[clang][bytecode] Start implementing __builtin_bit_cast (#112126)

This is a subset of #68288, with hopefully narrower scope. It does not
support bitcasting to non-integral types yet.
Bitfields are supported, but only if they result in a full byte-sized
final buffer. It does not support casting from null-pointers yet or
casting from indeterminate bits.


The tests are from #68288 and partially from #74775.

The `BitcastBuffer` struct is currently always working in single bits,
but I plan to (try to) optimize this for the common full-byte case.

Author: tbaederr

clang/lib/AST/ByteCode/Boolean.h

@@ -81,6 +81,16 @@ class Boolean final {
 
   Boolean truncate(unsigned TruncBits) const { return *this; }
 
+  static Boolean bitcastFromMemory(const std::byte *Buff, unsigned BitWidth) {
+    // Boolean width is currently always 8 for all supported targets. If this
+    // changes we need to get the bool width from the target info.
+    assert(BitWidth == 8);
+    bool Val = static_cast<bool>(*Buff);
+    return Boolean(Val);
+  }
+
+  void bitcastToMemory(std::byte *Buff) { std::memcpy(Buff, &V, sizeof(V)); }
+
   void print(llvm::raw_ostream &OS) const { OS << (V ? "true" : "false"); }
   std::string toDiagnosticString(const ASTContext &Ctx) const {
     std::string NameStr;

clang/lib/AST/ByteCode/Compiler.cpp

@@ -470,6 +470,9 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
     return this->emitDecayPtr(*FromT, *ToT, CE);
   }
 
+  case CK_LValueToRValueBitCast:
+    return this->emitBuiltinBitCast(CE);
+
   case CK_IntegralToBoolean:
   case CK_FixedPointToBoolean:
   case CK_BooleanToSignedIntegral:
@@ -6426,6 +6429,66 @@ bool Compiler<Emitter>::emitDummyPtr(const DeclTy &D, const Expr *E) {
       return this->emitDecayPtr(PT_Ptr, PT, E);
     return false;
   }
+  return true;
+}
+
+//  This function is constexpr if and only if To, From, and the types of
+//  all subobjects of To and From are types T such that...
+//  (3.1) - is_union_v<T> is false;
+//  (3.2) - is_pointer_v<T> is false;
+//  (3.3) - is_member_pointer_v<T> is false;
+//  (3.4) - is_volatile_v<T> is false; and
+//  (3.5) - T has no non-static data members of reference type
+template <class Emitter>
+bool Compiler<Emitter>::emitBuiltinBitCast(const CastExpr *E) {
+  const Expr *SubExpr = E->getSubExpr();
+  QualType FromType = SubExpr->getType();
+  QualType ToType = E->getType();
+  std::optional<PrimType> ToT = classify(ToType);
+
+  assert(!DiscardResult && "Implement DiscardResult mode for bitcasts.");
+
+  if (ToType->isNullPtrType()) {
+    if (!this->discard(SubExpr))
+      return false;
+
+    return this->emitNullPtr(nullptr, E);
+  }
+
+  if (FromType->isNullPtrType() && ToT) {
+    if (!this->discard(SubExpr))
+      return false;
+
+    return visitZeroInitializer(*ToT, ToType, E);
+  }
+  assert(!ToType->isReferenceType());
+
+  // Get a pointer to the value-to-cast on the stack.
+  if (!this->visit(SubExpr))
+    return false;
+
+  if (!ToT || ToT == PT_Ptr) {
+    // Conversion to an array or record type.
+    assert(false && "Implement bitcast to pointers.");
+  }
+  assert(ToT);
+
+  const llvm::fltSemantics *TargetSemantics = nullptr;
+  if (ToT == PT_Float)
+    TargetSemantics = &Ctx.getFloatSemantics(ToType);
+
+  // Conversion to a primitive type. FromType can be another
+  // primitive type, or a record/array.
+  bool ToTypeIsUChar = (ToType->isSpecificBuiltinType(BuiltinType::UChar) ||
+                        ToType->isSpecificBuiltinType(BuiltinType::Char_U));
+  uint32_t ResultBitWidth = std::max(Ctx.getBitWidth(ToType), 8u);
+
+  if (!this->emitBitCast(*ToT, ToTypeIsUChar || ToType->isStdByteType(),
+                         ResultBitWidth, TargetSemantics, E))
+    return false;
+
+  if (DiscardResult)
+    return this->emitPop(*ToT, E);
 
   return true;
 }

clang/lib/AST/ByteCode/Compiler.h

@@ -374,6 +374,7 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
   unsigned collectBaseOffset(const QualType BaseType,
                              const QualType DerivedType);
   bool emitLambdaStaticInvokerBody(const CXXMethodDecl *MD);
+  bool emitBuiltinBitCast(const CastExpr *E);
   bool compileConstructor(const CXXConstructorDecl *Ctor);
   bool compileDestructor(const CXXDestructorDecl *Dtor);
 

clang/lib/AST/ByteCode/Floating.h

@@ -135,6 +135,11 @@ class Floating final {
     return Floating(APFloat(Sem, API));
   }
 
+  void bitcastToMemory(std::byte *Buff) {
+    llvm::APInt API = F.bitcastToAPInt();
+    llvm::StoreIntToMemory(API, (uint8_t *)Buff, bitWidth() / 8);
+  }
+
   // === Serialization support ===
   size_t bytesToSerialize() const {
     return sizeof(llvm::fltSemantics *) +

clang/lib/AST/ByteCode/Integral.h

@@ -70,6 +70,7 @@ template <unsigned Bits, bool Signed> class Integral final {
   // The primitive representing the integral.
   using ReprT = typename Repr<Bits, Signed>::Type;
   ReprT V;
+  static_assert(std::is_trivially_copyable_v<ReprT>);
 
