[clang] Implement constexpr __builtin_bit_cast for complex types (#109981)

Fixes #94620

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -7237,6 +7237,7 @@ class APValueToBufferConverter {
 
     case APValue::ComplexInt:
     case APValue::ComplexFloat:
+      return visitComplex(Val, Ty, Offset);
     case APValue::FixedPoint:
       // FIXME: We should support these.
 
@@ -7323,6 +7324,31 @@ class APValueToBufferConverter {
     return true;
   }
 
+  bool visitComplex(const APValue &Val, QualType Ty, CharUnits Offset) {
+    const ComplexType *ComplexTy = Ty->castAs<ComplexType>();
+    QualType EltTy = ComplexTy->getElementType();
+    CharUnits EltSizeChars = Info.Ctx.getTypeSizeInChars(EltTy);
+    bool IsInt = Val.isComplexInt();
+
+    if (IsInt) {
+      if (!visitInt(Val.getComplexIntReal(), EltTy,
+                    Offset + (0 * EltSizeChars)))
+        return false;
+      if (!visitInt(Val.getComplexIntImag(), EltTy,
+                    Offset + (1 * EltSizeChars)))
+        return false;
+    } else {
+      if (!visitFloat(Val.getComplexFloatReal(), EltTy,
+                      Offset + (0 * EltSizeChars)))
+        return false;
+      if (!visitFloat(Val.getComplexFloatImag(), EltTy,
+                      Offset + (1 * EltSizeChars)))
+        return false;
+    }
+
+    return true;
+  }
+
   bool visitVector(const APValue &Val, QualType Ty, CharUnits Offset) {
     const VectorType *VTy = Ty->castAs<VectorType>();
     QualType EltTy = VTy->getElementType();
@@ -7595,6 +7621,23 @@ class BufferToAPValueConverter {
     return ArrayValue;
   }
 
+  std::optional<APValue> visit(const ComplexType *Ty, CharUnits Offset) {
+    QualType ElementType = Ty->getElementType();
+    CharUnits ElementWidth = Info.Ctx.getTypeSizeInChars(ElementType);
+    bool IsInt = ElementType->isIntegerType();
+
+    std::optional<APValue> Values[2];
+    for (unsigned I = 0; I != 2; ++I) {
+      Values[I] = visitType(Ty->getElementType(), Offset + I * ElementWidth);
+      if (!Values[I])
+        return std::nullopt;
+    }
+
+    if (IsInt)
+      return APValue(Values[0]->getInt(), Values[1]->getInt());
+    return APValue(Values[0]->getFloat(), Values[1]->getFloat());
+  }
+
   std::optional<APValue> visit(const VectorType *VTy, CharUnits Offset) {
     QualType EltTy = VTy->getElementType();
     unsigned NElts = VTy->getNumElements();

clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp

@@ -511,3 +511,19 @@ constexpr bool9 bad_short_to_bool9 = __builtin_bit_cast(bool9, static_cast<unsig
 constexpr bool17 bad_int_to_bool17 = __builtin_bit_cast(bool17, 0x0001CAFEU);
 
 }
+
+namespace test_complex {
+  constexpr _Complex unsigned test_int_complex = { 0x0C05FEFE, 0xCAFEBABE };
+  static_assert(round_trip<_Complex unsigned>(0xCAFEBABE0C05FEFEULL), "");
+  static_assert(bit_cast<unsigned long long>(test_int_complex) == (LITTLE_END
+                                                                   ? 0xCAFEBABE0C05FEFE
+                                                                   : 0x0C05FEFECAFEBABE), "");
+  static_assert(sizeof(double) == 2 * sizeof(float));
+  struct TwoFloats { float A; float B; };
+  constexpr _Complex float test_float_complex = {1.0f, 2.0f};
+  constexpr TwoFloats TF = __builtin_bit_cast(TwoFloats, test_float_complex);
+  static_assert(TF.A == 1.0f && TF.B == 2.0f);
+
+  constexpr double D = __builtin_bit_cast(double, test_float_complex);
+  constexpr int M = __builtin_bit_cast(int, test_int_complex); // expected-error {{__builtin_bit_cast source size does not equal destination size}}
+}