[clang] [clang][bytecode] Handle bitcasts involving bitfields (PR #116843)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,66 @@
+//===--------------------- 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 };
+
+/// Returns the value of the bit in the given sequence of bytes.
+static inline bool bitof(const std::byte *B, unsigned BitIndex) {
+  return (B[BitIndex / 8] & (std::byte{1} << (BitIndex % 8))) != std::byte{0};
+}
+
+/// Returns whether \p N is a full byte offset or size.
+static inline bool fullByte(unsigned N) { return N % 8 == 0; }
+
+/// Track what bits have been initialized to known values and which ones
+/// have indeterminate value.
+/// All offsets are in bits.
+struct BitcastBuffer {
+  size_t FinalBitSize = 0;
+  std::unique_ptr<std::byte[]> Data;
+
+  BitcastBuffer(size_t FinalBitSize) : FinalBitSize(FinalBitSize) {
+    assert(fullByte(FinalBitSize));
+    unsigned ByteSize = FinalBitSize / 8;
+    Data = std::make_unique<std::byte[]>(ByteSize);
+  }
+
+  /// Returns the buffer size in bits.
+  size_t size() const { return FinalBitSize; }
+
+  /// Returns \c true if all bits in the buffer have been initialized.
+  bool allInitialized() const {
+    // FIXME: Implement.
+    return true;
----------------
AaronBallman wrote:

We are calling this function from `clang::interp::DoBitCast()`; should this be implemented now?

https://github.com/llvm/llvm-project/pull/116843