Squashed commit of the following:

commit 8d41d93
Author: Pol Marcet Sardà <[email protected]>
Date:   Sat Apr 20 12:19:49 2024 +0200

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

commit 9493c0f
Author: Pol Marcet Sardà <[email protected]>
Date:   Sun Mar 31 18:18:45 2024 +0200

    Following the review of sethp, I have made the following changes:

    -- Added diagnostic for the undefined shuffle of -1
    -- Validated support for _BitInt
    -- A bunch of other minnor tweaks here and there

commit 8273abc
Author: Pol Marcet Sardà <[email protected]>
Date:   Thu Jan 4 12:31:08 2024 +0100

    Fix typo in file name

commit ff68f23
Author: Pol Marcet Sardà <[email protected]>
Date:   Thu Jan 4 11:26:08 2024 +0100

    Address suggestions from RKSimon

commit c14783d
Author: Pol Marcet Sardà <[email protected]>
Date:   Sat Dec 30 13:59:00 2023 +0100

    [clang] Constexpr for __builtin_shufflevector and __builtin_convertvector

    Summary:

    This patch adds constexpr support for __builtin_shufflevector
    and __builtin_convertvector.

    A small oddity encountered was that the arg to the intrinsics may be an
    lvalue without any sort of implicit cast of any kind. I solved this
    through the EvaluateVectorOrLValue function, which treats the lvalue as
    if it was in an rvalue cast, which gets me the desired vector.

Co-Authored-By: Seth Pellegrino <[email protected]>

Author: Destroyerrrocket

clang/docs/LanguageExtensions.rst

@@ -2931,7 +2931,7 @@ Query for this feature with ``__has_builtin(__builtin_dump_struct)``
 ``__builtin_shufflevector`` is used to express generic vector
 permutation/shuffle/swizzle operations.  This builtin is also very important
 for the implementation of various target-specific header files like
-``<xmmintrin.h>``.
+``<xmmintrin.h>``. This builtin can be used within constant expressions.
 
 **Syntax**:
 
@@ -2958,7 +2958,7 @@ 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**:
@@ -2971,6 +2971,7 @@ 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
@@ -2985,7 +2986,8 @@ Query for this feature with ``__has_builtin(__builtin_shufflevector)``.
 
 ``__builtin_convertvector`` is used to express generic vector
 type-conversion operations. The input vector and the output vector
-type must have the same number of elements.
+type must have the same number of elements. This builtin can be used within
+constant expressions.
 
 **Syntax**:
 

clang/docs/ReleaseNotes.rst

@@ -233,6 +233,9 @@ Non-comprehensive list of changes in this release
 * ``-fdenormal-fp-math=preserve-sign`` is no longer implied by ``-ffast-math``
   on x86 systems.
 
+- Builtins ``__builtin_shufflevector()`` and ``__builtin_convertvector()`` may
+  now be used within constant expressions.
+
 New Compiler Flags
 ------------------
 - ``-fsanitize=implicit-bitfield-conversion`` checks implicit truncation and

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10330,9 +10330,13 @@ def err_shufflevector_nonconstant_argument : Error<
 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 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

@@ -2706,7 +2706,11 @@ static bool checkFloatingPointResult(EvalInfo &Info, const Expr *E,
 static bool HandleFloatToFloatCast(EvalInfo &Info, const Expr *E,
                                    QualType SrcType, QualType DestType,
                                    APFloat &Result) {
-  assert(isa<CastExpr>(E) || isa<CompoundAssignOperator>(E));
+  assert((isa<CastExpr>(E) || isa<CompoundAssignOperator>(E) ||
+          isa<ConvertVectorExpr>(E)) &&
+         "HandleFloatToFloatCast has been checked with only CastExpr, "
+         "CompoundAssignOperator and ConvertVectorExpr. Please either validate "
+         "the new expression or address the root cause of this usage.");
   llvm::RoundingMode RM = getActiveRoundingMode(Info, E);
   APFloat::opStatus St;
   APFloat Value = Result;
@@ -10710,8 +10714,11 @@ namespace {
     bool VisitUnaryImag(const UnaryOperator *E);
     bool VisitBinaryOperator(const BinaryOperator *E);
     bool VisitUnaryOperator(const UnaryOperator *E);
+    bool VisitConvertVectorExpr(const ConvertVectorExpr *E);
+    bool VisitShuffleVectorExpr(const ShuffleVectorExpr *E);
+
     // FIXME: Missing: conditional operator (for GNU
-    //                 conditional select), shufflevector, ExtVectorElementExpr
+    //                 conditional select), ExtVectorElementExpr
   };
 } // end anonymous namespace
 
