[Sema] Warn when shifting a negative value.

Example:
 % ./clang -Wshift-negative-value emit.c
emit.c:3:14: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
  int a = -1 << 3;
          ~~ ^
1 warning generated.

PR:		24026
Differential Revision:	 http://reviews.llvm.org/D10938
Reviewed by:	rsmith

llvm-svn: 241478

1 files changed, 12 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index c023c8523a3..01a4d488057 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -7937,9 +7937,19 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &LHS, ExprResult &RHS,
   // representable in the result type, so never warn for those.
   llvm::APSInt Left;
   if (LHS.get()->isValueDependent() ||
-      !LHS.get()->isIntegerConstantExpr(Left, S.Context) ||
-      LHSType->hasUnsignedIntegerRepresentation())
+      LHSType->hasUnsignedIntegerRepresentation() ||
+      !LHS.get()->EvaluateAsInt(Left, S.Context))
     return;
+
+  // If LHS does not have a signed type and non-negative value
+  // then, the behavior is undefined. Warn about it.
+  if (Left.isNegative()) {
+    S.DiagRuntimeBehavior(Loc, LHS.get(),
+                          S.PDiag(diag::warn_shift_lhs_negative)
+                            << LHS.get()->getSourceRange());
+    return;
+  }
+
   llvm::APInt ResultBits =
       static_cast<llvm::APInt&>(Right) + Left.getMinSignedBits();
   if (LeftBits.uge(ResultBits))