[-Wunreachable-code] always treat 'case:' and 'default:' cases as reachable.

This is a heuristic.  Many switch statements, although they look covered
over an enum, may actually handle at runtime more values than in the enum.

This is overly conservative, as there are some cases that clearly
can be ruled as being clearly unreachable, e.g. 'switch (42) { case 1: ... }'.
We can refine this later.

llvm-svn: 202436

clang/lib/Analysis/ReachableCode.cpp

@@ -337,12 +337,6 @@ void DeadCodeScan::reportDeadCode(const CFGBlock *B,
   if (isTrivialReturnPrecededByNoReturn(B, S))
     return;
 
-  // Was this an unreachable 'default' case?  Such cases are covered
-  // by -Wcovered-switch-default, if the user so desires.
-  const Stmt *Label = B->getLabel();
-  if (Label && isa<DefaultStmt>(Label))
-    return;
-
   SourceRange R1, R2;
   SourceLocation Loc = GetUnreachableLoc(S, R1, R2);
   CB.HandleUnreachable(Loc, R1, R2);
@@ -374,8 +368,32 @@ unsigned ScanReachableFromBlock(const CFGBlock *Start,
     
     // Look at the successors and mark then reachable.
     for (CFGBlock::const_succ_iterator I = item->succ_begin(), 
-         E = item->succ_end(); I != E; ++I)
-      if (const CFGBlock *B = *I) {
+         E = item->succ_end(); I != E; ++I) {
+      const CFGBlock *B = *I;
+      if (!B) {
+        //
+        // For switch statements, treat all cases as being reachable.
+        // There are many cases where a switch can contain values that
+        // are not in an enumeration but they are still reachable because
+        // other values are possible.
+        //
+        // Note that this is quite conservative.  If one saw:
+        //
+        //  switch (1) {
+        //    case 2: ...
+        //
+        // we should be able to say that 'case 2' is unreachable.  To do
+        // this we can either put more heuristics here, or possibly retain
+        // that information in the CFG itself.
+        //
+        if (const CFGBlock *UB = I->getPossiblyUnreachableBlock()) {
+          const Stmt *Label = UB->getLabel();
+          if (Label && isa<SwitchCase>(Label)) {
+            B = UB;
+          }
+        }
+      }
+      if (B) {
         unsigned blockID = B->getBlockID();
         if (!Reachable[blockID]) {
           Reachable.set(blockID);
@@ -383,6 +401,7 @@ unsigned ScanReachableFromBlock(const CFGBlock *Start,
           ++count;
         }
       }
+    }
   }
   return count;
 }

clang/test/Sema/warn-unreachable.c

@@ -173,6 +173,9 @@ void unreachable_default(MyEnum e) {
   case Value1:
     calledFun();
     break;
+  case 2: // expected-warning {{case value not in enumerated type 'MyEnum'}}
+    calledFun();
+    break;
   default:
     calledFun(); // no-warning
     break;