@@ -10962,6 +10969,122 @@ bool VectorExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
   return Success(APValue(ResultElements.data(), ResultElements.size()), E);
 }
 
+static bool handleVectorElementCast(EvalInfo &Info, const FPOptions FPO,
+                                    const Expr *E, QualType SourceTy,
+                                    QualType DestTy, APValue const &Original,
+                                    APValue &Result) {
+  if (SourceTy->isIntegerType()) {
+    if (DestTy->isRealFloatingType()) {
+      Result = APValue(APFloat(0.0));
+      return HandleIntToFloatCast(Info, E, FPO, SourceTy, Original.getInt(),
+                                  DestTy, Result.getFloat());
+    }
+    if (DestTy->isIntegerType()) {
+      Result = APValue(
+          HandleIntToIntCast(Info, E, DestTy, SourceTy, Original.getInt()));
+      return true;
+    }
+  } else if (SourceTy->isRealFloatingType()) {
+    if (DestTy->isRealFloatingType()) {
+      Result = Original;
+      return HandleFloatToFloatCast(Info, E, SourceTy, DestTy,
+                                    Result.getFloat());
+    }
+    if (DestTy->isIntegerType()) {
+      Result = APValue(APSInt());
+      return HandleFloatToIntCast(Info, E, SourceTy, Original.getFloat(),
+                                  DestTy, Result.getInt());
+    }
+  }
+
+  Info.FFDiag(E, diag::err_convertvector_constexpr_unsupported_vector_cast)
+      << SourceTy << DestTy;
+  return false;
+}
+
+bool VectorExprEvaluator::VisitConvertVectorExpr(const ConvertVectorExpr *E) {
+  APValue Source;
+  QualType SourceVecType = E->getSrcExpr()->getType();
+  if (!EvaluateAsRValue(Info, E->getSrcExpr(), Source))
+    return false;
+
+  QualType DestTy = E->getType()->castAs<VectorType>()->getElementType();
+  QualType SourceTy = SourceVecType->castAs<VectorType>()->getElementType();
+
+  const FPOptions FPO = E->getFPFeaturesInEffect(Info.Ctx.getLangOpts());
+
+  auto SourceLen = Source.getVectorLength();
+  SmallVector<APValue, 4> ResultElements;
+  ResultElements.reserve(SourceLen);
+  for (unsigned EltNum = 0; EltNum < SourceLen; ++EltNum) {
+    APValue Elt;
+    if (!handleVectorElementCast(Info, FPO, E, SourceTy, DestTy,
+                                 Source.getVectorElt(EltNum), Elt))
+      return false;
+    ResultElements.push_back(std::move(Elt));
+  }
+
+  return Success(APValue(ResultElements.data(), ResultElements.size()), E);
+}
+
+static bool handleVectorShuffle(EvalInfo &Info, const ShuffleVectorExpr *E,
+                                QualType ElemType, APValue const &VecVal1,
+                                APValue const &VecVal2, unsigned EltNum,
+                                APValue &Result) {
+  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'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 >= TotalElementsInInputVector1 + TotalElementsInInputVector2)
+    llvm_unreachable("Out of bounds shuffle index");
+
+  if (index >= TotalElementsInInputVector1)
+    Result = VecVal2.getVectorElt(index - TotalElementsInInputVector1);
+  else
+    Result = VecVal1.getVectorElt(index);
+  return true;
+}
+
+bool VectorExprEvaluator::VisitShuffleVectorExpr(const ShuffleVectorExpr *E) {
+  APValue VecVal1;
+  const Expr *Vec1 = E->getExpr(0);
+  if (!EvaluateAsRValue(Info, Vec1, VecVal1))
+    return false;
+  APValue VecVal2;
+  const Expr *Vec2 = E->getExpr(1);
+  if (!EvaluateAsRValue(Info, Vec2, VecVal2))
+    return false;
+
+  VectorType const *DestVecTy = E->getType()->castAs<VectorType>();
+  QualType DestElTy = DestVecTy->getElementType();
+
+  auto TotalElementsInOutputVector = DestVecTy->getNumElements();
+
+  SmallVector<APValue, 4> ResultElements;
+  ResultElements.reserve(TotalElementsInOutputVector);
+  for (unsigned EltNum = 0; EltNum < TotalElementsInOutputVector; ++EltNum) {
+    APValue Elt;
+    if (!handleVectorShuffle(Info, E, DestElTy, VecVal1, VecVal2, EltNum, Elt))
+      return false;
+    ResultElements.push_back(std::move(Elt));
+  }
+
+  return Success(APValue(ResultElements.data(), ResultElements.size()), E);
+}
+
 //===----------------------------------------------------------------------===//
 // Array Evaluation
 //===----------------------------------------------------------------------===//