[clang][Interp] Implement __builtin_fmax

[cor3ntin]

LGTM

clang/lib/AST/Interp/InterpBuiltin.cpp

else if (LHS.isNan() || RHS < LHS)
     Result = RHS;
   else
     Result = LHS;
 
   S.Stk.push<Floating>(Result);
   return true;
 }
 
+static bool interp__builtin_fmax(InterpState &S, CodePtr OpPC,
+                                 const InterpFrame *Frame,
+                                 const Function *Func) {
+  const Floating &LHS = getParam<Floating>(Frame, 0);
+  const Floating &RHS = getParam<Floating>(Frame, 1);
+
+  Floating Result;
+
+  // When comparing zeroes, return +0.0 if one of the zeroes is positive.
+  if (LHS.isZero() && RHS.isZero() && LHS.isNegative())
+    Result = RHS;
+  else if (LHS.isNan() || RHS > LHS)
+    Result = RHS;
+  else
+    Result = LHS;
+
+  S.Stk.push<Floating>(Result);
+  return true;
+}
+
 /// Defined as __builtin_isnan(...), to accommodate the fact that it can
 /// take a float, double, long double, etc.
 /// But for us, that's all a Floating anyway.
 static bool interp__builtin_isnan(InterpState &S, CodePtr OpPC,
                                   const InterpFrame *Frame, const Function *F) {
   const Floating &Arg = S.Stk.peek<Floating>();
 
   S.Stk.push<Integral<32, true>>(Integral<32, true>::from(Arg.isNan()));
bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F) {
   case Builtin::BI__builtin_fminf:
   case Builtin::BI__builtin_fminl:
   case Builtin::BI__builtin_fminf16:
   case Builtin::BI__builtin_fminf128:
     if (interp__builtin_fmin(S, OpPC, Frame, F))
       return Ret<PT_Float>(S, OpPC, Dummy);
     break;
 
+  case Builtin::BI__builtin_fmax:
+  case Builtin::BI__builtin_fmaxf:
+  case Builtin::BI__builtin_fmaxl:
+  case Builtin::BI__builtin_fmaxf16:
+  case Builtin::BI__builtin_fmaxf128:
+    if (interp__builtin_fmax(S, OpPC, Frame, F))
+      return Ret<PT_Float>(S, OpPC, Dummy);
+    break;
+
   case Builtin::BI__builtin_isnan:
     if (interp__builtin_isnan(S, OpPC, Frame, F))
       return Ret<PT_Sint32>(S, OpPC, Dummy);
     break;
 
   case Builtin::BI__builtin_isinf:
     if (interp__builtin_isinf(S, OpPC, Frame, F, /*Sign=*/false))
       return Ret<PT_Sint32>(S, OpPC, Dummy);

clang/test/Sema/constant-builtins-fmax.cpp

 // RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify -fexperimental-new-constant-interpreter %s
 // expected-no-diagnostics
 
 constexpr double NaN = __builtin_nan("");
 constexpr double Inf = __builtin_inf();
 constexpr double NegInf = -__builtin_inf();
 
 #define FMAX_TEST_SIMPLE(T, FUNC)                           \
     static_assert(T(6.7890) == FUNC(T(1.2345), T(6.7890))); \