Make dead return statement detection more robust against changes in the CFG.

This change is a precondition to the proposed change to handle temporary
dtors correctly.

The idea is to explicitly search for the next return that doesn't have other
paths into it (that is, if the current block is dead, the block containing the
return must be dead, too). Thus, introducing non-control-flow block
transitions will not break the logic.

llvm-svn: 209531

clang/lib/Analysis/ReachableCode.cpp

@@ -59,10 +59,13 @@ static bool isTrivialDoWhile(const CFGBlock *B, const Stmt *S) {
 }
 
 static bool isDeadReturn(const CFGBlock *B, const Stmt *S) {
-  // Look to see if the block ends with a 'return', and see if 'S'
-  // is a substatement.  The 'return' may not be the last element in
-  // the block because of destructors.
-  for (CFGBlock::const_reverse_iterator I = B->rbegin(), E = B->rend();
+  // Look to see if the current control flow ends with a 'return', and see if
+  // 'S' is a substatement. The 'return' may not be the last element in the
+  // block, or may be in a subsequent block because of destructors.
+  const CFGBlock *Current = B;
+  while (true) {
+    for (CFGBlock::const_reverse_iterator I = Current->rbegin(),
+                                          E = Current->rend();
          I != E; ++I) {
       if (Optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
         if (const ReturnStmt *RS = dyn_cast<ReturnStmt>(CS->getStmt())) {
@@ -72,19 +75,41 @@ static bool isDeadReturn(const CFGBlock *B, const Stmt *S) {
             RE = RE->IgnoreParenCasts();
             if (RE == S)
               return true;
-          ParentMap PM(const_cast<Expr*>(RE));
+            ParentMap PM(const_cast<Expr *>(RE));
             // If 'S' is in the ParentMap, it is a subexpression of
-          // the return statement.  Note also that we are restricting
-          // to looking at return statements in the same CFGBlock,
-          // so this will intentionally not catch cases where the
-          // return statement contains nested control-flow.
+            // the return statement.
             return PM.getParent(S);
           }
         }
         break;
       }
     }
+    // Note also that we are restricting the search for the return statement
+    // to stop at control-flow; only part of a return statement may be dead,
+    // without the whole return statement being dead.
+    if (Current->getTerminator().isTemporaryDtorsBranch()) {
+      // Temporary destructors have a predictable control flow, thus we want to
+      // look into the next block for the return statement.
+      // We look into the false branch, as we know the true branch only contains
+      // the call to the destructor.
+      assert(Current->succ_size() == 2);
+      Current = *(Current->succ_begin() + 1);
+    } else if (!Current->getTerminator() && Current->succ_size() == 1) {
+      // If there is only one successor, we're not dealing with outgoing control
+      // flow. Thus, look into the next block.
+      Current = *Current->succ_begin();
+      if (Current->pred_size() > 1) {
+        // If there is more than one predecessor, we're dealing with incoming
+        // control flow - if the return statement is in that block, it might
+        // well be reachable via a different control flow, thus it's not dead.
         return false;
+      }
+    } else {
+      // We hit control flow or a dead end. Stop searching.
+      return false;
+    }
+  }
+  llvm_unreachable("Broke out of infinite loop.");
 }
 
 static SourceLocation getTopMostMacro(SourceLocation Loc, SourceManager &SM) {