# Changeset 1299 for docs

Ignore:
Timestamp:
Aug 8, 2011, 2:52:40 PM (8 years ago)
Message:

Minor edits.

Location:
docs/PACT2011
Files:
2 edited

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Unmodified
 r1298 \end{figure} To transform byte-oriented character data to parallel bit stream representation, source data is first loaded into SIMD registered in sequential order. It is then converted to the transposed basis bit stream representation through a series of packs, shifts, and logical bitwise operations. Using the SIMD capabilities of current commodity processors, the transposition of source data to basis bit stream format incurs an amortized cost of approximately 1 cycle per byte \cite{CameronHerdyLin2008}. To transform byte-oriented character data to parallel bit stream representation, source data is first loaded into SIMD registered in sequential order. It is then converted to the transposed basis bit stream representation through a series of parallel SIMD pack, shift, and logical bitwise operations. Using the SIMD capabilities of current commodity processors, the transposition of source data to basis bit stream format incurs an amortized cost of approximately 1 cycle per byte \cite{CameronHerdyLin2008}. Throughout the XML parsing process we must identify key XML characters. For example, the opening angle bracket character <'. For this purpose, we combine the basis bit streams using bitwise logic and compute character-class bit streams. For example, the $j$-th character is an open angle bracket <' if and only if the $j$-th bit of $B_2, B_3, B_4, B_5 =$ 1 and the $j$-th bit of $B_0, B_1, B_6, B_7 =$ 0. Character-class streams mark the positions of source characters as a single $1$-bit. Each bit position in the computed bit stream is in one-to-one correspondence with its source byte position.  Once generated, single cycle built-in {\em bitscan} operations are used to locate the positions of key XML character throughout the parsing process. Utilizing $M$ SIMD registers of width $W$, it is possible to scan through $W$ characters in parallel. The register width $W$ varies between 64-bit for MMX, 128-bit for SSE, and 256-bit for AVX. Throughout the XML parsing process we must identify key XML characters. For example, the opening angle bracket character <'. For this purpose, we combine the basis bit streams using bitwise logic and compute character-class bit streams. For example, the $j$-th character is an open angle bracket <' if and only if the $j$-th bit of $B_2, B_3, B_4, B_5 =$ 1 and the $j$-th bit of $B_0, B_1, B_6, B_7 =$ 0. Character-class streams mark the positions of source characters as a single $1$-bit. Each bit position in the computed bit stream is in one-to-one correspondence with its source byte position.  Once generated, single cycle built-in {\em bitscan} operations are used to locate the positions of key XML characters throughout the parsing process. Utilizing $M$ SIMD registers of width $W$, it is possible to scan through $W$ characters in parallel. The register width $W$ varies between 64-bit for MMX, 128-bit for SSE, and 256-bit for AVX. A common operation in XML parsing is XML start tag validation. Starts tags begin with <' and end with either />'' or >'' (depending whether the element tag is an empty element tag or not, respectively). Figure \ref{fig:Parabix1StarttagExample} conceptually demonstrates start tag validation as performed in Parabix1 using character-class streams together with the processor built-in bit scan operation. The first bit stream $M_0$ is created and the parser begins scanning the source data for an open angle bracket <', starting at position 1. Since the source data begins with <', $M_0$ is assigned a cursor position of 1. The $advance$ operation then shifts $M_0$'s cursor position by 1, resulting in the creation of a new bit stream, $M_1$, with the cursor position at 2. The following $bitscan$ operation takes the cursor position from $M_1$ and sequentially scans every position until it locates either an >'. It finds a `>' at position 4 and returns that as the new cursor position for $M_2$. Calculating $M_3$ advances the cursor again, and the $bitscan$ used to create $M_4$ locates the new opening angle bracket. This process continues in sequence until until all start tags are validated. Unlike traditional parsers, these sequential operations are accelerated significantly since the {\em bitscan} operation can skip up to $w$ positions, where $w$ is the processor word width in bits. This approach has recently been applied to Unicode transcoding and XML parsing to good effect, with research prototypes showing substantial speed-ups over even the best of byte-at-a-time alternatives \cite{CameronHerdyLin2008, Herdy2008, Cameron2009}.