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220 lines
6.1 KiB
C
220 lines
6.1 KiB
C
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//===- BitstreamReader.h - Low-level bitstream reader interface -*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by Chris Lattner and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This header defines the BitstreamReader class. This class can be used to
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// read an arbitrary bitstream, regardless of its contents.
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//
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//===----------------------------------------------------------------------===//
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#ifndef BITSTREAM_READER_H
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#define BITSTREAM_READER_H
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#include "llvm/Bitcode/BitCodes.h"
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#include "llvm/ADT/SmallVector.h"
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#include <cassert>
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namespace llvm {
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class BitstreamReader {
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const unsigned char *NextChar;
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const unsigned char *LastChar;
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/// CurWord - This is the current data we have pulled from the stream but have
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/// not returned to the client.
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uint32_t CurWord;
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/// BitsInCurWord - This is the number of bits in CurWord that are valid. This
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/// is always from [0...31] inclusive.
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unsigned BitsInCurWord;
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// CurCodeSize - This is the declared size of code values used for the current
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// block, in bits.
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unsigned CurCodeSize;
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/// BlockScope - This tracks the codesize of parent blocks.
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SmallVector<unsigned, 8> BlockScope;
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public:
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BitstreamReader(const unsigned char *Start, const unsigned char *End)
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: NextChar(Start), LastChar(End) {
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assert(((End-Start) & 3) == 0 &&"Bitcode stream not a multiple of 4 bytes");
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CurWord = 0;
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BitsInCurWord = 0;
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CurCodeSize = 2;
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}
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bool AtEndOfStream() const { return NextChar == LastChar; }
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uint32_t Read(unsigned NumBits) {
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// If the field is fully contained by CurWord, return it quickly.
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if (BitsInCurWord >= NumBits) {
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uint32_t R = CurWord & ((1U << NumBits)-1);
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CurWord >>= NumBits;
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BitsInCurWord -= NumBits;
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return R;
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}
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// If we run out of data, stop at the end of the stream.
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if (LastChar == NextChar) {
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CurWord = 0;
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BitsInCurWord = 0;
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return 0;
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}
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unsigned R = CurWord;
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// Read the next word from the stream.
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CurWord = (NextChar[0] << 0) | (NextChar[1] << 8) |
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(NextChar[2] << 16) | (NextChar[3] << 24);
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NextChar += 4;
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// Extract NumBits-BitsInCurWord from what we just read.
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unsigned BitsLeft = NumBits-BitsInCurWord;
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// Be careful here, BitsLeft is in the range [1..32] inclusive.
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R |= (CurWord & (~0U >> (32-BitsLeft))) << BitsInCurWord;
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// BitsLeft bits have just been used up from CurWord.
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if (BitsLeft != 32)
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CurWord >>= BitsLeft;
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else
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CurWord = 0;
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BitsInCurWord = 32-BitsLeft;
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return R;
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}
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uint32_t ReadVBR(unsigned NumBits) {
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uint32_t Piece = Read(NumBits);
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if ((Piece & (1U << NumBits-1)) == 0)
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return Piece;
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uint32_t Result = 0;
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unsigned NextBit = 0;
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while (1) {
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Result |= (Piece & ((1U << (NumBits-1))-1)) << NextBit;
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if ((Piece & (1U << NumBits-1)) == 0)
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return Result;
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NextBit += NumBits-1;
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Piece = Read(NumBits);
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}
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}
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uint64_t ReadVBR64(unsigned NumBits) {
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uint64_t Piece = Read(NumBits);
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if ((Piece & (1U << NumBits-1)) == 0)
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return Piece;
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uint64_t Result = 0;
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unsigned NextBit = 0;
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while (1) {
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Result |= (Piece & ((1U << (NumBits-1))-1)) << NextBit;
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if ((Piece & (1U << NumBits-1)) == 0)
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return Result;
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NextBit += NumBits-1;
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Piece = Read(NumBits);
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}
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}
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void SkipToWord() {
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BitsInCurWord = 0;
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CurWord = 0;
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}
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unsigned ReadCode() {
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return Read(CurCodeSize);
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}
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//===--------------------------------------------------------------------===//
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// Block Manipulation
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//===--------------------------------------------------------------------===//
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// Block header:
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// [ENTER_SUBBLOCK, blockid, newcodelen, <align4bytes>, blocklen]
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/// ReadSubBlockID - Having read the ENTER_SUBBLOCK code, read the BlockID for
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/// the block.
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unsigned ReadSubBlockID() {
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return ReadVBR(bitc::BlockIDWidth);
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}
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/// SkipBlock - Having read the ENTER_SUBBLOCK abbrevid and a BlockID, skip
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/// over the body of this block. If the block record is malformed, return
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/// true.
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bool SkipBlock() {
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// Read and ignore the codelen value. Since we are skipping this block, we
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// don't care what code widths are used inside of it.
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ReadVBR(bitc::CodeLenWidth);
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SkipToWord();
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unsigned NumWords = Read(bitc::BlockSizeWidth);
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// Check that the block wasn't partially defined, and that the offset isn't
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// bogus.
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if (AtEndOfStream() || NextChar+NumWords*4 > LastChar)
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return true;
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NextChar += NumWords*4;
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return false;
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}
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/// EnterSubBlock - Having read the ENTER_SUBBLOCK abbrevid, read and enter
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/// the block, returning the BlockID of the block we just entered.
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bool EnterSubBlock() {
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BlockScope.push_back(CurCodeSize);
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// Get the codesize of this block.
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CurCodeSize = ReadVBR(bitc::CodeLenWidth);
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SkipToWord();
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unsigned NumWords = Read(bitc::BlockSizeWidth);
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// Validate that this block is sane.
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if (CurCodeSize == 0 || AtEndOfStream() || NextChar+NumWords*4 > LastChar)
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return true;
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return false;
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}
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bool ReadBlockEnd() {
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if (BlockScope.empty()) return true;
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// Block tail:
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// [END_BLOCK, <align4bytes>]
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SkipToWord();
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CurCodeSize = BlockScope.back();
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BlockScope.pop_back();
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return false;
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}
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//===--------------------------------------------------------------------===//
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// Record Processing
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//===--------------------------------------------------------------------===//
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unsigned ReadRecord(unsigned AbbrevID, SmallVectorImpl<uint64_t> &Vals) {
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if (AbbrevID == bitc::UNABBREV_RECORD) {
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unsigned Code = ReadVBR(6);
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unsigned NumElts = ReadVBR(6);
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for (unsigned i = 0; i != NumElts; ++i)
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Vals.push_back(ReadVBR64(6));
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return Code;
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}
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assert(0 && "Reading with abbrevs not implemented!");
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}
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};
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} // End llvm namespace
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#endif
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