[clang] 54db162 - Revert "[clang][bytecode] Handle bitcasts involving bitfields (#116843)"

Timm Bäder via cfe-commits cfe-commits at lists.llvm.org
Wed Dec 4 02:44:17 PST 2024


Author: Timm Bäder
Date: 2024-12-04T11:43:43+01:00
New Revision: 54db16221c92eb52efbea90ad5b5d2a1d00cda3e

URL: https://github.com/llvm/llvm-project/commit/54db16221c92eb52efbea90ad5b5d2a1d00cda3e
DIFF: https://github.com/llvm/llvm-project/commit/54db16221c92eb52efbea90ad5b5d2a1d00cda3e.diff

LOG: Revert "[clang][bytecode] Handle bitcasts involving bitfields (#116843)"

This reverts commit 4b5e7fa4de54e00df007ae5e2675393fd046aa59.

This breaks builders:
https://lab.llvm.org/buildbot/#/builders/154/builds/8464

I guess some more testing on 32 bit hosts is needed.

Added: 
    

Modified: 
    clang/lib/AST/ByteCode/Boolean.h
    clang/lib/AST/ByteCode/Integral.h
    clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp
    clang/lib/AST/CMakeLists.txt
    clang/test/AST/ByteCode/builtin-bit-cast.cpp
    clang/unittests/AST/ByteCode/CMakeLists.txt

Removed: 
    clang/lib/AST/ByteCode/BitcastBuffer.cpp
    clang/lib/AST/ByteCode/BitcastBuffer.h
    clang/test/AST/ByteCode/builtin-bit-cast-bitfields.cpp
    clang/unittests/AST/ByteCode/BitcastBuffer.cpp


################################################################################
diff  --git a/clang/lib/AST/ByteCode/BitcastBuffer.cpp b/clang/lib/AST/ByteCode/BitcastBuffer.cpp
deleted file mode 100644
index 0cc97b0b6bf190..00000000000000
--- a/clang/lib/AST/ByteCode/BitcastBuffer.cpp
+++ /dev/null
@@ -1,95 +0,0 @@
-//===-------------------- Bitcastbuffer.cpp ---------------------*- C++ -*-===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#include "BitcastBuffer.h"
-
-using namespace clang;
-using namespace clang::interp;
-
-/// Returns the value of the bit in the given sequence of bytes.
-static inline bool bitof(const std::byte *B, Bits BitIndex) {
-  return (B[BitIndex.roundToBytes()] &
-          (std::byte{1} << BitIndex.getOffsetInByte())) != std::byte{0};
-}
-
-void BitcastBuffer::pushData(const std::byte *In, Bits BitOffset, Bits BitWidth,
-                             Endian TargetEndianness) {
-  for (unsigned It = 0; It != BitWidth.getQuantity(); ++It) {
-    bool BitValue = bitof(In, Bits(It));
-    if (!BitValue)
-      continue;
-
-    Bits DstBit;
-    if (TargetEndianness == Endian::Little)
-      DstBit = BitOffset + Bits(It);
-    else
-      DstBit = size() - BitOffset - BitWidth + Bits(It);
-
-    size_t DstByte = DstBit.roundToBytes();
-    Data[DstByte] |= std::byte{1} << DstBit.getOffsetInByte();
-  }
-}
-
-std::unique_ptr<std::byte[]>
-BitcastBuffer::copyBits(Bits BitOffset, Bits BitWidth, Bits FullBitWidth,
-                        Endian TargetEndianness) const {
-  assert(BitWidth.getQuantity() <= FullBitWidth.getQuantity());
-  assert(FullBitWidth.isFullByte());
-  auto Out = std::make_unique<std::byte[]>(FullBitWidth.roundToBytes());
-
-  for (unsigned It = 0; It != BitWidth.getQuantity(); ++It) {
-    Bits BitIndex;
-    if (TargetEndianness == Endian::Little)
-      BitIndex = BitOffset + Bits(It);
-    else
-      BitIndex = size() - BitWidth - BitOffset + Bits(It);
-
-    bool BitValue = bitof(Data.get(), BitIndex);
-    if (!BitValue)
-      continue;
-
-    Bits DstBit = Bits(It);
-    size_t DstByte = DstBit.roundToBytes();
-    Out[DstByte] |= std::byte{1} << DstBit.getOffsetInByte();
-  }
-
-  return Out;
-}
-
-#if 0
-  template<typename T>
-  static std::string hex(T t) {
-    std::stringstream stream;
-    stream << std::hex << (int)t;
-    return std::string(stream.str());
-  }
-
-
-  void BitcastBuffer::dump(bool AsHex = true) const {
-    llvm::errs() << "LSB\n  ";
-    unsigned LineLength = 0;
-    for (unsigned I = 0; I != (FinalBitSize / 8); ++I) {
-      std::byte B = Data[I];
-      if (AsHex) {
-        std::stringstream stream;
-        stream << std::hex << (int)B;
-        llvm::errs() << stream.str();
-        LineLength += stream.str().size() + 1;
-      } else {
-        llvm::errs() << std::bitset<8>((int)B).to_string();
-        LineLength += 8 + 1;
-        // llvm::errs() << (int)B;
-      }
-      llvm::errs() << ' ';
-    }
-    llvm::errs() << '\n';
-
-    for (unsigned I = 0; I != LineLength; ++I)
-      llvm::errs() << ' ';
-    llvm::errs() << "MSB\n";
-  }
-#endif

diff  --git a/clang/lib/AST/ByteCode/BitcastBuffer.h b/clang/lib/AST/ByteCode/BitcastBuffer.h
deleted file mode 100644
index 8442df5c60cf56..00000000000000
--- a/clang/lib/AST/ByteCode/BitcastBuffer.h
+++ /dev/null
@@ -1,88 +0,0 @@
-//===--------------------- BitcastBuffer.h ----------------------*- C++ -*-===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#ifndef LLVM_CLANG_AST_INTERP_BITCAST_BUFFER_H
-#define LLVM_CLANG_AST_INTERP_BITCAST_BUFFER_H
-
-#include <cassert>
-#include <cstddef>
-#include <memory>
-
-namespace clang {
-namespace interp {
-
-enum class Endian { Little, Big };
-
-/// A quantity in bits.
-struct Bits {
-  size_t N = 0;
-  Bits() = default;
-  static Bits zero() { return Bits(0); }
-  explicit Bits(size_t Quantity) : N(Quantity) {}
-  size_t getQuantity() const { return N; }
-  size_t roundToBytes() const { return N / 8; }
-  size_t getOffsetInByte() const { return N % 8; }
-  bool isFullByte() const { return N % 8 == 0; }
-  bool nonZero() const { return N != 0; }
-
-  Bits operator-(Bits Other) { return Bits(N - Other.N); }
-  Bits operator+(Bits Other) { return Bits(N + Other.N); }
-  Bits operator+=(size_t O) {
-    N += O;
-    return *this;
-  }
-
-  bool operator>=(Bits Other) { return N >= Other.N; }
-};
-
-/// A quantity in bytes.
-struct Bytes {
-  size_t N;
-  explicit Bytes(size_t Quantity) : N(Quantity) {}
-  size_t getQuantity() const { return N; }
-  Bits toBits() const { return Bits(N * 8); }
-};
-
-/// Track what bits have been initialized to known values and which ones
-/// have indeterminate value.
-struct BitcastBuffer {
-  Bits FinalBitSize;
-  std::unique_ptr<std::byte[]> Data;
-
-  BitcastBuffer(Bits FinalBitSize) : FinalBitSize(FinalBitSize) {
-    assert(FinalBitSize.isFullByte());
-    unsigned ByteSize = FinalBitSize.roundToBytes();
-    Data = std::make_unique<std::byte[]>(ByteSize);
-  }
-
-  /// Returns the buffer size in bits.
-  Bits size() const { return FinalBitSize; }
-
-  /// Returns \c true if all bits in the buffer have been initialized.
-  bool allInitialized() const {
-    // FIXME: Implement.
-    return true;
-  }
-
-  /// Push \p BitWidth bits at \p BitOffset from \p In into the buffer.
-  /// \p TargetEndianness is the endianness of the target we're compiling for.
-  /// \p In must hold at least \p BitWidth many bits.
-  void pushData(const std::byte *In, Bits BitOffset, Bits BitWidth,
-                Endian TargetEndianness);
-
-  /// Copy \p BitWidth bits at offset \p BitOffset from the buffer.
-  /// \p TargetEndianness is the endianness of the target we're compiling for.
-  ///
-  /// The returned output holds exactly (\p FullBitWidth / 8) bytes.
-  std::unique_ptr<std::byte[]> copyBits(Bits BitOffset, Bits BitWidth,
-                                        Bits FullBitWidth,
-                                        Endian TargetEndianness) const;
-};
-
-} // namespace interp
-} // namespace clang
-#endif

