From f92f6454efbd2b6cacbc563ffa131e165eba06a0 Mon Sep 17 00:00:00 2001
From: Eli Friedman <eli.friedman@gmail.com>
Date: Thu, 24 May 2012 21:05:41 +0000
Subject: [PATCH] A minor tweak to the new volatile lvalue warning: don't warn
 on "(void)x", where "x" refers to a local variable.  This should silence a
 useless warning in compiler-rt and other places.

llvm-svn: 157414
---
 clang/lib/AST/Expr.cpp        | 47 ++++++++++++++++++++-----------------------
 clang/test/SemaCXX/unused.cpp |  2 ++
 2 files changed, 24 insertions(+), 25 deletions(-)

diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 729030b..111f338 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -1883,43 +1883,40 @@ bool Expr::isUnusedResultAWarning(const Expr *&WarnE, SourceLocation &Loc,
     return true;
   }
   case CStyleCastExprClass: {
-    // Ignore an explicit cast to void, as long as the operand isn't a
+    // Ignore an explicit cast to void unless the operand is a non-trivial
     // volatile lvalue.
-    const CStyleCastExpr *CE = cast<CStyleCastExpr>(this);
-    if (CE->getCastKind() == CK_ToVoid) {
-      if (CE->getSubExpr()->isGLValue() &&
-          CE->getSubExpr()->getType().isVolatileQualified())
-        return CE->getSubExpr()->isUnusedResultAWarning(WarnE, Loc,
-                                                        R1, R2, Ctx);
-      return false;
-    }
-    WarnE = this;
-    Loc = CE->getLParenLoc();
-    R1 = CE->getSubExpr()->getSourceRange();
-    return true;
-  }
-  case CXXFunctionalCastExprClass: {
-    // Ignore an explicit cast to void, as long as the operand isn't a
-    // volatile lvalue.
-    const CXXFunctionalCastExpr *CE = cast<CXXFunctionalCastExpr>(this);
+    const CastExpr *CE = cast<CastExpr>(this);
     if (CE->getCastKind() == CK_ToVoid) {
       if (CE->getSubExpr()->isGLValue() &&
-          CE->getSubExpr()->getType().isVolatileQualified())
-        return CE->getSubExpr()->isUnusedResultAWarning(WarnE, Loc,
-                                                        R1, R2, Ctx);
+          CE->getSubExpr()->getType().isVolatileQualified()) {
+        const DeclRefExpr *DRE =
+            dyn_cast<DeclRefExpr>(CE->getSubExpr()->IgnoreParens());
+        if (!(DRE && isa<VarDecl>(DRE->getDecl()) &&
+              cast<VarDecl>(DRE->getDecl())->hasLocalStorage())) {
+          return CE->getSubExpr()->isUnusedResultAWarning(WarnE, Loc,
+                                                          R1, R2, Ctx);
+        }
+      }
       return false;
     }
-    
+
     // If this is a cast to a constructor conversion, check the operand.
     // Otherwise, the result of the cast is unused.
     if (CE->getCastKind() == CK_ConstructorConversion)
       return CE->getSubExpr()->isUnusedResultAWarning(WarnE, Loc, R1, R2, Ctx);
+
     WarnE = this;
-    Loc = CE->getTypeBeginLoc();
-    R1 = CE->getSubExpr()->getSourceRange();
+    if (const CXXFunctionalCastExpr *CXXCE =
+            dyn_cast<CXXFunctionalCastExpr>(this)) {
+      Loc = CXXCE->getTypeBeginLoc();
+      R1 = CXXCE->getSubExpr()->getSourceRange();
+    } else {
+      const CStyleCastExpr *CStyleCE = cast<CStyleCastExpr>(this);
+      Loc = CStyleCE->getLParenLoc();
+      R1 = CStyleCE->getSubExpr()->getSourceRange();
+    }
     return true;
   }
-
   case ImplicitCastExprClass: {
     const CastExpr *ICE = cast<ImplicitCastExpr>(this);
 
diff --git a/clang/test/SemaCXX/unused.cpp b/clang/test/SemaCXX/unused.cpp
index b330d6e..54898c8 100644
--- a/clang/test/SemaCXX/unused.cpp
+++ b/clang/test/SemaCXX/unused.cpp
@@ -30,5 +30,7 @@ namespace derefvolatile {
   void f(volatile char* x) {
     *x; // expected-warning {{expression result unused; assign into a variable to force a volatile load}}
     (void)*x; // expected-warning {{expression result unused; assign into a variable to force a volatile load}}
+    volatile char y = 10;
+    (void)y; // don't warn here, because it's a common pattern.
   }
 }
-- 
2.7.4