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

[Manuel Klimek]

Hi jordan_rose, krememek,

This change is a precondition to the proposed change
http://reviews.llvm.org/D3627. I think it's also generally helpful, as it
reduces the number of invariants of the CFG this code relies on.

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).

http://reviews.llvm.org/D3638

Files:
  lib/Analysis/ReachableCode.cpp

Index: lib/Analysis/ReachableCode.cpp
===================================================================
--- lib/Analysis/ReachableCode.cpp
+++ lib/Analysis/ReachableCode.cpp
@@ -59,32 +59,57 @@
 }
 
 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();
-       I != E; ++I) {
-    if (Optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
-      if (const ReturnStmt *RS = dyn_cast<ReturnStmt>(CS->getStmt())) {
-        if (RS == S)
-          return true;
-        if (const Expr *RE = RS->getRetValue()) {
-          RE = RE->IgnoreParenCasts();
-          if (RE == S)
+  // 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())) {
+          if (RS == S)
             return true;
-          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.
-          return PM.getParent(S);
+          if (const Expr *RE = RS->getRetValue()) {
+            RE = RE->IgnoreParenCasts();
+            if (RE == S)
+              return true;
+            ParentMap PM(const_cast<Expr *>(RE));
+            // If 'S' is in the ParentMap, it is a subexpression of
+            // 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 will be dead,
+    // without the whole return statement being dead.
+    if (Current->getTerminator().isTemporaryDtorsBranch()) {
+      // Temporary destructors are not 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;
       }
-      break;
+    } else {
+      // We hit control flow or a dead end. Stop searching.
+      return false;
     }
   }
-  return false;
+  llvm_unreachable("Broke out of infinite loop.");
 }
 
 static SourceLocation getTopMostMacro(SourceLocation Loc, SourceManager &SM) {
-------------- next part --------------
A non-text attachment was scrubbed...
Name: D3638.9151.patch
Type: text/x-patch
Size: 3698 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20140507/47f809fe/attachment.bin>