diff  --git a/clang/lib/AST/ByteCode/Boolean.h b/clang/lib/AST/ByteCode/Boolean.h
index 8380e85865ac55..78d75e75c7531a 100644
--- a/clang/lib/AST/ByteCode/Boolean.h
+++ b/clang/lib/AST/ByteCode/Boolean.h
@@ -82,7 +82,9 @@ class Boolean final {
   Boolean truncate(unsigned TruncBits) const { return *this; }
 
   static Boolean bitcastFromMemory(const std::byte *Buff, unsigned BitWidth) {
-    // Just load the first byte.
+    // 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);
   }

diff  --git a/clang/lib/AST/ByteCode/Integral.h b/clang/lib/AST/ByteCode/Integral.h
index 26585799e5eadc..cb81b5a6989240 100644
--- a/clang/lib/AST/ByteCode/Integral.h
+++ b/clang/lib/AST/ByteCode/Integral.h
@@ -183,7 +183,6 @@ template <unsigned Bits, bool Signed> class Integral final {
   }
 
   Integral truncate(unsigned TruncBits) const {
-    assert(TruncBits >= 1);
     if (TruncBits >= Bits)
       return *this;
     const ReprT BitMask = (ReprT(1) << ReprT(TruncBits)) - 1;

diff  --git a/clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp b/clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp
index 2fae7f873ab11b..b1230f92ddf1d4 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp
@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 #include "InterpBuiltinBitCast.h"
-#include "BitcastBuffer.h"
 #include "Boolean.h"
 #include "Context.h"
 #include "Floating.h"
@@ -22,19 +21,9 @@
 using namespace clang;
 using namespace clang::interp;
 
-/// Implement __builtin_bit_cast and related operations.
-/// Since our internal representation for data is more complex than
-/// something we can simply memcpy or memcmp, we first bitcast all the data
-/// into a buffer, which we then later use to copy the data into the target.
-
-// TODO:
-//  - Try to minimize heap allocations.
-//  - Optimize the common case of only pushing and pulling full
-//    bytes to/from the buffer.
-
 /// Used to iterate over pointer fields.
 using DataFunc = llvm::function_ref<bool(const Pointer &P, PrimType Ty,
-                                         Bits BitOffset, bool PackedBools)>;
+                                         size_t BitOffset, bool PackedBools)>;
 
 #define BITCAST_TYPE_SWITCH(Expr, B)                                           \
   do {                                                                         \
@@ -72,72 +61,156 @@ using DataFunc = llvm::function_ref<bool(const Pointer &P, PrimType Ty,
     }                                                                          \
   } while (0)
 
+static bool bitof(std::byte B, unsigned BitIndex) {
+  return (B & (std::byte{1} << BitIndex)) != std::byte{0};
+}
+
 static void swapBytes(std::byte *M, size_t N) {
   for (size_t I = 0; I != (N / 2); ++I)
     std::swap(M[I], M[N - 1 - I]);
 }
 
-/// We use this to recursively iterate over all fields and elements of a pointer
+/// Track what bits have been initialized to known values and which ones
+/// have indeterminate value.
+/// All offsets are in bits.
+struct BitcastBuffer {
+  size_t SizeInBits = 0;
+  llvm::SmallVector<std::byte> Data;
+
+  BitcastBuffer() = default;
+
+  size_t size() const { return SizeInBits; }
+
+  const std::byte *data() const { return Data.data(); }
+
+  std::byte *getBytes(unsigned BitOffset) const {
+    assert(BitOffset % 8 == 0);
+    assert(BitOffset < SizeInBits);
+    return const_cast<std::byte *>(data() + (BitOffset / 8));
+  }
+
+  bool allInitialized() const {
+    // FIXME: Implement.
+    return true;
+  }
+
+  bool atByteBoundary() const { return (Data.size() * 8) == SizeInBits; }
+
+  void pushBit(bool Value) {
+    if (atByteBoundary())
+      Data.push_back(std::byte{0});
+
+    if (Value)
+      Data.back() |= (std::byte{1} << (SizeInBits % 8));
+    ++SizeInBits;
+  }
+
+  void pushData(const std::byte *data, size_t BitWidth, bool BigEndianTarget) {
+    bool OnlyFullBytes = BitWidth % 8 == 0;
+    unsigned NBytes = BitWidth / 8;
+
+    size_t BitsHandled = 0;
+    // Read all full bytes first
+    for (size_t I = 0; I != NBytes; ++I) {
+      std::byte B =
+          BigEndianTarget ? data[NBytes - OnlyFullBytes - I] : data[I];
+      for (unsigned X = 0; X != 8; ++X) {
+        pushBit(bitof(B, X));
+        ++BitsHandled;
+      }
+    }
+
+    if (BitsHandled == BitWidth)
+      return;
+
+    // Rest of the bits.
+    assert((BitWidth - BitsHandled) < 8);
+    std::byte B = BigEndianTarget ? data[0] : data[NBytes];
+    for (size_t I = 0, E = (BitWidth - BitsHandled); I != E; ++I) {
+      pushBit(bitof(B, I));
+      ++BitsHandled;
+    }
+
+    assert(BitsHandled == BitWidth);
+  }
+};
+
+/// We use this to recursively iterate over all fields and elemends of a pointer
 /// and extract relevant data for a bitcast.
-static bool enumerateData(const Pointer &P, const Context &Ctx, Bits Offset,
-                          Bits BitsToRead, DataFunc F) {
+static bool enumerateData(const Pointer &P, const Context &Ctx, size_t Offset,
+                          DataFunc F) {
   const Descriptor *FieldDesc = P.getFieldDesc();
   assert(FieldDesc);
 
   // Primitives.
   if (FieldDesc->isPrimitive())
-    return F(P, FieldDesc->getPrimType(), Offset, /*PackedBools=*/false);
+    return F(P, FieldDesc->getPrimType(), Offset, false);
 
