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recommit r173072 (preparing bitstream reader to read a machine word at a time,

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instead of always 32-bits at a time) with two changes:

1. Make Read(0) always return zero without affecting the state of our cursor.
2. Hack word_t to always be 32 bits, as staging.

These two caveats will change shortly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174800 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2013-02-09 06:52:14 +00:00
parent 882b9276cb
commit e001f27e5e
1 changed files with 51 additions and 21 deletions
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72
include/llvm/Bitcode/BitstreamReader.h
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@ -170,12 +170,17 @@ class BitstreamCursor {
BitstreamReader *BitStream;
size_t NextChar;
/// CurWord - This is the current data we have pulled from the stream but have
/// not returned to the client.
uint32_t CurWord;
/// CurWord/word_t - This is the current data we have pulled from the stream
/// but have not returned to the client. This is specifically and
/// intentionally defined to follow the word size of the host machine for
/// efficiency. We use word_t in places that are aware of this to make it
/// perfectly explicit what is going on.
typedef uint32_t word_t;
word_t CurWord;
/// BitsInCurWord - This is the number of bits in CurWord that are valid. This
/// is always from [0...31] inclusive.
/// is always from [0...31/63] inclusive (depending on word size).
unsigned BitsInCurWord;
// CurCodeSize - This is the declared size of code values used for the current
@ -318,8 +323,8 @@ public:
/// JumpToBit - Reset the stream to the specified bit number.
void JumpToBit(uint64_t BitNo) {
uintptr_t ByteNo = uintptr_t(BitNo/8) & ~3;
uintptr_t WordBitNo = uintptr_t(BitNo) & 31;
uintptr_t ByteNo = uintptr_t(BitNo/8) & ~(sizeof(word_t)-1);
unsigned WordBitNo = unsigned(BitNo & (sizeof(word_t)*8-1));
assert(canSkipToPos(ByteNo) && "Invalid location");
// Move the cursor to the right word.
@ -328,16 +333,22 @@ public:
CurWord = 0;
// Skip over any bits that are already consumed.
if (WordBitNo)
Read(static_cast<unsigned>(WordBitNo));
if (WordBitNo) {
if (sizeof(word_t) > 4)
Read64(WordBitNo);
else
Read(WordBitNo);
}
}
uint32_t Read(unsigned NumBits) {
assert(NumBits <= 32 && "Cannot return more than 32 bits!");
if (NumBits == 0) return 0;
// If the field is fully contained by CurWord, return it quickly.
if (BitsInCurWord >= NumBits) {
uint32_t R = CurWord & ((1U << NumBits)-1);
uint32_t R = uint32_t(CurWord) & (~0U >> (32-NumBits));
CurWord >>= NumBits;
BitsInCurWord -= NumBits;
return R;
@ -350,24 +361,32 @@ public:
return 0;
}
unsigned R = CurWord;
uint32_t R = uint32_t(CurWord);
// Read the next word from the stream.
CurWord = getWord(NextChar);
NextChar += 4;
uint8_t buf[sizeof(word_t)] = {0};
BitStream->getBitcodeBytes().readBytes(NextChar, sizeof(buf), buf, NULL);
typedef support::detail::packed_endian_specific_integral
<word_t, support::little, support::unaligned> Endian_T;
CurWord = *reinterpret_cast<Endian_T*>(buf);
NextChar += sizeof(word_t);
// Extract NumBits-BitsInCurWord from what we just read.
unsigned BitsLeft = NumBits-BitsInCurWord;
// Be careful here, BitsLeft is in the range [1..32] inclusive.
R |= (CurWord & (~0U >> (32-BitsLeft))) << BitsInCurWord;
// Be careful here, BitsLeft is in the range [1..32]/[1..64] inclusive.
R |= uint32_t((CurWord & (word_t(~0ULL) >> (sizeof(word_t)*8-BitsLeft)))
<< BitsInCurWord);
// BitsLeft bits have just been used up from CurWord.
if (BitsLeft != 32)
// BitsLeft bits have just been used up from CurWord. BitsLeft is in the
// range [1..32]/[1..64] so be careful how we shift.
if (BitsLeft != sizeof(word_t)*8)
CurWord >>= BitsLeft;
else
CurWord = 0;
BitsInCurWord = 32-BitsLeft;
BitsInCurWord = sizeof(word_t)*8-BitsLeft;
return R;
}
@ -416,10 +435,21 @@ public:
}
}
private:
void SkipToFourByteBoundary() {
// If word_t is 64-bits and if we've read less than 32 bits, just dump
// the bits we have up to the next 32-bit boundary.
if (sizeof(word_t) > 4 &&
BitsInCurWord > 32) {
CurWord >>= BitsInCurWord-32;
BitsInCurWord = 32;
return;
}
BitsInCurWord = 0;
CurWord = 0;
}
public:
unsigned ReadCode() {
return Read(CurCodeSize);
@ -443,15 +473,15 @@ public:
// don't care what code widths are used inside of it.
ReadVBR(bitc::CodeLenWidth);
SkipToFourByteBoundary();
unsigned NumWords = Read(bitc::BlockSizeWidth);
unsigned NumFourBytes = Read(bitc::BlockSizeWidth);
// Check that the block wasn't partially defined, and that the offset isn't
// bogus.
size_t SkipTo = NextChar + NumWords*4;
if (AtEndOfStream() || !canSkipToPos(SkipTo))
size_t SkipTo = GetCurrentBitNo() + NumFourBytes*4*8;
if (AtEndOfStream() || !canSkipToPos(SkipTo/8))
return true;
NextChar = SkipTo;
JumpToBit(SkipTo);
return false;
}