summary

Current LCG doesn't check aliased functions. So if an internal function has a public alias it will not be added to CG SCC, but it is still reachable from outside through the alias.
So this patch adds aliased functions to SCC.

[tejohnson]

lib/Analysis/LazyCallGraph.cpp

192 ↗

(On Diff #192507)

Presumably aliases with internal linkage themselves can be ignored.

[chandlerc]

Good catch!

lib/Analysis/LazyCallGraph.cpp

191 ↗

(On Diff #192518)

I would rewrite the comment to more closely match the patch description: "Externally visible aliases of internal functions are also viable entry edges to the module."

192–200 ↗

(On Diff #192518)

All of this should go above I think, right after we walk the module. We want to have the complete entry edge set before we visit globals and do the reference visit walk.

196–197 ↗

(On Diff #192518)

Include the fact that it is being added due to an alias, and the name of that alias?

test/Analysis/LazyCallGraph/alias.ll

3 ↗

(On Diff #192518)

Also test that internal aliases *don't* become reachable?

[Carrot]

ping

[chandlerc]

LGTM!

test/Analysis/LazyCallGraph/alias.ll

23 ↗

(On Diff #192705)

nit: can drop this comment.

30 ↗

(On Diff #192705)

nit: same

38 ↗

(On Diff #192705)

nit: same

[Carrot]

Will check in this version.

llvm/trunk/lib/Analysis/LazyCallGraph.cpp

for (Function &F : M) {
 
     // External linkage defined functions have edges to them from other
     // modules.
     LLVM_DEBUG(dbgs() << "  Adding '" << F.getName()
                       << "' to entry set of the graph.\n");
     addEdge(EntryEdges.Edges, EntryEdges.EdgeIndexMap, get(F), Edge::Ref);
   }
 
+  // Externally visible aliases of internal functions are also viable entry
+  // edges to the module.
+  for (auto &A : M.aliases()) {
+    if (A.hasLocalLinkage())
+      continue;
+    if (Function* F = dyn_cast<Function>(A.getAliasee())) {
+      LLVM_DEBUG(dbgs() << "  Adding '" << F->getName()
+                        << "' with alias '" << A.getName()
+                        << "' to entry set of the graph.\n");
+      addEdge(EntryEdges.Edges, EntryEdges.EdgeIndexMap, get(*F), Edge::Ref);
+    }
+  }
+
   // Now add entry nodes for functions reachable via initializers to globals.
   SmallVector<Constant *, 16> Worklist;
   SmallPtrSet<Constant *, 16> Visited;
   for (GlobalVariable &GV : M.globals())
     if (GV.hasInitializer())
       if (Visited.insert(GV.getInitializer()).second)
         Worklist.push_back(GV.getInitializer());
 

llvm/trunk/test/Analysis/LazyCallGraph/alias.ll

+; RUN: opt -disable-output -passes=print-lcg %s 2>&1 | FileCheck %s
+;
+; Aliased function should be reachable in CGSCC.
+
+target triple = "x86_64-grtev4-linux-gnu"
+
+; CHECK:        Edges in function: foo
+; CHECK:        Edges in function: bar
+; CHECK:        Edges in function: baz
+
+; CHECK:        RefSCC with 1 call SCCs:
+; CHECK-NEXT:     SCC with 1 functions:
+; CHECK-NEXT:       bar
+
+; CHECK:       RefSCC with 1 call SCCs:
+; CHECK-NEXT:    SCC with 1 functions:
+; CHECK-NEXT:      foo
+
+; CHECK-NOT:       baz
+
+@alias1 = weak dso_local alias i8* (i8*), i8* (i8*)* @foo
+
+define dso_local i8* @foo(i8* %returned) {
+  ret i8* %returned
+}
+
+@alias2 = weak dso_local alias i8* (i8*), i8* (i8*)* @bar
+
+define internal i8* @bar(i8* %returned) {
+  ret i8* %returned
+}
+
+; Internal alias is not reachable.
+@alias3 = internal alias i8* (i8*), i8* (i8*)* @baz
+
+define internal i8* @baz(i8* %returned) {
+  ret i8* %returned
+}