   /// Primitive representing limits.
   static const auto Min = std::numeric_limits<ReprT>::min();
@@ -154,6 +155,18 @@ template <unsigned Bits, bool Signed> class Integral final {
     return Compare(V, RHS.V);
   }
 
+  void bitcastToMemory(std::byte *Dest) const {
+    std::memcpy(Dest, &V, sizeof(V));
+  }
+
+  static Integral bitcastFromMemory(const std::byte *Src, unsigned BitWidth) {
+    assert(BitWidth == sizeof(ReprT) * 8);
+    ReprT V;
+
+    std::memcpy(&V, Src, sizeof(ReprT));
+    return Integral(V);
+  }
+
   std::string toDiagnosticString(const ASTContext &Ctx) const {
     std::string NameStr;
     llvm::raw_string_ostream OS(NameStr);

clang/lib/AST/ByteCode/IntegralAP.h

@@ -171,6 +171,12 @@ template <bool Signed> class IntegralAP final {
     return IntegralAP<false>(Copy);
   }
 
+  void bitcastToMemory(std::byte *Dest) const { assert(false); }
+
+  static IntegralAP bitcastFromMemory(const std::byte *Src, unsigned BitWidth) {
+    return IntegralAP();
+  }
+
   ComparisonCategoryResult compare(const IntegralAP &RHS) const {
     assert(Signed == RHS.isSigned());
     assert(bitWidth() == RHS.bitWidth());

clang/lib/AST/ByteCode/Interp.cpp

@@ -1574,6 +1574,23 @@ bool CastPointerIntegralAPS(InterpState &S, CodePtr OpPC, uint32_t BitWidth) {
   return true;
 }
 
+bool CheckBitCast(InterpState &S, CodePtr OpPC, bool HasIndeterminateBits,
+                  bool TargetIsUCharOrByte) {
+  // This is always fine.
+  if (!HasIndeterminateBits)
+    return true;
+
+  // Indeterminate bits can only be bitcast to unsigned char or std::byte.
+  if (TargetIsUCharOrByte)
+    return true;
+
+  const Expr *E = S.Current->getExpr(OpPC);
+  QualType ExprType = E->getType();
+  S.FFDiag(E, diag::note_constexpr_bit_cast_indet_dest)
+      << ExprType << S.getLangOpts().CharIsSigned << E->getSourceRange();
+  return false;
+}
+
 // https://github.com/llvm/llvm-project/issues/102513
 #if defined(_WIN32) && !defined(__clang__) && !defined(NDEBUG)
 #pragma optimize("", off)

clang/lib/AST/ByteCode/Interp.h

@@ -20,6 +20,7 @@
 #include "Floating.h"
 #include "Function.h"
 #include "FunctionPointer.h"
+#include "InterpBuiltinBitCast.h"
 #include "InterpFrame.h"
 #include "InterpStack.h"
 #include "InterpState.h"
@@ -162,6 +163,8 @@ bool CallPtr(InterpState &S, CodePtr OpPC, uint32_t ArgSize,
              const CallExpr *CE);
 bool CheckLiteralType(InterpState &S, CodePtr OpPC, const Type *T);
 bool InvalidShuffleVectorIndex(InterpState &S, CodePtr OpPC, uint32_t Index);
+bool CheckBitCast(InterpState &S, CodePtr OpPC, bool HasIndeterminateBits,
+                  bool TargetIsUCharOrByte);
 
 template <typename T>
 static bool handleOverflow(InterpState &S, CodePtr OpPC, const T &SrcValue) {
@@ -3039,6 +3042,34 @@ bool CheckNewTypeMismatchArray(InterpState &S, CodePtr OpPC, const Expr *E) {
   return CheckNewTypeMismatch(S, OpPC, E, static_cast<uint64_t>(Size));
 }
 bool InvalidNewDeleteExpr(InterpState &S, CodePtr OpPC, const Expr *E);
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+inline bool BitCast(InterpState &S, CodePtr OpPC, bool TargetIsUCharOrByte,
+                    uint32_t ResultBitWidth, const llvm::fltSemantics *Sem) {
+  const Pointer &FromPtr = S.Stk.pop<Pointer>();
+
+  if (!CheckLoad(S, OpPC, FromPtr))
+    return false;
+
+  size_t BuffSize = ResultBitWidth / 8;
+  llvm::SmallVector<std::byte> Buff(BuffSize);
+  bool HasIndeterminateBits = false;
+
+  if (!DoBitCast(S, OpPC, FromPtr, Buff.data(), BuffSize, HasIndeterminateBits))
+    return false;
+
+  if (!CheckBitCast(S, OpPC, HasIndeterminateBits, TargetIsUCharOrByte))
+    return false;
+
+  if constexpr (std::is_same_v<T, Floating>) {
+    assert(false && "Implement bitcasting to a floating type");
+  } else {
+    assert(!Sem);
+    S.Stk.push<T>(T::bitcastFromMemory(Buff.data(), ResultBitWidth));
+  }
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 // Read opcode arguments
 //===----------------------------------------------------------------------===//

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -10,6 +10,7 @@
 #include "Compiler.h"
 #include "EvalEmitter.h"
 #include "Interp.h"
+#include "InterpBuiltinBitCast.h"
 #include "PrimType.h"
 #include "clang/AST/OSLog.h"
 #include "clang/AST/RecordLayout.h"