An unreachable block may have a route to a reachable block, don't fast-path return that it can't.

[nicholas]

Summary

A block reachable from the entry block can't have any route to a block that's not reachable from the entry block (if it did, that route would make it reachable from the entry block). That is the intended performance optimization for isPotentiallyReachable. For the case where we ask whether an unreachable from entry block has a route to a reachable from entry block, we can't conclude one way or the other. Fix a bug where we claimed there could be no such route.

The fix in rL357425 ironically reintroduced the very bug it was fixing but only when a DominatorTree is provided. This fixes the remaining bug.

[asbirlea]

lgtm.

[nicholas]

rL357734

llvm/lib/Analysis/CFG.cpp

if (Worklist.empty()) {
       // We've proven that there's no path!
       return false;
     }
   } else {
     Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
   }
 
   if (DT) {
-    if (DT->isReachableFromEntry(A->getParent()) !=
-        DT->isReachableFromEntry(B->getParent()))
+    if (DT->isReachableFromEntry(A->getParent()) &&
+        !DT->isReachableFromEntry(B->getParent()))
       return false;
     if (!ExclusionSet || ExclusionSet->empty()) {
       if (A->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
           DT->isReachableFromEntry(B->getParent()))
         return true;
       if (B->getParent() == &A->getParent()->getParent()->getEntryBlock() &&
           DT->isReachableFromEntry(A->getParent()))
         return false;
     }
   }
 
   return isPotentiallyReachableFromMany(
       Worklist, const_cast<BasicBlock *>(B->getParent()), ExclusionSet, DT, LI);
 }

llvm/unittests/Analysis/CFGTest.cpp

ParseAssembly("declare i1 @switch()\n"
                 "diamond:\n"
                 "  br label %exit\n"
                 "exit:\n"
                 "  %B = bitcast i8 undef to i8\n"
                 "  ret void\n"
                 "}");
   ExpectPath(true);
 }
+
+TEST_F(IsPotentiallyReachableTest, UnreachableToReachable) {
+  ParseAssembly("define void @test() {\n"
+                "entry:\n"
+                "  br label %exit\n"
+                "unreachableblock:\n"
+                "  %A = bitcast i8 undef to i8\n"
+                "  br label %exit\n"
+                "exit:\n"
+                "  %B = bitcast i8 undef to i8\n"
+                "  ret void\n"
+                "}");
+  ExpectPath(true);
+}