Address some misc comments; added a diagnostic and expanded macros in

testing.

Author: Destroyerrrocket

clang/docs/LanguageExtensions.rst

@@ -2954,7 +2954,7 @@ for the implementation of various target-specific header files like
   // Concatenate every other element of 8-element vectors V1 and V2.
   __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
 
-  // Shuffle v1 with some elements being undefined
+  // Shuffle v1 with some elements being undefined. Not allowed in constexpr.
   __builtin_shufflevector(v1, v1, 3, -1, 1, -1)
 
 **Description**:
@@ -2967,6 +2967,7 @@ starting with the first vector, continuing into the second vector.  Thus, if
 ``vec1`` is a 4-element vector, index 5 would refer to the second element of
 ``vec2``. An index of -1 can be used to indicate that the corresponding element
 in the returned vector is a don't care and can be optimized by the backend.
+Values of -1 are not supported in constant expressions.
 
 The result of ``__builtin_shufflevector`` is a vector with the same element
 type as ``vec1``/``vec2`` but that has an element count equal to the number of

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10331,10 +10331,12 @@ def err_shufflevector_argument_too_large : Error<
   "index for __builtin_shufflevector must be less than the total number "
   "of vector elements">;
 def err_shufflevector_minus_one_is_undefined_behavior_constexpr : Error<
-  "index for __builtin_shufflevector must be within the bounds of the input vectors in a constexpr context. An index of -1 at position %0 was found. -1 is only allowed at runtime.">;
+  "index for __builtin_shufflevector not within the bounds of the input vectors; index of -1 found at position %0 not permitted in a constexpr context.">;
 
 def err_convertvector_non_vector : Error<
   "first argument to __builtin_convertvector must be a vector">;
+def err_convertvector_constexpr_unsupported_vector_cast : Error<
+  "unsupported vector cast from %0 to %1 in a constant expression.">;
 def err_builtin_non_vector_type : Error<
   "%0 argument to %1 must be of vector type">;
 def err_convertvector_incompatible_vector : Error<

clang/lib/AST/ExprConstant.cpp

@@ -10995,6 +10995,9 @@ static bool handleVectorElementCast(EvalInfo &Info, const FPOptions FPO,
                                   DestTy, Result.getInt());
     }
   }
+
+  Info.FFDiag(E, diag::err_convertvector_constexpr_unsupported_vector_cast)
+      << SourceTy << DestTy;
   return false;
 }
 
@@ -11027,24 +11030,28 @@ static bool handleVectorShuffle(EvalInfo &Info, const ShuffleVectorExpr *E,
                                 QualType ElemType, APValue const &VecVal1,
                                 APValue const &VecVal2, unsigned EltNum,
                                 APValue &Result) {
-  unsigned const TotalElementsInInputVector = VecVal1.getVectorLength();
+  unsigned const TotalElementsInInputVector1 = VecVal1.getVectorLength();
+  unsigned const TotalElementsInInputVector2 = VecVal2.getVectorLength();
 
   APSInt IndexVal = E->getShuffleMaskIdx(Info.Ctx, EltNum);
   int64_t index = IndexVal.getExtValue();
   // The spec says that -1 should be treated as undef for optimizations,
-  // but in constexpr we need to choose a value. We'll choose 0.
+  // but in constexpr we'd have to produce an APValue::Indeterminate,
+  // which is prohibited from being a top-level constant value. Emit a
+  // diagnostic instead.
   if (index == -1) {
     Info.FFDiag(
         E, diag::err_shufflevector_minus_one_is_undefined_behavior_constexpr)
         << EltNum;
     return false;
   }
 
-  if (index < 0 || index >= TotalElementsInInputVector * 2)
+  if (index < 0 ||
+      index >= TotalElementsInInputVector1 + TotalElementsInInputVector2)
     llvm_unreachable("Out of bounds shuffle index");
 
-  if (index >= TotalElementsInInputVector)
-    Result = VecVal2.getVectorElt(index - TotalElementsInInputVector);
+  if (index >= TotalElementsInInputVector1)
+    Result = VecVal2.getVectorElt(index - TotalElementsInInputVector1);
   else
     Result = VecVal1.getVectorElt(index);
   return true;
@@ -11061,9 +11068,6 @@ bool VectorExprEvaluator::VisitShuffleVectorExpr(const ShuffleVectorExpr *E) {
     return false;
 
   VectorType const *DestVecTy = E->getType()->castAs<VectorType>();
-  if (!DestVecTy)
-    return false;
-
   QualType DestElTy = DestVecTy->getElementType();
 
   auto TotalElementsInOutputVector = DestVecTy->getNumElements();