[clang] [Clang] Add constexpr eval for cmath builtins (PR #194327)

[Timm Baeder via cfe-commits]

================
@@ -700,6 +707,348 @@ static inline Floating abs(InterpState &S, const Floating &In) {
   return Output;
 }
 
+static bool interp__builtin_ceil(InterpState &S, CodePtr OpPC,
+                                 const InterpFrame *Frame,
+                                 const CallExpr *Call) {
+  const Floating &Val = S.Stk.pop<Floating>();
+  Floating Result = S.allocFloat(Val.getSemantics());
+  APFloat F = Val.getAPFloat();
+  unsigned BuiltinOp = Call->getBuiltinCallee();
+
+  llvm::RoundingMode RM;
+  switch (BuiltinOp) {
+  case Builtin::BI__builtin_ceil:
+  case Builtin::BI__builtin_ceilf:
+  case Builtin::BI__builtin_ceill:
+  case Builtin::BI__builtin_ceilf16:
+  case Builtin::BI__builtin_ceilf128:
+    RM = llvm::RoundingMode::TowardPositive;
+    break;
+  case Builtin::BI__builtin_floor:
+  case Builtin::BI__builtin_floorf:
+  case Builtin::BI__builtin_floorl:
+  case Builtin::BI__builtin_floorf16:
+  case Builtin::BI__builtin_floorf128:
+    RM = llvm::RoundingMode::TowardNegative;
+    break;
+  case Builtin::BI__builtin_trunc:
+  case Builtin::BI__builtin_truncf:
+  case Builtin::BI__builtin_truncl:
+  case Builtin::BI__builtin_truncf16:
+  case Builtin::BI__builtin_truncf128:
+    RM = llvm::RoundingMode::TowardZero;
+    break;
+  default:
+    llvm_unreachable("invalid builtin ID");
+  }
+
+  F.roundToIntegral(RM);
+  Result.copy(F);
+  S.Stk.push<Floating>(Result);
+  return true;
+}
+
+static bool interp__builtin_fdim(InterpState &S, CodePtr OpPC,
+                                 const InterpFrame *Frame,
+                                 const CallExpr *Call) {
+  const Floating &RHS = S.Stk.pop<Floating>();
+  const Floating &LHS = S.Stk.pop<Floating>();
+  APFloat L = LHS.getAPFloat();
+  APFloat R = RHS.getAPFloat();
+  APFloat Result(L.getSemantics());
+
+  if (L.compare(R) == APFloat::cmpGreaterThan) {
+    L.subtract(R, APFloat::rmNearestTiesToEven);
+    Result = L;
+  } else if (L.isNaN() || R.isNaN()) {
+    L.add(R, APFloat::rmNearestTiesToEven);
+    Result = L;
+  } else {
+    Result = APFloat::getZero(L.getSemantics());
+  }
+
+  Floating F = S.allocFloat(Result.getSemantics());
+  F.copy(Result);
+  S.Stk.push<Floating>(F);
+  return true;
+}
+
+static bool interp__builtin_fma(InterpState &S, CodePtr OpPC,
+                                const InterpFrame *Frame,
+                                const CallExpr *Call) {
+  const Floating &Z = S.Stk.pop<Floating>();
+  const Floating &Y = S.Stk.pop<Floating>();
+  const Floating &X = S.Stk.pop<Floating>();
+  APFloat Result = X.getAPFloat();
+
+  const FPOptions FPO = Call->getFPFeaturesInEffect(S.getLangOpts());
+  llvm::RoundingMode RM = FPO.getRoundingMode();
+  if (RM == llvm::RoundingMode::Dynamic)
+    RM = llvm::RoundingMode::NearestTiesToEven;
----------------
tbaederr wrote:

There's `getRoundingMode()` from `InterpHelpers.h` for this.

https://github.com/llvm/llvm-project/pull/194327