[clang][ExprConst] allow single element access of vector object to be constant expression

Supports both v[0] and v.x/v.r/v.s0 syntax.
Selecting multiple elements is left as a future work.

Author: yuanfang-chen

clang/docs/ReleaseNotes.rst

@@ -144,6 +144,9 @@ sections with improvements to Clang's support for those languages.
 
 C++ Language Changes
 --------------------
+- Allow single element access of vector object to be constant expression.
+  Supports the `V.xyzw` syntax and other tidbits as seen in OpenCL.
+  Selecting multiple elements is left as a future work.
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -221,6 +221,11 @@ namespace {
         ArraySize = 2;
         MostDerivedLength = I + 1;
         IsArray = true;
+      } else if (const auto *VT = Type->getAs<VectorType>()) {
+        Type = VT->getElementType();
+        ArraySize = VT->getNumElements();
+        MostDerivedLength = I + 1;
+        IsArray = true;
       } else if (const FieldDecl *FD = getAsField(Path[I])) {
         Type = FD->getType();
         ArraySize = 0;
@@ -437,6 +442,16 @@ namespace {
       MostDerivedArraySize = 2;
       MostDerivedPathLength = Entries.size();
     }
+    void addVectorUnchecked(QualType EltTy, uint64_t Size, uint64_t Idx) {
+      Entries.push_back(PathEntry::ArrayIndex(Idx));
+
+      // This is technically a most-derived object, though in practice this
+      // is unlikely to matter.
+      MostDerivedType = EltTy;
+      MostDerivedIsArrayElement = true;
+      MostDerivedArraySize = Size;
+      MostDerivedPathLength = Entries.size();
+    }
     void diagnoseUnsizedArrayPointerArithmetic(EvalInfo &Info, const Expr *E);
     void diagnosePointerArithmetic(EvalInfo &Info, const Expr *E,
                                    const APSInt &N);
@@ -1732,6 +1747,11 @@ namespace {
       if (checkSubobject(Info, E, Imag ? CSK_Imag : CSK_Real))
         Designator.addComplexUnchecked(EltTy, Imag);
     }
+    void addVectorElement(EvalInfo &Info, const Expr *E, QualType EltTy,
+                          uint64_t Size, uint64_t Idx) {
+      if (checkSubobject(Info, E, CSK_VectorElement))
+        Designator.addVectorUnchecked(EltTy, Size, Idx);
+    }
     void clearIsNullPointer() {
       IsNullPtr = false;
     }
@@ -3278,6 +3298,19 @@ static bool HandleLValueComplexElement(EvalInfo &Info, const Expr *E,
   return true;
 }
 
+static bool HandleLValueVectorElement(EvalInfo &Info, const Expr *E,
+                                      LValue &LVal, QualType EltTy,
+                                      uint64_t Size, uint64_t Idx) {
+  if (Idx) {
+    CharUnits SizeOfElement;
+    if (!HandleSizeof(Info, E->getExprLoc(), EltTy, SizeOfElement))
+      return false;
+    LVal.Offset += SizeOfElement * Idx;
+  }
+  LVal.addVectorElement(Info, E, EltTy, Size, Idx);
+  return true;
+}
+
 /// Try to evaluate the initializer for a variable declaration.
 ///
 /// \param Info   Information about the ongoing evaluation.
@@ -3823,6 +3856,21 @@ findSubobject(EvalInfo &Info, const Expr *E, const CompleteObject &Obj,
         return handler.found(Index ? O->getComplexFloatImag()
                                    : O->getComplexFloatReal(), ObjType);
       }
+    } else if (const auto *VT = ObjType->getAs<VectorType>()) {
+      uint64_t Index = Sub.Entries[I].getAsArrayIndex();
+      if (Index >= VT->getNumElements()) {
+        if (Info.getLangOpts().CPlusPlus11)
+          Info.FFDiag(E, diag::note_constexpr_access_past_end)
+            << handler.AccessKind;
+        else
+          Info.FFDiag(E);
+        return handler.failed();
+      }
+
+      ObjType = VT->getElementType();
+
+      assert(I == N - 1 && "extracting subobject of scalar?");
+      return handler.found(O->getVectorElt(Index), ObjType);
     } else if (const FieldDecl *Field = getAsField(Sub.Entries[I])) {
       if (Field->isMutable() &&
           !Obj.mayAccessMutableMembers(Info, handler.AccessKind)) {
@@ -8432,6 +8480,7 @@ class LValueExprEvaluator
   bool VisitCXXTypeidExpr(const CXXTypeidExpr *E);
   bool VisitCXXUuidofExpr(const CXXUuidofExpr *E);
   bool VisitArraySubscriptExpr(const ArraySubscriptExpr *E);
+  bool VisitExtVectorElementExpr(const ExtVectorElementExpr *E);
   bool VisitUnaryDeref(const UnaryOperator *E);
   bool VisitUnaryReal(const UnaryOperator *E);
   bool VisitUnaryImag(const UnaryOperator *E);
@@ -8755,15 +8804,63 @@ bool LValueExprEvaluator::VisitMemberExpr(const MemberExpr *E) {
   return LValueExprEvaluatorBaseTy::VisitMemberExpr(E);
 }
 
