[clang][bytecode] Implement fixed point casts (#110409)

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -697,6 +697,14 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
     const auto *TargetSemantics = &Ctx.getFloatSemantics(CE->getType());
     return this->emitCastFixedPointFloating(TargetSemantics, CE);
   }
+  case CK_FixedPointCast: {
+    if (!this->visit(SubExpr))
+      return false;
+    auto Sem = Ctx.getASTContext().getFixedPointSemantics(CE->getType());
+    uint32_t I;
+    std::memcpy(&I, &Sem, sizeof(Sem));
+    return this->emitCastFixedPoint(I, CE);
+  }
 
   case CK_ToVoid:
     return discard(SubExpr);

clang/lib/AST/ByteCode/FixedPoint.h

@@ -61,6 +61,11 @@ class FixedPoint final {
 
   FixedPoint truncate(unsigned BitWidth) const { return *this; }
 
+  FixedPoint toSemantics(const llvm::FixedPointSemantics &Sem,
+                         bool *Overflow) const {
+    return FixedPoint(V.convert(Sem, Overflow));
+  }
+
   llvm::APFloat toFloat(const llvm::fltSemantics *Sem) const {
     return V.convertToFloat(*Sem);
   }

clang/lib/AST/ByteCode/Interp.h

@@ -2161,6 +2161,31 @@ inline bool CastFP(InterpState &S, CodePtr OpPC, const llvm::fltSemantics *Sem,
   return true;
 }
 
+inline bool CastFixedPoint(InterpState &S, CodePtr OpPC, uint32_t FPS) {
+  FixedPointSemantics TargetSemantics(0, 0, false, false, false);
+  std::memcpy(&TargetSemantics, &FPS, sizeof(TargetSemantics));
+
+  const auto &Source = S.Stk.pop<FixedPoint>();
+
+  bool Overflow;
+  FixedPoint Result = Source.toSemantics(TargetSemantics, &Overflow);
+
+  if (Overflow) {
+    const Expr *E = S.Current->getExpr(OpPC);
+    if (S.checkingForUndefinedBehavior()) {
+      S.getASTContext().getDiagnostics().Report(
+          E->getExprLoc(), diag::warn_fixedpoint_constant_overflow)
+          << Result.toDiagnosticString(S.getASTContext()) << E->getType();
+    }
+    S.CCEDiag(E, diag::note_constexpr_overflow) << Result << E->getType();
+    if (!S.noteUndefinedBehavior())
+      return false;
+  }
+
+  S.Stk.push<FixedPoint>(Result);
+  return true;
+}
+
 /// Like Cast(), but we cast to an arbitrary-bitwidth integral, so we need
 /// to know what bitwidth the result should be.
 template <PrimType Name, class T = typename PrimConv<Name>::T>

clang/lib/AST/ByteCode/Opcodes.td

@@ -626,6 +626,10 @@ def CastFP : Opcode {
   let Args = [ArgFltSemantics, ArgRoundingMode];
 }
 
+def CastFixedPoint : Opcode {
+  let Args = [ArgUint32];
+}
+
 def FixedSizeIntegralTypes : TypeClass {
   let Types = [Uint8, Sint8, Uint16, Sint16, Uint32, Sint32, Uint64, Sint64, Bool];
 }

clang/test/AST/ByteCode/fixed-point.cpp

@@ -47,3 +47,9 @@ namespace BinOps {
                                       // ref-error {{is not an integral constant expression}} \
                                       // ref-note {{is outside the range of representable values}}
 }
+
+namespace FixedPointCasts {
+  constexpr _Fract B = 0.3;
+  constexpr _Accum A = B;
+  constexpr _Fract C = A;
+}