Wire up CFG improvements for while when the condition is known.

llvm-svn: 76522

clang/lib/Analysis/CFG.cpp

@@ -701,7 +701,7 @@ CFGBlock *CFGBuilder::VisitDeclSubExpr(Decl* D) {
 }
 
 CFGBlock* CFGBuilder::VisitIfStmt(IfStmt* I) {
-  // See if this is a known constant first.
+  // See if this is a known constant.
   bool KnownTrue = false;
   bool KnownFalse = false;
   Expr::EvalResult Result;
@@ -1102,6 +1102,18 @@ CFGBlock* CFGBuilder::VisitWhileStmt(WhileStmt* W) {
   // "while" is a control-flow statement.  Thus we stop processing the current
   // block.
 
+  // See if this is a known constant.
+  bool KnownTrue = false;
+  bool KnownFalse = false;
+  Expr::EvalResult Result;
+  if (W->getCond()->Evaluate(Result, *Context)
+      && Result.Val.isInt()) {
+    if (Result.Val.getInt().getBoolValue())
+      KnownTrue = true;
+    else
+      KnownFalse = true;
+  }
+
   CFGBlock* LoopSuccessor = NULL;
 
   if (Block) {
@@ -1170,13 +1182,21 @@ CFGBlock* CFGBuilder::VisitWhileStmt(WhileStmt* W) {
         return 0;
     }
 
-    // Add the loop body entry as a successor to the condition.
-    ExitConditionBlock->addSuccessor(BodyBlock);
+    if (KnownFalse)
+      ExitConditionBlock->addSuccessor(0);
+    else {
+      // Add the loop body entry as a successor to the condition.
+      ExitConditionBlock->addSuccessor(BodyBlock);
+    }
   }
 
-  // Link up the condition block with the code that follows the loop.  (the
-  // false branch).
-  ExitConditionBlock->addSuccessor(LoopSuccessor);
+  if (KnownTrue)
+    ExitConditionBlock->addSuccessor(0);
+  else {
+    // Link up the condition block with the code that follows the loop.  (the
+    // false branch).
+    ExitConditionBlock->addSuccessor(LoopSuccessor);
+  }
 
   // There can be no more statements in the condition block since we loop back
   // to this block.  NULL out Block to force lazy creation of another block.

clang/test/Analysis/dead-stores.c

@@ -199,6 +199,8 @@ void f22() {
   int y8 = 4;
   int y9 = 4;
   int y10 = 4;
+  int y11 = 4;
+  int y12 = 4;
 
   ++x; // expected-warning{{never read}}
   ++y1;
@@ -211,6 +213,8 @@ void f22() {
   ++y8;
   ++y9;
   ++y10;
+  ++y11;
+  ++y12;
 
   switch (j) {
   case 1:
@@ -265,5 +269,18 @@ void f22() {
   case 9:
     (void)(1 || x);
     (void)y10;
+    break;
+  case 10:
+    while (1) {
+      (void)y11;
+    }
+    (void)x;
+    break;
+  case 11:
+    while (0) {
+      (void)x;
+    }
+    (void)y12;
+    break;
   }
 }