Sema: show shift result in hexadecimal

Change the output for -Wshift-overflow and
-Wshift-sign-overflow to an unsigned hexadecimal. It makes
more sense for looking at bits than a signed decimal does.
Also, change the diagnostic's wording from "overrides"
to "sets".

This uses a new optional argument in APInt::toString()
that adds the '0x' prefix to hexademical numbers.

This fixes PR 9651.

Patch by nobled@dreamwidth.org!

llvm-svn: 133033

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2566,11 +2566,11 @@ def warn_remainder_by_zero : Warning<"remainder by zero is undefined">;
 def warn_shift_negative : Warning<"shift count is negative">;
 def warn_shift_gt_typewidth : Warning<"shift count >= width of type">;
 def warn_shift_result_gt_typewidth : Warning<
-  "shift result (%0) requires %1 bits to represent, but %2 only has %3 bits">,
-   InGroup<DiagGroup<"shift-overflow">>;
-def warn_shift_result_overrides_sign_bit : Warning<
-  "shift result (%0) overrides the sign bit of the shift expression's type "
-  "(%1) and becomes negative">,
+  "signed shift result (%0) requires %1 bits to represent, but %2 only has "
+  "%3 bits">, InGroup<DiagGroup<"shift-overflow">>;
+def warn_shift_result_sets_sign_bit : Warning<
+  "signed shift result (%0) sets the sign bit of the shift expression's "
+  "type (%1) and becomes negative">,
   InGroup<DiagGroup<"shift-sign-overflow">>, DefaultIgnore;
 
 def warn_precedence_bitwise_rel : Warning<

clang/lib/Sema/SemaExpr.cpp

@@ -7394,19 +7394,24 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &lex, ExprResult &rex,
   llvm::APSInt Result = Left.extend(ResultBits.getLimitedValue());
   Result = Result.shl(Right);
 
+  // Print the bit representation of the signed integer as an unsigned
+  // hexadecimal number.
+  llvm::SmallString<40> HexResult;
+  Result.toString(HexResult, 16, /*Signed =*/false, /*Literal =*/true);
+
   // If we are only missing a sign bit, this is less likely to result in actual
   // bugs -- if the result is cast back to an unsigned type, it will have the
   // expected value. Thus we place this behind a different warning that can be
   // turned off separately if needed.
   if (LeftBits == ResultBits - 1) {
-    S.Diag(Loc, diag::warn_shift_result_overrides_sign_bit)
-        << Result.toString(10) << LHSTy
+    S.Diag(Loc, diag::warn_shift_result_sets_sign_bit)
+        << HexResult.str() << LHSTy
         << lex.get()->getSourceRange() << rex.get()->getSourceRange();
     return;
   }
 
   S.Diag(Loc, diag::warn_shift_result_gt_typewidth)
-    << Result.toString(10) << Result.getMinSignedBits() << LHSTy
+    << HexResult.str() << Result.getMinSignedBits() << LHSTy
     << Left.getBitWidth() << lex.get()->getSourceRange() << rex.get()->getSourceRange();
 }
 

clang/test/Sema/shift.c

@@ -38,7 +38,7 @@ void test() {
   int i;
   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 {{overrides the sign bit of the shift expression}}
+  i = 1 << (WORD_BIT - 1); // expected-warning {{sets the sign bit of the shift expression}}
   i = -1 << (WORD_BIT - 1);
   i = 0 << (WORD_BIT - 1);
   i = (char)1 << (WORD_BIT - 2);