[OpenCL] Add ExtVectorElementExpr constant evaluation (PR42387)

[svenvh]

Summary

Add constexpr evaluation for ExtVectorElementExpr nodes by evaluating
the underlying vector expression.

Add basic folding too, for the case that Evaluate does not return an
LValue.

[Anastasia]

clang/lib/AST/ExprConstant.cpp

7057

can Val not be vector at this point?

[svenvh]

clang/lib/AST/ExprConstant.cpp

7057

Yes, for example when evaluating const int2 fromConstexprFunc = addOne(int2(2)); from the included test case we get an LValue (that won't be folded by the code below).

[Anastasia]

LGTM! Thanks!

clang/lib/AST/ExprConstant.cpp

7057

I see. Would it be folded elsewhere or not folded at all?

clang/lib/AST/ExprConstant.cpp

bool VisitMemberExpr(const MemberExpr *E) {
     SubobjectDesignator Designator(BaseTy);
     Designator.addDeclUnchecked(FD);
 
     APValue Result;
     return extractSubobject(Info, E, Obj, Designator, Result) &&
            DerivedSuccess(Result, E);
   }
 
+  bool VisitExtVectorElementExpr(const ExtVectorElementExpr *E) {
+    APValue Val;
+    if (!Evaluate(Val, Info, E->getBase()))
+      return false;
+
+    if (Val.isVector()) {
+      SmallVector<uint32_t, 4> Indices;
+      E->getEncodedElementAccess(Indices);
+      if (Indices.size() == 1) {
+        // Return scalar.
+        return DerivedSuccess(Val.getVectorElt(Indices[0]), E);
+      } else {
+        // Construct new APValue vector.
+        SmallVector<APValue, 4> Elts;
+        for (unsigned I = 0; I < Indices.size(); ++I) {
+          Elts.push_back(Val.getVectorElt(Indices[I]));
+        }
+        APValue VecResult(Elts.data(), Indices.size());
+        return DerivedSuccess(VecResult, E);
+      }
+    }
+
+    return false;
+  }
+
   bool VisitCastExpr(const CastExpr *E) {
     switch (E->getCastKind()) {
     default:
       break;
 
     case CK_AtomicToNonAtomic: {
       APValue AtomicVal;
       // This does not need to be done in place even for class/array types:

clang/test/CodeGenOpenCLCXX/constexpr.cl

 // RUN: %clang_cc1 %s -triple spir-unknown-unknown -cl-std=clc++ -O0 -emit-llvm -o - | FileCheck %s
 
+typedef int int2 __attribute__((ext_vector_type(2)));
+typedef int int4 __attribute__((ext_vector_type(4)));
+
 struct Storage final {
   constexpr const float& operator[](const int index) const noexcept {
     return InternalStorage[index];
   }
 
   const float InternalStorage[1];
 };
 
 constexpr Storage getStorage() {
   return Storage{{1.0f}};
 }
 
 constexpr float compute() {
   constexpr auto s = getStorage();
   return 2.0f / (s[0]);
 }
 
 constexpr float FloatConstant = compute();
 
 // CHECK-LABEL: define spir_kernel void @foo
 // CHECK: store float 2.000000e+00
 kernel void foo(global float *x) {
   *x = FloatConstant;
 }
+
+// Test evaluation of constant vectors.
+// CHECK-LABEL: define spir_kernel void @vecEval
+// CHECK: store i32 3
+// CHECK: store <2 x i32> <i32 22, i32 33>, <2 x i32>
+
+const int oneElt = int4(3).x;
+const int2 twoElts = (int4)(11, 22, 33, 44).yz;
+
+kernel void vecEval(global int *x, global int2 *y) {
+  *x = oneElt;
+  *y = twoElts;
+}
+
+// Test evaluation of vectors initialized through a constexpr function.
+// CHECK-LABEL: define spir_kernel void @vecEval2
+// CHECK: store <2 x i32>
+constexpr int2 addOne(int2 x) {
+  return (int2)(x.x + 1, x.y + 1);
+}
+const int2 fromConstexprFunc = addOne(int2(2));
+
+kernel void vecEval2(global int2 *x) {
+  *x = fromConstexprFunc;
+}