   // Primitive arrays.
   if (FieldDesc->isPrimitiveArray()) {
+    bool BigEndianTarget = Ctx.getASTContext().getTargetInfo().isBigEndian();
     QualType ElemType = FieldDesc->getElemQualType();
     size_t ElemSizeInBits = Ctx.getASTContext().getTypeSize(ElemType);
     PrimType ElemT = *Ctx.classify(ElemType);
     // Special case, since the bools here are packed.
     bool PackedBools = FieldDesc->getType()->isExtVectorBoolType();
-    unsigned NumElems = FieldDesc->getNumElems();
     bool Ok = true;
-    for (unsigned I = P.getIndex(); I != NumElems; ++I) {
-      Ok = Ok && F(P.atIndex(I), ElemT, Offset, PackedBools);
-      Offset += PackedBools ? 1 : ElemSizeInBits;
-      if (Offset >= BitsToRead)
-        break;
+    for (unsigned I = 0; I != FieldDesc->getNumElems(); ++I) {
+      unsigned Index = BigEndianTarget ? (FieldDesc->getNumElems() - 1 - I) : I;
+      Ok = Ok && F(P.atIndex(Index), ElemT, Offset, PackedBools);
+      Offset += ElemSizeInBits;
     }
     return Ok;
   }
 
   // Composite arrays.
   if (FieldDesc->isCompositeArray()) {
+    bool BigEndianTarget = Ctx.getASTContext().getTargetInfo().isBigEndian();
     QualType ElemType = FieldDesc->getElemQualType();
     size_t ElemSizeInBits = Ctx.getASTContext().getTypeSize(ElemType);
     for (unsigned I = 0; I != FieldDesc->getNumElems(); ++I) {
-      enumerateData(P.atIndex(I).narrow(), Ctx, Offset, BitsToRead, F);
+      unsigned Index = BigEndianTarget ? (FieldDesc->getNumElems() - 1 - I) : I;
+      enumerateData(P.atIndex(Index).narrow(), Ctx, Offset, F);
       Offset += ElemSizeInBits;
-      if (Offset >= BitsToRead)
-        break;
     }
     return true;
   }
 
   // Records.
   if (FieldDesc->isRecord()) {
+    bool BigEndianTarget = Ctx.getASTContext().getTargetInfo().isBigEndian();
     const Record *R = FieldDesc->ElemRecord;
     const ASTRecordLayout &Layout =
         Ctx.getASTContext().getASTRecordLayout(R->getDecl());
     bool Ok = true;
 
-    for (const Record::Field &Fi : R->fields()) {
-      Pointer Elem = P.atField(Fi.Offset);
-      Bits BitOffset =
-          Offset + Bits(Layout.getFieldOffset(Fi.Decl->getFieldIndex()));
-      Ok = Ok && enumerateData(Elem, Ctx, BitOffset, BitsToRead, F);
-    }
-    for (const Record::Base &B : R->bases()) {
-      Pointer Elem = P.atField(B.Offset);
-      CharUnits ByteOffset =
-          Layout.getBaseClassOffset(cast<CXXRecordDecl>(B.Decl));
-      Bits BitOffset = Offset + Bits(Ctx.getASTContext().toBits(ByteOffset));
-      Ok = Ok && enumerateData(Elem, Ctx, BitOffset, BitsToRead, F);
+    auto enumerateFields = [&]() -> void {
+      for (unsigned I = 0, N = R->getNumFields(); I != N; ++I) {
+        const Record::Field *Fi =
+            R->getField(BigEndianTarget ? (N - 1 - I) : I);
+        Pointer Elem = P.atField(Fi->Offset);
+        size_t BitOffset =
+            Offset + Layout.getFieldOffset(Fi->Decl->getFieldIndex());
+        Ok = Ok && enumerateData(Elem, Ctx, BitOffset, F);
+      }
+    };
+    auto enumerateBases = [&]() -> void {
+      for (unsigned I = 0, N = R->getNumBases(); I != N; ++I) {
+        const Record::Base *B = R->getBase(BigEndianTarget ? (N - 1 - I) : I);
+        Pointer Elem = P.atField(B->Offset);
+        CharUnits ByteOffset =
+            Layout.getBaseClassOffset(cast<CXXRecordDecl>(B->Decl));
+        size_t BitOffset = Offset + Ctx.getASTContext().toBits(ByteOffset);
+        Ok = Ok && enumerateData(Elem, Ctx, BitOffset, F);
+      }
+    };
+
+    if (BigEndianTarget) {
+      enumerateFields();
+      enumerateBases();
+    } else {
+      enumerateBases();
+      enumerateFields();
     }
 
     return Ok;
@@ -147,8 +220,8 @@ static bool enumerateData(const Pointer &P, const Context &Ctx, Bits Offset,
 }
 
 static bool enumeratePointerFields(const Pointer &P, const Context &Ctx,
-                                   Bits BitsToRead, DataFunc F) {
-  return enumerateData(P, Ctx, Bits::zero(), BitsToRead, F);
+                                   DataFunc F) {
+  return enumerateData(P, Ctx, 0, F);
 }
 
 //  This function is constexpr if and only if To, From, and the types of
@@ -222,57 +295,62 @@ static bool CheckBitcastType(InterpState &S, CodePtr OpPC, QualType T,
 static bool readPointerToBuffer(const Context &Ctx, const Pointer &FromPtr,
                                 BitcastBuffer &Buffer, bool ReturnOnUninit) {
   const ASTContext &ASTCtx = Ctx.getASTContext();
-  Endian TargetEndianness =
-      ASTCtx.getTargetInfo().isLittleEndian() ? Endian::Little : Endian::Big;
+  bool SwapData = (ASTCtx.getTargetInfo().isLittleEndian() !=
+                   llvm::sys::IsLittleEndianHost);
+  bool BigEndianTarget = ASTCtx.getTargetInfo().isBigEndian();
 
   return enumeratePointerFields(
-      FromPtr, Ctx, Buffer.size(),
-      [&](const Pointer &P, PrimType T, Bits BitOffset,
+      FromPtr, Ctx,
+      [&](const Pointer &P, PrimType T, size_t BitOffset,
           bool PackedBools) -> bool {
-        // if (!P.isInitialized()) {
-        // assert(false && "Implement uninitialized value tracking");
-        // return ReturnOnUninit;
-        // }
+        if (!P.isInitialized()) {
+          assert(false && "Implement uninitialized value tracking");
+          return ReturnOnUninit;
+        }
 
-        // assert(P.isInitialized());
+        assert(P.isInitialized());
         // nullptr_t is a PT_Ptr for us, but it's still not std::is_pointer_v.
         if (T == PT_Ptr)
           assert(false && "Implement casting to pointer types");
 
         CharUnits ObjectReprChars = ASTCtx.getTypeSizeInChars(P.getType());
-        Bits BitWidth = Bits(ASTCtx.toBits(ObjectReprChars));
-        Bits FullBitWidth = BitWidth;
-        auto Buff =
-            std::make_unique<std::byte[]>(ObjectReprChars.getQuantity());
+        unsigned BitWidth = ASTCtx.toBits(ObjectReprChars);
+        llvm::SmallVector<std::byte> Buff(ObjectReprChars.getQuantity());
         // Work around floating point types that contain unused padding bytes.
         // This is really just `long double` on x86, which is the only
         // fundamental type with padding bytes.
         if (T == PT_Float) {
           const Floating &F = P.deref<Floating>();
-          Bits NumBits = Bits(
-              llvm::APFloatBase::getSizeInBits(F.getAPFloat().getSemantics()));
-          assert(NumBits.isFullByte());
-          assert(NumBits.getQuantity() <= FullBitWidth.getQuantity());
-          F.bitcastToMemory(Buff.get());
+          unsigned NumBits =
+              llvm::APFloatBase::getSizeInBits(F.getAPFloat().getSemantics());
+          assert(NumBits % 8 == 0);
+          assert(NumBits <= (ObjectReprChars.getQuantity() * 8));
+          F.bitcastToMemory(Buff.data());
           // Now, only (maybe) swap the actual size of the float, excluding the
           // padding bits.
-          if (llvm::sys::IsBigEndianHost)
-            swapBytes(Buff.get(), NumBits.roundToBytes());
+          if (SwapData)
+            swapBytes(Buff.data(), NumBits / 8);
 
