[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

4 files changed, 18 insertions, 5 deletions

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 750219483c6..39b602c5514 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -4740,6 +4740,8 @@ def warn_division_by_zero : Warning<"division by zero is undefined">,
   InGroup<DivZero>;
 def warn_remainder_by_zero : Warning<"remainder by zero is undefined">,
   InGroup<DivZero>;
+def warn_shift_lhs_negative : Warning<"shifting a negative signed value is undefined">,
+  InGroup<DiagGroup<"shift-negative-value">>;
 def warn_shift_negative : Warning<"shift count is negative">,
   InGroup<DiagGroup<"shift-count-negative">>;
 def warn_shift_gt_typewidth : Warning<"shift count >= width of type">,
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))
diff --git a/clang/test/CXX/expr/expr.const/p2-0x.cpp b/clang/test/CXX/expr/expr.const/p2-0x.cpp
index d027c7a388b..2adefd928af 100644
--- a/clang/test/CXX/expr/expr.const/p2-0x.cpp
+++ b/clang/test/CXX/expr/expr.const/p2-0x.cpp
@@ -157,7 +157,7 @@ namespace UndefinedBehavior {
   constexpr int shl_unsigned_negative = unsigned(-3) << 1; // ok
   constexpr int shl_unsigned_into_sign = 1u << 31; // ok
   constexpr int shl_unsigned_overflow = 1024u << 31; // ok
-  constexpr int shl_signed_negative = (-3) << 1; // expected-error {{constant expression}} expected-note {{left shift of negative value -3}}
+  constexpr int shl_signed_negative = (-3) << 1; // expected-warning {{shifting a negative signed value is undefined}} // expected-error {{constant expression}} expected-note {{left shift of negative value -3}}
   constexpr int shl_signed_ok = 1 << 30; // ok
   constexpr int shl_signed_into_sign = 1 << 31; // ok (DR1457)
   constexpr int shl_signed_into_sign_2 = 0x7fffffff << 1; // ok (DR1457)
diff --git a/clang/test/Sema/shift.c b/clang/test/Sema/shift.c
index d3555448851..07c5fe5280e 100644
--- a/clang/test/Sema/shift.c
+++ b/clang/test/Sema/shift.c
@@ -39,7 +39,8 @@ void test() {
   i = 1 << (WORD_BIT - 2);
   i = 2 << (WORD_BIT - 1); // expected-warning {{bits to represent, but 'int' only has}}
   i = 1 << (WORD_BIT - 1); // expected-warning {{sets the sign bit of the shift expression}}
-  i = -1 << (WORD_BIT - 1);
+  i = -1 << (WORD_BIT - 1); // expected-warning {{shifting a negative signed value is undefined}}
+  i = -1 << 0; // expected-warning {{shifting a negative signed value is undefined}}
   i = 0 << (WORD_BIT - 1);
   i = (char)1 << (WORD_BIT - 2);
 
@@ -48,7 +49,7 @@ void test() {
   u = 5U << (WORD_BIT - 1);
 
   long long int lli;
-  lli = INT_MIN << 2; // expected-warning {{bits to represent, but 'int' only has}}
+  lli = INT_MIN << 2; // expected-warning {{shifting a negative signed value is undefined}}
   lli = 1LL << (sizeof(long long) * CHAR_BIT - 2);
 }