Dead stores checker: Don't flag dead stores for self-assignments (common escape hatch for 'unused variable' warnings).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62010 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 15 insertions, 3 deletions

diff --git a/lib/Analysis/CheckDeadStores.cpp b/lib/Analysis/CheckDeadStores.cpp
index 51943d5016..cad19f4f93 100644
--- a/lib/Analysis/CheckDeadStores.cpp
+++ b/lib/Analysis/CheckDeadStores.cpp
@@ -128,16 +128,23 @@ public:
       
       if (DeclRefExpr* DR = dyn_cast<DeclRefExpr>(B->getLHS()))
         if (VarDecl *VD = dyn_cast<VarDecl>(DR->getDecl())) {
+          Expr* RHS = B->getRHS()->IgnoreParenCasts();
+          
           // Special case: check for assigning null to a pointer.
           //  This is a common form of defensive programming.          
           if (VD->getType()->isPointerType()) {
-            if (IntegerLiteral* L =
-                  dyn_cast<IntegerLiteral>(B->getRHS()->IgnoreParenCasts()))
+            if (IntegerLiteral* L = dyn_cast<IntegerLiteral>(RHS))
               // FIXME: Probably should have an Expr::isNullPointerConstant.              
               if (L->getValue() == 0)
                 return;
           }
-
+          // Special case: self-assignments.  These are often used to shut up
+          //  "unused variable" compiler warnings.
+          if (DeclRefExpr* RhsDR = dyn_cast<DeclRefExpr>(RHS))
+            if (VD == dyn_cast<VarDecl>(RhsDR->getDecl()))
+              return;
+            
+          // Otherwise, issue a warning.
           DeadStoreKind dsk = 
             Parents.isSubExpr(B)
             ? Enclosing 
diff --git a/test/Analysis/dead-stores.c b/test/Analysis/dead-stores.c
index 71c4e3b28b..7f99499aab 100644
--- a/test/Analysis/dead-stores.c
+++ b/test/Analysis/dead-stores.c
@@ -127,3 +127,8 @@ int f16(int x) {
   return x;
 }
 
+// Self-assignments should not be flagged as dead stores.
+int f17() {
+  int x = 1;
+  x = x; // no-warning
+}