         } else {
           if (const FieldDecl *FD = P.getField(); FD && FD->isBitField())
-            BitWidth = Bits(std::min(FD->getBitWidthValue(ASTCtx),
-                                     (unsigned)FullBitWidth.getQuantity()));
+            BitWidth = FD->getBitWidthValue(ASTCtx);
           else if (T == PT_Bool && PackedBools)
-            BitWidth = Bits(1);
-
-          BITCAST_TYPE_SWITCH(T, { P.deref<T>().bitcastToMemory(Buff.get()); });
-
-          if (llvm::sys::IsBigEndianHost)
-            swapBytes(Buff.get(), FullBitWidth.roundToBytes());
+            BitWidth = 1;
+
+          BITCAST_TYPE_SWITCH(T, {
+            T Val = P.deref<T>();
+            Val.bitcastToMemory(Buff.data());
+          });
+          if (SwapData)
+            swapBytes(Buff.data(), ObjectReprChars.getQuantity());
         }
 
-        Buffer.pushData(Buff.get(), BitOffset, BitWidth, TargetEndianness);
+        if (BitWidth != (Buff.size() * 8) && BigEndianTarget) {
+          Buffer.pushData(Buff.data() + (Buff.size() - 1 - (BitWidth / 8)),
+                          BitWidth, BigEndianTarget);
+        } else {
+          Buffer.pushData(Buff.data(), BitWidth, BigEndianTarget);
+        }
         return true;
       });
 }
@@ -284,21 +362,16 @@ bool clang::interp::DoBitCast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
   assert(Ptr.isBlockPointer());
   assert(Buff);
 
-  Bits BitSize = Bytes(BuffSize).toBits();
-  BitcastBuffer Buffer(BitSize);
+  BitcastBuffer Buffer;
   if (!CheckBitcastType(S, OpPC, Ptr.getType(), /*IsToType=*/false))
     return false;
 
   bool Success = readPointerToBuffer(S.getContext(), Ptr, Buffer,
                                      /*ReturnOnUninit=*/false);
-  HasIndeterminateBits = !Buffer.allInitialized();
+  assert(Buffer.size() == BuffSize * 8);
 
-  const ASTContext &ASTCtx = S.getASTContext();
-  Endian TargetEndianness =
-      ASTCtx.getTargetInfo().isLittleEndian() ? Endian::Little : Endian::Big;
-  auto B = Buffer.copyBits(Bits::zero(), BitSize, BitSize, TargetEndianness);
-
-  std::memcpy(Buff, B.get(), BuffSize);
+  HasIndeterminateBits = !Buffer.allInitialized();
+  std::memcpy(Buff, Buffer.data(), BuffSize);
 
   if (llvm::sys::IsBigEndianHost)
     swapBytes(Buff, BuffSize);
@@ -320,60 +393,43 @@ bool clang::interp::DoBitCastPtr(InterpState &S, CodePtr OpPC,
   if (!CheckBitcastType(S, OpPC, FromType, /*IsToType=*/false))
     return false;
 
-  const ASTContext &ASTCtx = S.getASTContext();
-
-  CharUnits ObjectReprChars = ASTCtx.getTypeSizeInChars(ToType);
-  BitcastBuffer Buffer(Bits(ASTCtx.toBits(ObjectReprChars)));
+  BitcastBuffer Buffer;
   readPointerToBuffer(S.getContext(), FromPtr, Buffer,
                       /*ReturnOnUninit=*/false);
 
   // Now read the values out of the buffer again and into ToPtr.
-  Endian TargetEndianness =
-      ASTCtx.getTargetInfo().isLittleEndian() ? Endian::Little : Endian::Big;
+  const ASTContext &ASTCtx = S.getASTContext();
+  size_t BitOffset = 0;
   bool Success = enumeratePointerFields(
-      ToPtr, S.getContext(), Buffer.size(),
-      [&](const Pointer &P, PrimType T, Bits BitOffset,
-          bool PackedBools) -> bool {
-        CharUnits ObjectReprChars = ASTCtx.getTypeSizeInChars(P.getType());
-        Bits FullBitWidth = Bits(ASTCtx.toBits(ObjectReprChars));
+      ToPtr, S.getContext(),
+      [&](const Pointer &P, PrimType T, size_t _, bool PackedBools) -> bool {
         if (T == PT_Float) {
+          CharUnits ObjectReprChars = ASTCtx.getTypeSizeInChars(P.getType());
           const auto &Semantics = ASTCtx.getFloatTypeSemantics(P.getType());
-          Bits NumBits = Bits(llvm::APFloatBase::getSizeInBits(Semantics));
-          assert(NumBits.isFullByte());
-          assert(NumBits.getQuantity() <= FullBitWidth.getQuantity());
-          auto M = Buffer.copyBits(BitOffset, NumBits, FullBitWidth,
-                                   TargetEndianness);
+          unsigned NumBits = llvm::APFloatBase::getSizeInBits(Semantics);
+          assert(NumBits % 8 == 0);
+          assert(NumBits <= ASTCtx.toBits(ObjectReprChars));
+          std::byte *M = Buffer.getBytes(BitOffset);
 
           if (llvm::sys::IsBigEndianHost)
-            swapBytes(M.get(), NumBits.roundToBytes());
+            swapBytes(M, NumBits / 8);
 
-          P.deref<Floating>() = Floating::bitcastFromMemory(M.get(), Semantics);
+          P.deref<Floating>() = Floating::bitcastFromMemory(M, Semantics);
           P.initialize();
+          BitOffset += ASTCtx.toBits(ObjectReprChars);
           return true;
         }
 
-        Bits BitWidth;
-        if (const FieldDecl *FD = P.getField(); FD && FD->isBitField())
-          BitWidth = Bits(std::min(FD->getBitWidthValue(ASTCtx),
-                                   (unsigned)FullBitWidth.getQuantity()));
-        else if (T == PT_Bool && PackedBools)
-          BitWidth = Bits(1);
-        else
-          BitWidth = FullBitWidth;
+        BITCAST_TYPE_SWITCH_FIXED_SIZE(T, {
+          std::byte *M = Buffer.getBytes(BitOffset);
 
-        auto Memory = Buffer.copyBits(BitOffset, BitWidth, FullBitWidth,
-                                      TargetEndianness);
-        if (llvm::sys::IsBigEndianHost)
-          swapBytes(Memory.get(), FullBitWidth.roundToBytes());
+          if (llvm::sys::IsBigEndianHost)
+            swapBytes(M, T::bitWidth() / 8);
 