+bool LValueExprEvaluator::VisitExtVectorElementExpr(
+    const ExtVectorElementExpr *E) {
+  bool Success = true;
+
+  APValue Val;
+  if (!Evaluate(Val, Info, E->getBase())) {
+    if (!Info.noteFailure())
+      return false;
+    Success = false;
+  }
+
+  SmallVector<uint32_t, 4> Indices;
+  E->getEncodedElementAccess(Indices);
+  // FIXME: support accessing more than one element
+  if (Indices.size() > 1)
+    return false;
+
+  if (Success) {
+    Result.setFrom(Info.Ctx, Val);
+    const auto *VT = E->getBase()->getType()->castAs<VectorType>();
+    HandleLValueVectorElement(Info, E, Result, VT->getElementType(),
+                              VT->getNumElements(), Indices[0]);
+  }
+
+  return Success;
+}
+
 bool LValueExprEvaluator::VisitArraySubscriptExpr(const ArraySubscriptExpr *E) {
-  // FIXME: Deal with vectors as array subscript bases.
-  if (E->getBase()->getType()->isVectorType() ||
-      E->getBase()->getType()->isSveVLSBuiltinType())
+  if (E->getBase()->getType()->isSveVLSBuiltinType())
     return Error(E);
 
   APSInt Index;
   bool Success = true;
 
+  if (const auto *VT = E->getBase()->getType()->getAs<VectorType>()) {
+    APValue Val;
+    if (!Evaluate(Val, Info, E->getBase())) {
+      if (!Info.noteFailure())
+        return false;
+      Success = false;
+    }
+
+    if (!EvaluateInteger(E->getIdx(), Index, Info)) {
+      if (!Info.noteFailure())
+        return false;
+      Success = false;
+    }
+
+    if (Success) {
+      Result.setFrom(Info.Ctx, Val);
+      HandleLValueVectorElement(Info, E, Result, VT->getElementType(),
+                                VT->getNumElements(), Index.getExtValue());
+    }
+
+    return Success;
+  }
+
   // C++17's rules require us to evaluate the LHS first, regardless of which
   // side is the base.
   for (const Expr *SubExpr : {E->getLHS(), E->getRHS()}) {

clang/lib/AST/Interp/State.h

@@ -44,7 +44,8 @@ enum CheckSubobjectKind {
   CSK_ArrayToPointer,
   CSK_ArrayIndex,
   CSK_Real,
-  CSK_Imag
+  CSK_Imag,
+  CSK_VectorElement
 };
 
 namespace interp {

clang/test/CodeGenCXX/temporaries.cpp

@@ -64,6 +64,27 @@ namespace RefTempSubobject {
   constexpr const SelfReferential &sr = SelfReferential();
 }
 
