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rework the Bitstream reader to actually work a machine word at a time…

…, instead of 32-bits at a time.

This cuts in half the number of virtual methods called to refill that word when compiling on a 64-bit
host, and will make 64-bit read operations faster.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173072 91177308-0d34-0410-b5e6-96231b3b80d8
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commit 02b206f6d817b005799cbeca119376c034679212 1 parent 69582cf
Chris Lattner authored
Showing with 49 additions and 21 deletions.
  1. +49 −21 include/llvm/Bitcode/BitstreamReader.h
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70 include/llvm/Bitcode/BitstreamReader.h
@@ -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 size_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 @@ class BitstreamCursor {
/// 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.
@@ -328,8 +333,12 @@ class BitstreamCursor {
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);
+ }
}
@@ -337,7 +346,7 @@ class BitstreamCursor {
assert(NumBits <= 32 && "Cannot return more than 32 bits!");
// 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 +359,32 @@ class BitstreamCursor {
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 +433,21 @@ class BitstreamCursor {
}
}
+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 +471,15 @@ class BitstreamCursor {
// 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;
}
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