-        BITCAST_TYPE_SWITCH_FIXED_SIZE(T, {
-          if (BitWidth.nonZero())
-            P.deref<T>() = T::bitcastFromMemory(Memory.get(), T::bitWidth())
-                               .truncate(BitWidth.getQuantity());
-          else
-            P.deref<T>() = T::zero();
+          P.deref<T>() = T::bitcastFromMemory(M, T::bitWidth());
+          P.initialize();
+          BitOffset += T::bitWidth();
         });
-        P.initialize();
         return true;
       });
 

diff  --git a/clang/lib/AST/CMakeLists.txt b/clang/lib/AST/CMakeLists.txt
index cb13c5225b713b..52c6a45de9a26c 100644
--- a/clang/lib/AST/CMakeLists.txt
+++ b/clang/lib/AST/CMakeLists.txt
@@ -65,7 +65,6 @@ add_clang_library(clangAST
   ExternalASTSource.cpp
   FormatString.cpp
   InheritViz.cpp
-  ByteCode/BitcastBuffer.cpp
   ByteCode/ByteCodeEmitter.cpp
   ByteCode/Compiler.cpp
   ByteCode/Context.cpp

diff  --git a/clang/test/AST/ByteCode/builtin-bit-cast-bitfields.cpp b/clang/test/AST/ByteCode/builtin-bit-cast-bitfields.cpp
deleted file mode 100644
index 1bb44f4f13443b..00000000000000
--- a/clang/test/AST/ByteCode/builtin-bit-cast-bitfields.cpp
+++ /dev/null
@@ -1,437 +0,0 @@
-// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -fexperimental-new-constant-interpreter %s
-// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -triple aarch64_be-linux-gnu -fexperimental-new-constant-interpreter %s
-// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -fexperimental-new-constant-interpreter -triple powerpc64le-unknown-unknown -mabi=ieeelongdouble %s
-// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -fexperimental-new-constant-interpreter -triple powerpc64-unknown-unknown -mabi=ieeelongdouble %s
-
-#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
-#  define LITTLE_END 1
-#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-#  define LITTLE_END 0
-#else
-#  error "huh?"
-#endif
-
-typedef decltype(nullptr) nullptr_t;
-typedef __INTPTR_TYPE__ intptr_t;
-typedef unsigned __INT16_TYPE__ uint16_t;
-typedef unsigned __INT32_TYPE__ uint32_t;
-typedef unsigned __INT64_TYPE__ uint64_t;
-
-static_assert(sizeof(int) == 4);
-static_assert(sizeof(long long) == 8);
-
-template <class To, class From>
-constexpr To bit_cast(const From &from) {
-  static_assert(sizeof(To) == sizeof(From));
-  return __builtin_bit_cast(To, from);
-}
-
-template <class Intermediate, class Init>
-constexpr bool check_round_trip(const Init &init) {
-  return bit_cast<Init>(bit_cast<Intermediate>(init)) == init;
-}
-
-template <class Intermediate, class Init>
-constexpr Init round_trip(const Init &init) {
-  return bit_cast<Init>(bit_cast<Intermediate>(init));
-}
-
-namespace std {
-enum byte : unsigned char {};
-} // namespace std
-
-template <int N, typename T = unsigned char, int Pad = 0>
-struct bits {
-  T : Pad;
-  T bits : N;
-
-  constexpr bool operator==(const T& rhs) const {
-    return bits == rhs;
-  }
-};
-
-template <int N, typename T, int P>
-constexpr bool operator==(const struct bits<N, T, P>& lhs, const struct bits<N, T, P>& rhs) {
-  return lhs.bits == rhs.bits;
-}
-
-template<int N>
-struct bytes {
-  using size_t = unsigned int;
-  unsigned char d[N];
-
-  constexpr unsigned char operator[](size_t index) {
-    if (index < N)
-      return d[index];
-    return -1;
-  }
-};
-
-namespace Sanity {
-  /// This is just one byte, and we extract 2 bits from it.
-  ///
-  /// 3 is 0000'0011.
-  /// For both LE and BE, the buffer will contain exactly that
-  /// byte, unaltered and not reordered in any way. It contains all 8 bits.
-  static_assert(__builtin_bit_cast(bits<2>, (unsigned char)3) == (LITTLE_END ? 3 : 0));
-
-  /// Similarly, we have one full byte of data, with the two most-significant
-  /// bits set:
-  /// 192 is 1100'0000
-  static_assert(__builtin_bit_cast(bits<2>, (unsigned char)192) == (LITTLE_END ? 0 : 3));
-
-
-  /// Here we are instead bitcasting two 1-bits into a destination of 8 bits.
-  /// On LE, we should pick the two least-significant bits. On BE, the opposite.
-  /// NOTE: Can't verify this with gcc.
-  constexpr auto B1 = bits<2>{3};
-  static_assert(__builtin_bit_cast(unsigned char, B1) == (LITTLE_END ? 3 : 192));
-
-  /// This should be 0000'0110.
-  /// On LE, this should result in 6.
-  /// On BE, 1100'0000 = 192.
-  constexpr auto B2 = bits<3>{6};
-  static_assert(__builtin_bit_cast(unsigned char, B2) == (LITTLE_END ? 6 : 192));
-
-  constexpr auto B3 = bits<4>{6};
-  static_assert(__builtin_bit_cast(unsigned char, B3) == (LITTLE_END ? 6 : 96));
-
-  struct B {
-    std::byte b0 : 4;
-    std::byte b1 : 4;
-  };
-
-  /// We can properly decompose one byte (8 bit) int two 4-bit bitfields.
-  constexpr struct { unsigned char b0; } T = {0xee};
-  constexpr B MB = __builtin_bit_cast(B, T);
-  static_assert(MB.b0 == 0xe);
-  static_assert(MB.b1 == 0xe);
-}
-
-namespace BitFields {
-  struct BitFields {
-    unsigned a : 2;
-    unsigned b : 30;
-  };
-
-  constexpr unsigned A = __builtin_bit_cast(unsigned, BitFields{3, 16});
-  static_assert(A == (LITTLE_END ? 67 : 3221225488));
-
-  struct S {
-    unsigned a : 2;
-    unsigned b : 28;
-    unsigned c : 2;
-  };
-
-  constexpr S s = __builtin_bit_cast(S, 0xFFFFFFFF);
-  static_assert(s.a == 3);
-  static_assert(s.b == 268435455);
-  static_assert(s.c == 3);
-
-  void bitfield_indeterminate() {
-    struct BF { unsigned char z : 2; };
-    enum byte : unsigned char {};
-
-    constexpr BF bf = {0x3};
-    /// Requires bitcasts to composite types.
-    static_assert(bit_cast<bits<2>>(bf).bits == bf.z);
-    static_assert(bit_cast<unsigned char>(bf));
-
-    static_assert(__builtin_bit_cast(byte, bf));
-
-    struct M {
-      // ref-note at +1 {{subobject declared here}}
-      unsigned char mem[sizeof(BF)];
-    };
-    // ref-error at +2 {{initialized by a constant expression}}
-    // ref-note at +1 {{not initialized}}
-    constexpr M m = bit_cast<M>(bf);
-
-    constexpr auto f = []() constexpr {
-      // bits<24, unsigned int, LITTLE_END ? 0 : 8> B = {0xc0ffee};
-      constexpr struct { unsigned short b1; unsigned char b0;  } B = {0xc0ff, 0xee};
-      return bit_cast<bytes<4>>(B);
-    };
-
-    static_assert(f()[0] + f()[1] + f()[2] == 0xc0 + 0xff + 0xee);
-    {
-      // ref-error at +2 {{initialized by a constant expression}}
-      // ref-note at +1 {{read of uninitialized object is not allowed in a constant expression}}
-      constexpr auto _bad = f()[3];
-    }
-
-    struct B {
-      unsigned short s0 : 8;
-      unsigned short s1 : 8;
-      std::byte b0 : 4;
-      std::byte b1 : 4;
-      std::byte b2 : 4;
-    };
-    constexpr auto g = [f]() constexpr {
-      return bit_cast<B>(f());
-    };
-    static_assert(g().s0 + g().s1 + g().b0 + g().b1 == 0xc0 + 0xff + 0xe + 0xe);
-    {
-      // ref-error at +2 {{initialized by a constant expression}}
-      // ref-note at +1 {{read of uninitialized object is not allowed in a constant expression}}
-      constexpr auto _bad = g().b2;
-    }
-  }
-}
-
-namespace BoolVectors {
-  typedef bool bool32 __attribute__((ext_vector_type(32)));
-  constexpr auto v = bit_cast<bool32>(0xa1c0ffee);
-#if LITTLE_END
-  static_assert(!v[0]);