+namespace Vector {
+  typedef __attribute__((vector_size(16))) int vi4a;
+  typedef __attribute__((ext_vector_type(4))) int vi4b;
+  struct S {
+    vi4a v;
+    vi4b w;
+  };
+
+  int &&r = S().v[1];
+  // CHECK: @_ZGRN6Vector1rE_ = internal global i32 0, align 4
+  // CHECK: @_ZN6Vector1rE = constant ptr @_ZGRN6Vector1rE_, align 8
+
+  int &&s = S().w[1];
+  // CHECK: @_ZGRN6Vector1sE_ = internal global i32 0, align 4
+  // CHECK: @_ZN6Vector1sE = constant ptr @_ZGRN6Vector1sE_, align 8
+
+  int &&t = S().w.y;
+  // CHECK: @_ZGRN6Vector1tE_ = internal global i32 0, align 4
+  // CHECK: @_ZN6Vector1tE = constant ptr @_ZGRN6Vector1tE_, align 8
+}
+
 struct A {
   A();
   ~A();
@@ -666,28 +687,6 @@ namespace Bitfield {
   int &&r = S().a;
 }
 
-namespace Vector {
-  typedef __attribute__((vector_size(16))) int vi4a;
-  typedef __attribute__((ext_vector_type(4))) int vi4b;
-  struct S {
-    vi4a v;
-    vi4b w;
-  };
-  // CHECK: alloca
-  // CHECK: extractelement
-  // CHECK: store i32 {{.*}}, ptr @_ZGRN6Vector1rE_
-  // CHECK: store ptr @_ZGRN6Vector1rE_, ptr @_ZN6Vector1rE,
-  int &&r = S().v[1];
-
-  // CHECK: alloca
-  // CHECK: extractelement
-  // CHECK: store i32 {{.*}}, ptr @_ZGRN6Vector1sE_
-  // CHECK: store ptr @_ZGRN6Vector1sE_, ptr @_ZN6Vector1sE,
-  int &&s = S().w[1];
-  // FIXME PR16204: The following code leads to an assertion in Sema.
-  //int &&s = S().w.y;
-}
-
 namespace ImplicitTemporaryCleanup {
   struct A { A(int); ~A(); };
   void g();

clang/test/SemaCXX/constexpr-vectors-access-elements.cpp

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 %s -Wno-uninitialized -std=c++17 -fsyntax-only -verify
+
+namespace Vector {
+
+using TwoIntsVecSize __attribute__((vector_size(8))) = int;
+
+constexpr TwoIntsVecSize a = {1,2};
+static_assert(a[1] == 2);
+static_assert(a[2]); // expected-error {{not an integral constant expression}} expected-note {{read of dereferenced one-past-the-end pointer}}
+
+}
+
+namespace ExtVector {
+
+using FourIntsExtVec __attribute__((ext_vector_type(4))) = int;
+
+constexpr FourIntsExtVec b = {1,2,3,4};
+static_assert(b[0] == 1 && b[1] == 2 && b[2] == 3 && b[3] == 4);
+static_assert(b.s0 == 1 && b.s1 == 2 && b.s2 == 3 && b.s3 == 4);
+static_assert(b.x == 1 && b.y == 2 && b.z == 3 && b.w == 4);
+static_assert(b.r == 1 && b.g == 2 && b.b == 3 && b.a == 4);
+static_assert(b[5]); // expected-error {{not an integral constant expression}} expected-note {{read of dereferenced one-past-the-end pointer}}
+
+// FIXME: support selecting multiple elements
+static_assert(b.lo.lo == 1); // expected-error {{not an integral constant expression}}
+// static_assert(b.lo.lo==1 && b.lo.hi==2 && b.hi.lo == 3 && b.hi.hi == 4);
+// static_assert(b.odd[0]==1 && b.odd[1]==2 && b.even[0] == 3 && b.even[1] == 4);
+
+}