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//===- BitstreamWriter.h - Low-level bitstream writer interface -*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by Chris Lattner and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This header defines the BitstreamWriter class. This class can be used to
// write an arbitrary bitstream, regardless of its contents.
//
//===----------------------------------------------------------------------===//
#ifndef BITSTREAM_WRITER_H
#define BITSTREAM_WRITER_H
#include "llvm/Bitcode/BitCodes.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/DataTypes.h"
#include <cassert>
#include <vector>
namespace llvm {
class BitstreamWriter {
std::vector<unsigned char> &Out;
/// CurBit - Always between 0 and 31 inclusive, specifies the next bit to use.
unsigned CurBit;
/// CurValue - The current value. Only bits < CurBit are valid.
uint32_t CurValue;
// CurCodeSize - This is the declared size of code values used for the current
// block, in bits.
unsigned CurCodeSize;
struct Block {
unsigned PrevCodeSize;
unsigned StartSizeWord;
Block(unsigned PCS, unsigned SSW) : PrevCodeSize(PCS), StartSizeWord(SSW) {}
};
/// BlockScope - This tracks the current blocks that we have entered.
std::vector<Block> BlockScope;
public:
BitstreamWriter(std::vector<unsigned char> &O)
: Out(O), CurBit(0), CurValue(0), CurCodeSize(2) {}
~BitstreamWriter() {
assert(CurBit == 0 && "Unflused data remaining");
assert(BlockScope.empty() && "Block imbalance");
}
//===--------------------------------------------------------------------===//
// Basic Primitives for emitting bits to the stream.
//===--------------------------------------------------------------------===//
void Emit(uint32_t Val, unsigned NumBits) {
assert(NumBits <= 32 && "Invalid value size!");
assert((Val & ~(~0U >> (32-NumBits))) == 0 && "High bits set!");
CurValue |= Val << CurBit;
if (CurBit + NumBits < 32) {
CurBit += NumBits;
return;
}
// Add the current word.
unsigned V = CurValue;
Out.push_back((unsigned char)(V >> 0));
Out.push_back((unsigned char)(V >> 8));
Out.push_back((unsigned char)(V >> 16));
Out.push_back((unsigned char)(V >> 24));
if (CurBit)
CurValue = Val >> (32-CurBit);
else
CurValue = 0;
CurBit = (CurBit+NumBits) & 31;
}
void Emit64(uint64_t Val, unsigned NumBits) {
if (NumBits <= 32)
Emit((uint32_t)Val, NumBits);
else {
Emit((uint32_t)Val, 32);
Emit((uint32_t)(Val >> 32), NumBits-32);
}
}
void FlushToWord() {
if (CurBit) {
unsigned V = CurValue;
Out.push_back((unsigned char)(V >> 0));
Out.push_back((unsigned char)(V >> 8));
Out.push_back((unsigned char)(V >> 16));
Out.push_back((unsigned char)(V >> 24));
CurBit = 0;
CurValue = 0;
}
}
void EmitVBR(uint32_t Val, unsigned NumBits) {
uint32_t Threshold = 1U << (NumBits-1);
// Emit the bits with VBR encoding, NumBits-1 bits at a time.
while (Val >= Threshold) {
Emit((Val & ((1 << (NumBits-1))-1)) | (1 << (NumBits-1)), NumBits);
Val >>= NumBits-1;
}
Emit(Val, NumBits);
}
void EmitVBR64(uint64_t Val, unsigned NumBits) {
if ((uint32_t)Val == Val)
return EmitVBR((uint32_t)Val, NumBits);
uint64_t Threshold = 1U << (NumBits-1);
// Emit the bits with VBR encoding, NumBits-1 bits at a time.
while (Val >= Threshold) {
Emit(((uint32_t)Val & ((1 << (NumBits-1))-1)) |
(1 << (NumBits-1)), NumBits);
Val >>= NumBits-1;
}
Emit((uint32_t)Val, NumBits);
}
/// EmitCode - Emit the specified code.
void EmitCode(unsigned Val) {
Emit(Val, CurCodeSize);
}
//===--------------------------------------------------------------------===//
// Block Manipulation
//===--------------------------------------------------------------------===//
void EnterSubblock(unsigned BlockID, unsigned CodeLen) {
// Block header:
// [ENTER_SUBBLOCK, blockid, newcodelen, <align4bytes>, blocklen]
EmitCode(bitc::ENTER_SUBBLOCK);
EmitVBR(BlockID, bitc::BlockIDWidth);
EmitVBR(CodeLen, bitc::CodeLenWidth);
FlushToWord();
BlockScope.push_back(Block(CurCodeSize, Out.size()/4));
// Emit a placeholder, which will be replaced when the block is popped.
Emit(0, bitc::BlockSizeWidth);
CurCodeSize = CodeLen;
}
void ExitBlock() {
assert(!BlockScope.empty() && "Block scope imbalance!");
Block B = BlockScope.back();
BlockScope.pop_back();
// Block tail:
// [END_BLOCK, <align4bytes>]
EmitCode(bitc::END_BLOCK);
FlushToWord();
// Compute the size of the block, in words, not counting the size field.
unsigned SizeInWords = Out.size()/4-B.StartSizeWord - 1;
unsigned ByteNo = B.StartSizeWord*4;
// Update the block size field in the header of this sub-block.
Out[ByteNo++] = (unsigned char)(SizeInWords >> 0);
Out[ByteNo++] = (unsigned char)(SizeInWords >> 8);
Out[ByteNo++] = (unsigned char)(SizeInWords >> 16);
Out[ByteNo++] = (unsigned char)(SizeInWords >> 24);
// Restore the outer block's code size.
CurCodeSize = B.PrevCodeSize;
}
//===--------------------------------------------------------------------===//
// Record Emission
//===--------------------------------------------------------------------===//
/// EmitRecord - Emit the specified record to the stream, using an abbrev if
/// we have one to compress the output.
void EmitRecord(unsigned Code, SmallVectorImpl<uint64_t> &Vals,
unsigned Abbrev = 0) {
if (Abbrev) {
assert(0 && "abbrevs not implemented yet!");
} else {
// If we don't have an abbrev to use, emit this in its fully unabbreviated
// form.
EmitCode(bitc::UNABBREV_RECORD);
EmitVBR(Code, 6);
EmitVBR(Vals.size(), 6);
for (unsigned i = 0, e = Vals.size(); i != e; ++i)
EmitVBR64(Vals[i], 6);
}
}
/// EmitRecord - Emit the specified record to the stream, using an abbrev if
/// we have one to compress the output.
void EmitRecord(unsigned Code, SmallVectorImpl<unsigned> &Vals,
unsigned Abbrev = 0) {
if (Abbrev) {
assert(0 && "abbrevs not implemented yet!");
} else {
// If we don't have an abbrev to use, emit this in its fully unabbreviated
// form.
EmitCode(bitc::UNABBREV_RECORD);
EmitVBR(Code, 6);
EmitVBR(Vals.size(), 6);
for (unsigned i = 0, e = Vals.size(); i != e; ++i)
EmitVBR(Vals[i], 6);
}
}
};
} // End llvm namespace
#endif
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