-  static_assert(v[1]);
-  static_assert(v[2]);
-  static_assert(v[3]);
-  static_assert(!v[4]);
-  static_assert(v[5]);
-  static_assert(v[6]);
-  static_assert(v[7]);
-
-  static_assert(v[8]);
-  static_assert(v[9]);
-  static_assert(v[10]);
-  static_assert(v[11]);
-  static_assert(v[12]);
-  static_assert(v[13]);
-  static_assert(v[14]);
-  static_assert(v[15]);
-
-  static_assert(!v[16]);
-  static_assert(!v[17]);
-  static_assert(!v[18]);
-  static_assert(!v[19]);
-  static_assert(!v[20]);
-  static_assert(!v[21]);
-  static_assert(v[22]);
-  static_assert(v[23]);
-
-  static_assert(v[24]);
-  static_assert(!v[25]);
-  static_assert(!v[26]);
-  static_assert(!v[27]);
-  static_assert(!v[28]);
-  static_assert(v[29]);
-  static_assert(!v[30]);
-  static_assert(v[31]);
-
-#else
-  static_assert(v[0]);
-  static_assert(!v[1]);
-  static_assert(v[2]);
-  static_assert(!v[3]);
-  static_assert(!v[4]);
-  static_assert(!v[5]);
-  static_assert(!v[6]);
-  static_assert(v[7]);
-
-  static_assert(v[8]);
-  static_assert(v[9]);
-  static_assert(!v[10]);
-  static_assert(!v[11]);
-  static_assert(!v[12]);
-  static_assert(!v[13]);
-  static_assert(!v[14]);
-  static_assert(!v[15]);
-
-  static_assert(v[16]);
-  static_assert(v[17]);
-  static_assert(v[18]);
-  static_assert(v[19]);
-  static_assert(v[20]);
-  static_assert(v[21]);
-  static_assert(v[22]);
-  static_assert(v[23]);
-
-  static_assert(v[24]);
-  static_assert(v[25]);
-  static_assert(v[26]);
-  static_assert(!v[27]);
-  static_assert(v[28]);
-  static_assert(v[29]);
-  static_assert(v[30]);
-  static_assert(!v[31]);
-#endif
-
-  struct pad {
-    unsigned short s;
-    unsigned char c;
-  };
-
-  constexpr auto p = bit_cast<pad>(v);
-  static_assert(p.s == (LITTLE_END ? 0xffee : 0xa1c0));
-  static_assert(p.c == (LITTLE_END ? 0xc0 : 0xff));
-}
-
-namespace TwoShorts {
-  struct B {
-    unsigned short s0 : 8;
-    unsigned short s1 : 8;
-  };
-  constexpr struct { unsigned short b1;} T = {0xc0ff};
-  constexpr B MB = __builtin_bit_cast(B, T);
-#if LITTLE_END
-    static_assert(MB.s0 == 0xff);
-    static_assert(MB.s1 == 0xc0);
-#else
-    static_assert(MB.s0 == 0xc0);
-    static_assert(MB.s1 == 0xff);
-
-#endif
-}
-
-typedef bool bool8 __attribute__((ext_vector_type(8)));
-typedef bool bool9 __attribute__((ext_vector_type(9)));
-typedef bool bool16 __attribute__((ext_vector_type(16)));
-typedef bool bool17 __attribute__((ext_vector_type(17)));
-typedef bool bool32 __attribute__((ext_vector_type(32)));
-typedef bool bool128 __attribute__((ext_vector_type(128)));
-
-static_assert(bit_cast<unsigned char>(bool8{1,0,1,0,1,0,1,0}) == (LITTLE_END ? 0x55 : 0xAA), "");
-constexpr bool8 b8 = __builtin_bit_cast(bool8, 0x55); // both-error {{'__builtin_bit_cast' source type 'int' does not match destination type 'bool8' (vector of 8 'bool' values) (4 vs 1 bytes)}}
-static_assert(check_round_trip<bool8>(static_cast<unsigned char>(0)), "");
-static_assert(check_round_trip<bool8>(static_cast<unsigned char>(1)), "");
-static_assert(check_round_trip<bool8>(static_cast<unsigned char>(0x55)), "");
-
-static_assert(bit_cast<unsigned short>(bool16{1,1,1,1,1,0,0,0, 1,1,1,1,0,1,0,0}) == (LITTLE_END ? 0x2F1F : 0xF8F4), "");
-
-static_assert(check_round_trip<bool16>(static_cast<short>(0xCAFE)), "");
-static_assert(check_round_trip<bool32>(static_cast<int>(0xCAFEBABE)), "");
-static_assert(check_round_trip<bool128>(static_cast<__int128_t>(0xCAFEBABE0C05FEFEULL)), "");
-
-static_assert(bit_cast<bits<8, uint16_t, 7>, uint16_t>(0xcafe) == (LITTLE_END ? 0x95 : 0x7f));
-static_assert(bit_cast<bits<4, uint16_t, 10>, uint16_t>(0xcafe) == (LITTLE_END ? 0x2 : 0xf));
-static_assert(bit_cast<bits<4, uint32_t, 19>, uint32_t>(0xa1cafe) == (LITTLE_END ? 0x4 : 0x5));
-
-struct S {
-  // little endian:
-  //    MSB .... .... LSB
-  //        |y|   |x|
-  //
-  // big endian
-  //    MSB .... .... LSB
-  //        |x|   |y|
-
-  unsigned char x : 4;
-  unsigned char y : 4;
-
-  constexpr bool operator==(S const &other) const {
-    return x == other.x && y == other.y;
-  }
-};
-
-constexpr S s{0xa, 0xb};
-static_assert(bit_cast<bits<8>>(s) == (LITTLE_END ? 0xba : 0xab));
-static_assert(bit_cast<bits<7>>(s) == (LITTLE_END
-                                            ? 0xba & 0x7f
-                                            : (0xab & 0xfe) >> 1));
-
-static_assert(round_trip<bits<8>>(s) == s);
-
-struct R {
-  unsigned int r : 31;
-  unsigned int : 0;
-  unsigned int : 32;
-  constexpr bool operator==(R const &other) const {
-    return r == other.r;
-  }
- };
-using T = bits<31, signed long long>;
-constexpr R r{0x4ac0ffee};
-constexpr T t = bit_cast<T>(r);
-static_assert(t == ((0xFFFFFFFF8 << 28) | 0x4ac0ffee)); // sign extension
-
-static_assert(round_trip<T>(r) == r);
-static_assert(round_trip<R>(t) == t);
-
-
-/// The oversized bitfield is an error on Windows and not just a warning.
-#if !defined(_WIN32)
-struct U {
-  // expected-warning at +1 {{exceeds the width of its type}}
-  uint32_t trunc : 33;
-  uint32_t u : 31;
-  constexpr bool operator==(U const &other) const {
-    return trunc == other.trunc && u == other.u;
-  }
-};
-struct V {
-  uint64_t notrunc : 32;
-  uint64_t : 1;
-  uint64_t v : 31;
-  constexpr bool operator==(V const &other) const {
-    return notrunc == other.notrunc && v == other.v;
-  }
-};
-
-constexpr U u{static_cast<unsigned int>(~0), 0x4ac0ffee};
-constexpr V v = bit_cast<V>(u);
-static_assert(v.v == 0x4ac0ffee);
-
-static_assert(round_trip<V>(u) == u);
-static_assert(round_trip<U>(v) == v);
-
-constexpr auto w = bit_cast<bits<12, unsigned long, 33>>(u);
-static_assert(w == (LITTLE_END
-                    ? 0x4ac0ffee & 0xFFF
-                    : (0x4ac0ffee & (0xFFF << (31 - 12))) >> (31-12)
-                  ));
-#endif
-
-
-namespace NestedStructures {
-  struct J {
-    struct {
-      uint16_t  k : 12;
-    } K;
-    struct {
-      uint16_t  l : 4;
-    } L;
-  };
-
-  static_assert(sizeof(J) == 4);
-  constexpr J j = bit_cast<J>(0x8c0ffee5);
-
-  static_assert(j.K.k == (LITTLE_END ? 0xee5 : 0x8c0));
-  static_assert(j.L.l == 0xf /* yay symmetry */);
-  static_assert(bit_cast<bits<4, uint16_t, 16>>(j) == 0xf);
-  struct N {
-    bits<12, uint16_t> k;
-    uint16_t : 16;
-  };
-  static_assert(bit_cast<N>(j).k == j.K.k);
-
-  struct M {
-    bits<4, uint16_t, 0> m[2];
-    constexpr bool operator==(const M& rhs) const {
-      return m[0] == rhs.m[0] && m[1] == rhs.m[1];
-    };
-  };
-  #if LITTLE_END == 1
-  constexpr uint16_t want[2] = {0x5, 0xf};
-  #else
-  constexpr uint16_t want[2] = {0x8000, 0xf000};
-  #endif
-
-  static_assert(bit_cast<M>(j) == bit_cast<M>(want));
-}
-
-namespace Enums {
-  // ensure we're packed into the top 2 bits
-  constexpr int pad = LITTLE_END ? 6 : 0;
-  struct X
-  {
-    char : pad;
-    enum class direction: char { left, right, up, down } direction : 2;
-  };
-
-  constexpr X x = { X::direction::down };
-  static_assert(bit_cast<bits<2, signed char, pad>>(x) == -1);
-  static_assert(bit_cast<bits<2, unsigned char, pad>>(x) == 3);
-  static_assert(
-    bit_cast<X>((unsigned char)0x40).direction == X::direction::right);
-}

