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//===--- BitPatternReader.h - Split bit patterns into chunks. -------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2019 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
#pragma once
#include "swift/Basic/ClusteredBitVector.h"
#include "llvm/ADT/APInt.h"
#include <optional>
namespace swift {
namespace irgen {
/// BitPatternReader allows an APInt to be read from in chunks.
/// Chunks may be read starting from either the least significant bit
/// (little-endian) or the most significant bit (big-endian).
///
/// This is useful when interpreting an APInt as a multi-byte mask
/// that needs to be applied to a value with a composite type.
///
/// Example:
///
/// // big-endian
/// auto x = BitPatternReader(APInt(32, 0x1234), false);
/// x.read(16) // 0x12
/// x.read(8) // 0x3
/// x.read(8) // 0x4
///
/// // little-endian
/// auto y = BitPatternReader(APInt(32, 0x1234), true);
/// y.read(16) // 0x34
/// y.read(8) // 0x2
/// y.read(8) // 0x1
///
class BitPatternReader {
using APInt = llvm::APInt;
const APInt Value;
const bool LittleEndian;
unsigned Offset = 0;
public:
/// If the reader is in little-endian mode then bits will be read
/// from the least significant to the most significant. Otherwise
/// they will be read from the most significant to the least
/// significant.
BitPatternReader(const APInt &value, bool littleEndian) :
Value(value),
LittleEndian(littleEndian) {}
/// Read the given number of bits from the unread part of the
/// underlying value and adjust the remaining value as appropriate.
APInt read(unsigned numBits) {
assert(numBits % 8 == 0);
assert(Value.getBitWidth() >= Offset + numBits);
unsigned offset = Offset;
if (!LittleEndian) {
offset = Value.getBitWidth() - offset - numBits;
}
Offset += numBits;
return Value.extractBits(numBits, offset);
}
// Skip the number of bits provided.
void skip(unsigned numBits) {
assert(numBits % 8 == 0);
assert(Value.getBitWidth() >= Offset + numBits);
Offset += numBits;
}
};
} // end namespace irgen
} // end namespace swift
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