[clang][Interp] Implement __arithmethic_fence for floating types

[tbaeder]

Ping

[tbaeder]

Ping

[aaron.ballman]

Hmmm, something is weird here. The original patch adding the LLVM intrinsic is https://reviews.llvm.org/D99675 and that says: The invocation of __arithmetic_fence is not a C/C++ constant expression, even if the operands are constant. yet https://reviews.llvm.org/D136036 changed arithmetic fence to be a constant expression. I think that was a mistake and we should back that bit of the change out. WDYT @Izaron ?

[tbaeder]

Ping @lzaron

clang/lib/AST/Interp/InterpBuiltin.cpp

static bool interp__builtin_fabs(InterpState &S, CodePtr OpPC,
                                  const InterpFrame *Frame,
                                  const Function *Func) {
   const Floating &Val = getParam<Floating>(Frame, 0);
 
   S.Stk.push<Floating>(Floating::abs(Val));
   return true;
 }
 
+/// The argument can be a floating, vector or complex type.
+/// The return type must be the same.
+static bool interp__builtin_arithmetic_fence(InterpState &S, CodePtr OpPC,
+                                             const InterpFrame *Frame,
+                                             const Function *Func) {
+  const Expr *E = S.Current->getExpr(OpPC);
+  const CallExpr *CE = cast<CallExpr>(E);
+  QualType ReturnType = CE->getType();
+
+  // TODO: Support vector and complex types.
+  if (!ReturnType->isFloatingType())
+    return false;
+
+  // We just ignore this.
+  const Floating &Val = S.Stk.peek<Floating>();
+  S.Stk.push<Floating>(Val);
+  return true;
+}
+
 bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F) {
   InterpFrame *Frame = S.Current;
   APValue Dummy;
 
   switch (F->getBuiltinID()) {
   case Builtin::BI__builtin_is_constant_evaluated:
     S.Stk.push<Boolean>(Boolean::from(S.inConstantContext()));
     return Ret<PT_Bool>(S, OpPC, Dummy);
bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F) {
   case Builtin::BI__builtin_fabs:
   case Builtin::BI__builtin_fabsf:
   case Builtin::BI__builtin_fabsl:
   case Builtin::BI__builtin_fabsf128:
     if (interp__builtin_fabs(S, OpPC, Frame, F))
       return Ret<PT_Float>(S, OpPC, Dummy);
     break;
 
+  case Builtin::BI__arithmetic_fence:
+    if (interp__builtin_arithmetic_fence(S, OpPC, Frame, F))
+      return Ret<PT_Float>(S, OpPC, Dummy);
+    break;
+
   default:
     return false;
   }
 
   return false;
 }
 
 } // namespace interp
 } // namespace clang

clang/test/AST/Interp/builtin-functions.cpp

namespace fpclassify {
   char classify_denorm2 [__builtin_fpclassify(-1, -1, -1, +1, -1, -1e-308)];
   char classify_zero    [__builtin_fpclassify(-1, -1, -1, -1, +1, 0.0)];
   char classify_neg_zero[__builtin_fpclassify(-1, -1, -1, -1, +1, -0.0)];
 }
 
 namespace fabs {
   static_assert(__builtin_fabs(-14.0) == 14.0, "");
 }
+
+namespace ArithmeticFence {
+  constexpr double d = __arithmetic_fence(13.0);
+  static_assert(d == 13.0, "");
+  constexpr float f = __arithmetic_fence(16.0);
+  static_assert(f == 16.0f, "");
+
+  /// TODO: Support complex and vector types in __arithmethic_fence.
+#if 0
+  _Complex double CD2 = {1.0, 2.0};
+  _Complex double CD = __arithmetic_fence(CD);
+#endif
+}