diff  --git a/clang/test/AST/ByteCode/builtin-bit-cast.cpp b/clang/test/AST/ByteCode/builtin-bit-cast.cpp
index e956675b18b85f..0fecde59cd57ce 100644
--- a/clang/test/AST/ByteCode/builtin-bit-cast.cpp
+++ b/clang/test/AST/ByteCode/builtin-bit-cast.cpp
@@ -186,6 +186,72 @@ namespace bitint {
                                 // ref-note {{initializer of 'IB' is not a constant expression}}
 }
 
+namespace BitFields {
+  struct BitFields {
+    unsigned a : 2;
+    unsigned b : 30;
+  };
+
+  constexpr unsigned A = __builtin_bit_cast(unsigned, BitFields{3, 16}); // ref-error {{must be initialized by a constant expression}} \
+                                                                         // ref-note {{not yet supported}} \
+                                                                         // ref-note {{declared here}}
+  static_assert(A == (LITTLE_END ? 67 : 3221225488)); // ref-error {{not an integral constant expression}} \
+                                                      // ref-note {{initializer of 'A'}}
+
+
+  void bitfield_indeterminate() {
+    struct BF { unsigned char z : 2; };
+    enum byte : unsigned char {};
+
+    constexpr BF bf = {0x3};
+    /// Requires bitcasts to composite types.
+    // static_assert(bit_cast<bits<2>>(bf).bits == bf.z);
+    // static_assert(bit_cast<unsigned char>(bf));
+
+#if 0
+    // static_assert(__builtin_bit_cast(byte, bf));
+
+    struct M {
+      // expected-note at +1 {{subobject declared here}}
+      unsigned char mem[sizeof(BF)];
+    };
+    // expected-error at +2 {{initialized by a constant expression}}
+    // expected-note at +1 {{not initialized}}
+    constexpr M m = bit_cast<M>(bf);
+
+    constexpr auto f = []() constexpr {
+      // bits<24, unsigned int, LITTLE_END ? 0 : 8> B = {0xc0ffee};
+      constexpr struct { unsigned short b1; unsigned char b0;  } B = {0xc0ff, 0xee};
+      return bit_cast<bytes<4>>(B);
+    };
+
+    static_assert(f()[0] + f()[1] + f()[2] == 0xc0 + 0xff + 0xee);
+    {
+      // expected-error at +2 {{initialized by a constant expression}}
+      // expected-note at +1 {{read of uninitialized object is not allowed in a constant expression}}
+      constexpr auto _bad = f()[3];
+    }
+
+    struct B {
+      unsigned short s0 : 8;
+      unsigned short s1 : 8;
+      std::byte b0 : 4;
+      std::byte b1 : 4;
+      std::byte b2 : 4;
+    };
+    constexpr auto g = [f]() constexpr {
+      return bit_cast<B>(f());
+    };
+    static_assert(g().s0 + g().s1 + g().b0 + g().b1 == 0xc0 + 0xff + 0xe + 0xe);
+    {
+      // expected-error at +2 {{initialized by a constant expression}}
+      // expected-note at +1 {{read of uninitialized object is not allowed in a constant expression}}
+      constexpr auto _bad = g().b2;
+    }
+#endif
+  }
+}
+
 namespace Classes {
   class A {
   public:
@@ -265,21 +331,6 @@ static_assert(check_round_trip<long long>(splice));
 #endif
 
 
-namespace Overread {
-  /// This used to crash becaus we were reading all elements of the
-  /// source array even though we should only be reading 1.
-  constexpr int a[] = {2,3, 4, 5};
-  constexpr int b = __builtin_bit_cast(int, *(a + 1));
-  static_assert(b == 3);
-
-  struct S {
-    int a;
-  };
-  constexpr S ss[] = {{1},{2}};
-  constexpr int c = __builtin_bit_cast(int, *(ss + 1));
-  static_assert(c == 2);
-}
-
 
 /// ---------------------------------------------------------------------------
 /// From here on, it's things copied from test/SemaCXX/constexpr-builtin-bit.cast.cpp
@@ -459,6 +510,27 @@ static_assert(bit_cast<unsigned long long>(test_vector) == (LITTLE_END
 static_assert(check_round_trip<uint2>(0xCAFEBABE0C05FEFEULL), "");
 static_assert(check_round_trip<byte8>(0xCAFEBABE0C05FEFEULL), "");
 
