[clang][Interp] Implement __builtin_fabs()

[cor3ntin]

LGTM

clang/lib/AST/Interp/Floating.h

public:
   }
 
   static Floating deserialize(const std::byte *Buff) {
     const llvm::fltSemantics *Sem;
     std::memcpy((void *)&Sem, Buff, sizeof(void *));
     return bitcastFromMemory(Buff + sizeof(void *), *Sem);
   }
 
+  static Floating abs(const Floating &F) {
+    APFloat V = F.F;
+    if (V.isNegative())
+      V.changeSign();
+    return Floating(V);
+  }
+
   // -------
 
   static APFloat::opStatus add(const Floating &A, const Floating &B,
                                llvm::RoundingMode RM, Floating *R) {
     *R = Floating(A.F);
     return R->F.add(B.F, RM);
   }
 

clang/lib/AST/Interp/InterpBuiltin.cpp

static bool interp__builtin_fpclassify(InterpState &S, CodePtr OpPC,
   unsigned Offset = align(primSize(PT_Float)) +
                     ((1 + (4 - Index)) * align(primSize(PT_Sint32)));
 
   const Integral<32, true> &I = S.Stk.peek<Integral<32, true>>(Offset);
   S.Stk.push<Integral<32, true>>(I);
   return true;
 }
 
+// The C standard says "fabs raises no floating-point exceptions,
+// even if x is a signaling NaN. The returned value is independent of
+// the current rounding direction mode."  Therefore constant folding can
+// proceed without regard to the floating point settings.
+// Reference, WG14 N2478 F.10.4.3
+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;
+}
+
 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);
case Builtin::BI__builtin_isfpclass:
     if (interp__builtin_isfpclass(S, OpPC, Frame, F))
       return Ret<PT_Sint32>(S, OpPC, Dummy);
     break;
   case Builtin::BI__builtin_fpclassify:
     if (interp__builtin_fpclassify(S, OpPC, Frame, F))
       return Ret<PT_Sint32>(S, OpPC, Dummy);
     break;
 
+  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;
+
   default:
     return false;
   }
 
   return false;
 }
 
 } // namespace interp
 } // namespace clang

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

namespace fpclassify {
   char classify_neg_inf [__builtin_fpclassify(-1, +1, -1, -1, -1, -__builtin_inf())];
   char classify_normal  [__builtin_fpclassify(-1, -1, +1, -1, -1, 1.539)];
   char classify_normal2 [__builtin_fpclassify(-1, -1, +1, -1, -1, 1e-307)];
   char classify_denorm  [__builtin_fpclassify(-1, -1, -1, +1, -1, 1e-308)];
   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, "");
+}