[Clang] Add __builtin_vectorelements to get number of elements in vector (#69010)

Adds a new `__builtin_vectorelements()` function which returns the
number of elements for a given vector either at compile-time for
fixed-sized vectors, e.g., created via `__attribute__((vector_size(N)))`
or at runtime via a call to `@llvm.vscale.i32()` for scalable vectors,
e.g., SVE or RISCV V.

The new builtin follows a similar path as `sizeof()`, as it essentially
does the same thing but for the number of elements in vector instead of
the number of bytes. This allows us to re-use a lot of the existing
logic to handle types etc.

A small side addition is `Type::isSizelessVectorType()`, which we need
to distinguish between sizeless vectors (SVE, RISCV V) and sizeless
types (WASM).

This is the [corresponding
discussion](https://discourse.llvm.org/t/new-builtin-function-to-get-number-of-lanes-in-simd-vectors/73911).

Author: lawben

clang/docs/LanguageExtensions.rst

@@ -619,6 +619,14 @@ Let ``T`` be one of the following types:
 
 For scalar types, consider the operation applied to a vector with a single element.
 
+*Vector Size*
+To determine the number of elements in a vector, use ``__builtin_vectorelements()``.
+For fixed-sized vectors, e.g., defined via ``__attribute__((vector_size(N)))`` or ARM
+NEON's vector types (e.g., ``uint16x8_t``), this returns the constant number of
+elements at compile-time. For scalable vectors, e.g., SVE or RISC-V V, the number of
+elements is not known at compile-time and is determined at runtime. This builtin can
+be used, e.g., to increment the loop-counter in vector-type agnostic loops.
+
 *Elementwise Builtins*
 
 Each builtin returns a vector equivalent to applying the specified operation

clang/docs/ReleaseNotes.rst

@@ -182,6 +182,12 @@ C23 Feature Support
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
+* Clang now has a ``__builtin_vectorelements()`` function that determines the number of elements in a vector.
+  For fixed-sized vectors, e.g., defined via ``__attribute__((vector_size(N)))`` or ARM NEON's vector types
+  (e.g., ``uint16x8_t``), this returns the constant number of elements at compile-time.
+  For scalable vectors, e.g., SVE or RISC-V V, the number of elements is not known at compile-time and is
+  determined at runtime.
+
 New Compiler Flags
 ------------------
 

clang/include/clang/AST/Type.h

@@ -2060,6 +2060,9 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
   bool isSizelessType() const;
   bool isSizelessBuiltinType() const;
 
+  /// Returns true for all scalable vector types.
+  bool isSizelessVectorType() const;
+
   /// Returns true for SVE scalable vector types.
   bool isSVESizelessBuiltinType() const;
 

clang/include/clang/Basic/Builtins.def

@@ -674,6 +674,7 @@ BUILTIN(__builtin_debugtrap, "v", "n")
 BUILTIN(__builtin_unreachable, "v", "nr")
 BUILTIN(__builtin_shufflevector, "v."   , "nct")
 BUILTIN(__builtin_convertvector, "v."   , "nct")
+BUILTIN(__builtin_vectorelements, "v."  , "nct")
 BUILTIN(__builtin_alloca, "v*z"   , "Fn")
 BUILTIN(__builtin_alloca_uninitialized, "v*z", "Fn")
 BUILTIN(__builtin_alloca_with_align, "v*zIz", "Fn")

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -394,6 +394,8 @@ def note_constexpr_unsupported_layout : Note<
   "type %0 has unexpected layout">;
 def note_constexpr_unsupported_flexible_array : Note<
   "flexible array initialization is not yet supported">;
+def note_constexpr_non_const_vectorelements : Note<
+  "cannot determine number of elements for sizeless vectors in a constant expression">;
 def err_experimental_clang_interp_failed : Error<
   "the experimental clang interpreter failed to evaluate an expression">;
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10169,8 +10169,8 @@ def err_shufflevector_argument_too_large : Error<
 
 def err_convertvector_non_vector : Error<
   "first argument to __builtin_convertvector must be a vector">;
-def err_convertvector_non_vector_type : Error<
-  "second argument to __builtin_convertvector must be a vector type">;
+def err_builtin_non_vector_type : Error<
+  "%0 argument to %1 must be of vector type">;
 def err_convertvector_incompatible_vector : Error<
   "first two arguments to __builtin_convertvector must have the same number of elements">;
 

clang/include/clang/Basic/TokenKinds.def

@@ -746,6 +746,7 @@ ALIAS("_pascal"      , __pascal   , KEYBORLAND)
 
 // Clang Extensions.
 KEYWORD(__builtin_convertvector          , KEYALL)
+UNARY_EXPR_OR_TYPE_TRAIT(__builtin_vectorelements, VectorElements, KEYALL)
 ALIAS("__char16_t"   , char16_t          , KEYCXX)
 ALIAS("__char32_t"   , char32_t          , KEYCXX)
 KEYWORD(__builtin_bit_cast               , KEYALL)

clang/lib/AST/ExprConstant.cpp

@@ -13595,6 +13595,20 @@ bool IntExprEvaluator::VisitUnaryExprOrTypeTraitExpr(
                     Info.Ctx.getOpenMPDefaultSimdAlign(E->getArgumentType()))
             .getQuantity(),
         E);
+  case UETT_VectorElements: {
+    QualType Ty = E->getTypeOfArgument();
+    // If the vector has a fixed size, we can determine the number of elements
+    // at compile time.
+    if (Ty->isVectorType())
+      return Success(Ty->castAs<VectorType>()->getNumElements(), E);
+
+    assert(Ty->isSizelessVectorType());
+    if (Info.InConstantContext)
+      Info.CCEDiag(E, diag::note_constexpr_non_const_vectorelements)
+          << E->getSourceRange();
+
+    return false;
+  }
   }
 
   llvm_unreachable("unknown expr/type trait");

clang/lib/AST/ItaniumMangle.cpp

@@ -5127,6 +5127,14 @@ void CXXNameMangler::mangleExpression(const Expr *E, unsigned Arity,
       Diags.Report(DiagID);
       return;
     }
+    case UETT_VectorElements: {
+      DiagnosticsEngine &Diags = Context.getDiags();
+      unsigned DiagID = Diags.getCustomDiagID(
+          DiagnosticsEngine::Error,
+          "cannot yet mangle __builtin_vectorelements expression");
+      Diags.Report(DiagID);
+      return;
+    }
     }
     break;
   }

clang/lib/AST/Type.cpp

@@ -2369,7 +2369,7 @@ bool Type::isIncompleteType(NamedDecl **Def) const {
 }
 
 bool Type::isSizelessBuiltinType() const {
-  if (isSVESizelessBuiltinType() || isRVVSizelessBuiltinType())
+  if (isSizelessVectorType())
     return true;
 
   if (const BuiltinType *BT = getAs<BuiltinType>()) {
@@ -2403,6 +2403,10 @@ bool Type::isWebAssemblyTableType() const {
 
 bool Type::isSizelessType() const { return isSizelessBuiltinType(); }
 
+bool Type::isSizelessVectorType() const {
+  return isSVESizelessBuiltinType() || isRVVSizelessBuiltinType();
+}
+
 bool Type::isSVESizelessBuiltinType() const {
   if (const BuiltinType *BT = getAs<BuiltinType>()) {
     switch (BT->getKind()) {