+typedef bool bool8 __attribute__((ext_vector_type(8)));
+typedef bool bool9 __attribute__((ext_vector_type(9)));
+typedef bool bool16 __attribute__((ext_vector_type(16)));
+typedef bool bool17 __attribute__((ext_vector_type(17)));
+typedef bool bool32 __attribute__((ext_vector_type(32)));
+typedef bool bool128 __attribute__((ext_vector_type(128)));
+
+static_assert(bit_cast<unsigned char>(bool8{1,0,1,0,1,0,1,0}) == (LITTLE_END ? 0x55 : 0xAA), "");
+constexpr bool8 b8 = __builtin_bit_cast(bool8, 0x55); // both-error {{'__builtin_bit_cast' source type 'int' does not match destination type 'bool8' (vector of 8 'bool' values) (4 vs 1 bytes)}}
+#if 0
+static_assert(check_round_trip<bool8>(static_cast<unsigned char>(0)), "");
+static_assert(check_round_trip<bool8>(static_cast<unsigned char>(1)), "");
+static_assert(check_round_trip<bool8>(static_cast<unsigned char>(0x55)), "");
+
+static_assert(bit_cast<unsigned short>(bool16{1,1,1,1,1,0,0,0, 1,1,1,1,0,1,0,0}) == (LITTLE_END ? 0x2F1F : 0xF8F4), "");
+
+static_assert(check_round_trip<bool16>(static_cast<short>(0xCAFE)), "");
+static_assert(check_round_trip<bool32>(static_cast<int>(0xCAFEBABE)), "");
+static_assert(check_round_trip<bool128>(static_cast<__int128_t>(0xCAFEBABE0C05FEFEULL)), "");
+#endif
+
 #if 0
 // expected-error at +2 {{constexpr variable 'bad_bool9_to_short' must be initialized by a constant expression}}
 // expected-note at +1 {{bit_cast involving type 'bool __attribute__((ext_vector_type(9)))' (vector of 9 'bool' values) is not allowed in a constant expression; element size 1 * element count 9 is not a multiple of the byte size 8}}
@@ -487,20 +559,3 @@ namespace test_complex {
   constexpr double D = __builtin_bit_cast(double, test_float_complex);
   constexpr int M = __builtin_bit_cast(int, test_int_complex); // both-error {{size of '__builtin_bit_cast' source type 'const _Complex unsigned int' does not match destination type 'int' (8 vs 4 bytes)}}
 }
-
-
-namespace OversizedBitField {
-#if defined(_WIN32)
-  /// This is an error (not just a warning) on Windows and the field ends up with a size of 1 instead of 4.
-#else
-  typedef unsigned __INT16_TYPE__ uint16_t;
-  typedef unsigned __INT32_TYPE__ uint32_t;
-  struct S {
-    uint16_t a : 20; // both-warning {{exceeds the width of its type}}
-  };
-
-  static_assert(sizeof(S) == 4);
-  static_assert(__builtin_bit_cast(S, (uint32_t)32).a == (LITTLE_END ? 32 : 0)); // ref-error {{not an integral constant expression}} \
-                                                                                 // ref-note {{constexpr bit_cast involving bit-field is not yet supported}}
-#endif
-}

diff  --git a/clang/unittests/AST/ByteCode/BitcastBuffer.cpp b/clang/unittests/AST/ByteCode/BitcastBuffer.cpp
deleted file mode 100644
index 02c38a22013630..00000000000000
--- a/clang/unittests/AST/ByteCode/BitcastBuffer.cpp
+++ /dev/null
@@ -1,87 +0,0 @@
-#include "../../../lib/AST/ByteCode/BitcastBuffer.h"
-#include "clang/AST/ASTContext.h"
-#include "gtest/gtest.h"
-#include <bitset>
-#include <cassert>
-#include <cmath>
-#include <memory>
-#include <string>
-
-using namespace clang;
-using namespace clang::interp;
-
-TEST(BitcastBuffer, PushData) {
-  BitcastBuffer Buff1(Bytes(sizeof(int)).toBits());
-
-  const unsigned V = 0xCAFEBABE;
-  Bits VSize = Bytes(sizeof(V)).toBits();
-  std::byte Data[sizeof(V)];
-  std::memcpy(Data, &V, sizeof(V));
-
-  Endian HostEndianness =
-      llvm::sys::IsLittleEndianHost ? Endian::Little : Endian::Big;
-
-  Buff1.pushData(Data, Bits::zero(), VSize, HostEndianness);
-
-  // The buffer is in host-endianness.
-  if (llvm::sys::IsLittleEndianHost) {
-    ASSERT_EQ(Buff1.Data[0], std::byte{0xbe});
-    ASSERT_EQ(Buff1.Data[1], std::byte{0xba});
-    ASSERT_EQ(Buff1.Data[2], std::byte{0xfe});
-    ASSERT_EQ(Buff1.Data[3], std::byte{0xca});
-  } else {
-    ASSERT_EQ(Buff1.Data[0], std::byte{0xca});
-    ASSERT_EQ(Buff1.Data[1], std::byte{0xfe});
-    ASSERT_EQ(Buff1.Data[2], std::byte{0xba});
-    ASSERT_EQ(Buff1.Data[3], std::byte{0xbe});
-  }
-
-  {
-    unsigned V2;
-    auto D = Buff1.copyBits(Bits::zero(), VSize, VSize, Endian::Little);
-    std::memcpy(&V2, D.get(), sizeof(V));
-    ASSERT_EQ(V, V2);
-
-    D = Buff1.copyBits(Bits::zero(), VSize, VSize, Endian::Big);
-    std::memcpy(&V2, D.get(), sizeof(V));
-    ASSERT_EQ(V, V2);
-  }
-
-  BitcastBuffer Buff2(Bytes(sizeof(int)).toBits());
-  {
-    short s1 = 0xCAFE;
-    short s2 = 0xBABE;
-    std::byte sdata[2];
-
-    std::memcpy(sdata, &s1, sizeof(s1));
-    Buff2.pushData(sdata, Bits::zero(), Bits(sizeof(s1) * 8), HostEndianness);
-    std::memcpy(sdata, &s2, sizeof(s2));
-    Buff2.pushData(sdata, Bits(sizeof(s1) * 8), Bits(sizeof(s2) * 8),
-                   HostEndianness);
-  }
-
-  if (llvm::sys::IsLittleEndianHost) {
-    ASSERT_EQ(Buff2.Data[0], std::byte{0xfe});
-    ASSERT_EQ(Buff2.Data[1], std::byte{0xca});
-    ASSERT_EQ(Buff2.Data[2], std::byte{0xbe});
-    ASSERT_EQ(Buff2.Data[3], std::byte{0xba});
-  } else {
-    ASSERT_EQ(Buff2.Data[0], std::byte{0xba});
-    ASSERT_EQ(Buff2.Data[1], std::byte{0xbe});
-    ASSERT_EQ(Buff2.Data[2], std::byte{0xca});
-    ASSERT_EQ(Buff2.Data[3], std::byte{0xfe});
-  }
-
-  {
-    unsigned V;
-    auto D = Buff2.copyBits(Bits::zero(), Bits(sizeof(V) * 8),
-                            Bits(sizeof(V) * 8), Endian::Little);
-    std::memcpy(&V, D.get(), sizeof(V));
-    ASSERT_EQ(V, 0xBABECAFE);
-
-    D = Buff2.copyBits(Bits::zero(), Bits(sizeof(V) * 8), Bits(sizeof(V) * 8),
-                       Endian::Big);
-    std::memcpy(&V, D.get(), sizeof(V));
-    ASSERT_EQ(V, 0xBABECAFE);
-  }
-}

diff  --git a/clang/unittests/AST/ByteCode/CMakeLists.txt b/clang/unittests/AST/ByteCode/CMakeLists.txt
index b862fb4834fbdc..ea727cdd4412be 100644
--- a/clang/unittests/AST/ByteCode/CMakeLists.txt
+++ b/clang/unittests/AST/ByteCode/CMakeLists.txt
@@ -1,5 +1,4 @@
 add_clang_unittest(InterpTests
-  BitcastBuffer.cpp
   Descriptor.cpp
   toAPValue.cpp
   )


        


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