[PATCH] D10938: [Sema] Warn when shifting a negative value

[Davide Italiano]

davide created this revision.
davide added a reviewer: rsmith.
davide added a subscriber: cfe-commits.

While I think there's no need to not mirror gcc warning, probably the message can be reworded in a slightly different way.
I tried, but suggestions are more than welcome. Fixes PR24026

% ./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.


Repository:
  rL LLVM

http://reviews.llvm.org/D10938

Files:
  include/clang/Basic/DiagnosticSemaKinds.td
  lib/Sema/SemaExpr.cpp
  test/CXX/expr/expr.const/p2-0x.cpp
  test/Sema/shift.c

Index: test/Sema/shift.c
===================================================================
--- test/Sema/shift.c
+++ test/Sema/shift.c
@@ -39,16 +39,16 @@
   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 = 0 << (WORD_BIT - 1);
   i = (char)1 << (WORD_BIT - 2);
 
   unsigned u;
   u = 1U << (WORD_BIT - 1);
   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);
 }
 
Index: test/CXX/expr/expr.const/p2-0x.cpp
===================================================================
--- test/CXX/expr/expr.const/p2-0x.cpp
+++ test/CXX/expr/expr.const/p2-0x.cpp
@@ -157,7 +157,7 @@
   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)
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -7937,9 +7937,19 @@
   // representable in the result type, so never warn for those.
   llvm::APSInt Left;
   if (LHS.get()->isValueDependent() ||
-      !LHS.get()->isIntegerConstantExpr(Left, S.Context) ||
+      !LHS.get()->EvaluateAsInt(Left, S.Context) ||
       LHSType->hasUnsignedIntegerRepresentation())
     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))
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -4740,6 +4740,8 @@
   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">,


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