Benchmarks

Benchmarks are not built by default, as some of them contain large automata that take considerable time to compile with GCC or Clang. To enable them, configure with --enable-benchmarks (Autotools) or -DRE2C_BUILD_BENCHMARKS=yes (CMake). There are a few different groups of benchmarks.

Submatch extraction in lexer generators:

$ cd ${BUILD_DIR}/benchmarks/c
$ ./run.py --repetitions ${REP_COUNT} --output=results.json

Submatch extraction in library algorithms based on deterministic automata:

$ cd ${BUILD_DIR}/benchmarks/c/libre2c/jit
$ ./bench_submatch_jit --benchmark_out_format=json --benchmark_out=results.json

Submatch extraction in library algorithms based on non-deterministic automata:

$ cd ${BUILD_DIR}/benchmarks/c/libre2c/nfa
$ ./bench_submatch_nfa --benchmark_out_format=json --benchmark_out=results.json

To generate a TeX bar chart (PGF plot) from the JSON output, use json2pgfplot.py script. It has a few options, such as --relative-to <algo> (which scales the timings relative to the specified algorithm) and --font <name> (which specifies the font to be used). The generated TeX file can be compiled to PDF, which can be further converted to SVG, etc.

$ ${SOURCE_DIR}/benchmarks/json2pgfplot.py --variant <aot | jit | nfa> results.json results.tex
$ pdflatex results.tex </dev/null >results.log
$ pdf2svg results.pdf results.svg

Submatch (lexer generators)

These benchmarks contain regular expressions with submatch markers that are compiled to C code by a lexer generator, and further compiled to native code by a C compiler (GCC and Clang). Compilation happens ahead of time, so it is not included in the run time. The currently supported generators are ragel, kleenex and re2c TDFA(0), TDFA(1) and sta-DFA algorithms (no flex, as it does not support submatch extraction). Regular expressions used in the benchmarks can be divided in two categories:

• real-world regular expressions (parsers for HTTP message headers, URI, email addresses, etc.) ranging from large to small, mostly unambiguous

• artificial regular expressions (a few different groups with emphasis on alternative, concatenation or repetition) in series of increasing complexity and ambiguity

The generated programs do string rewriting. They read text from stdin and write it to stdout with some characters inserted at the points of submatch extraction (this is to make sure that the generated program is correct). String rewriting is a form dictated by kleenex, as it is suited to this task and cannot generate free-form programs as ragel and re2c do. Both ragel and re2c programs are written with performance in mind and use buffered input. As for kleenex, there is no control over what it generates (user input consists of regular expressions only), but presumably it also optimizes the generated code.

Ragel programs are fast, but not always correct. This is because ragel cannot handle non-determinism in the general case. It is a fundamental limitation which cannot be solved with disambiguation operators, except for simple cases. Ragel has no notion of tags and tag variables, and its submatch extraction is based on actions — arbitrary blocks of code embedded in the regular expression. When ragel generates a DFA, it puts actions either on transitions or in states (depending on the action type). Non-determinism means that there are multiple parallel NFA paths with different actions that reach a given DFA state. Actions on different paths conflict with each other (in particular, an action can conflict with itself) and change the program state in unforeseen ways (e.g. overwrite variables set by another action). Disambiguation operators can remove some of the conflicting actions, but they cannot fork the program state.

Kleenex generates very large automata in some benchmarks, which cannot be compiled in reasonable time. On the whole, kleenex programs have some constant overhead compared to other programs. It can be explained by a different underlying automaton type (DSST), which is better suited to full parsing than submatch extraction.

Of the algorithms in re2c, TDFA(1) is the fastest and the most robust one. In unambiguous cases its performance is similar to ragel, sometimes slightly better or worse (it varies more with the C compiler than with the benchmark). The generated binary size is also close. In highly ambiguous cases TDFA(1) is slower than ragel, which happens because TDFA(1) handles non-determinism by keeping track of multiple possible tag values, while ragel doesn’t handle it in any special way. TDFA(0) is generally less efficient than TDFA(1), see the paper Tagged Deterministic Finite Automata with Lookahead for a detailed comparison. Sta-DFA performs well on small benchmarks, but it degrades quickly on large or ambiguous regular expressions, both in speed and in the automaton size.

The measurements have been taken on an Intel(R) Core(TM) i7-8750H CPU with 32 KiB L1 Data cache (x6 cores), 32 KiB L1 Instruction cache (x6 cores), 256 KiB L2 Unified cache (x6 cores), 9216 KiB L3 Unified cache, 32 GiB RAM. Software versions: ragel-7.0.4, kleenex built from Git at commit d474c60, re2c built from Git at commit 814d38f, GCC-11.2.0, Clang-13.0.0. The results may be out of date!

Time is measured in ms (on 100MB of text), binary size is measured in KB (binaries are stripped).

Submatch (libraries, DFA)

This group contains library algorithms for submatch extraction that are based on deterministic automata. These are regular expressions that undergo just-in-time determinization before matching, which is included in the run time. Currently the group contains only two algorithms, TDFA(1) and regless-TDFA(1) — a modification of TDFA(1) that replaces register operations with a record of tag history, and a second pass on the history that unfolds it and reconstructs submatch results. The benchmark also includes variations for POSIX and leftmost greedy disambiguation policies, since disambiguation is also included in the run time.

Regless-TDFA(1) bypasses a few expensive computation steps in regcomp(), which results in much faster determinization times (a few orders of magnitude faster on large regular expressions). For regexec() the results are divided: on real-world regular expressions TDFA(1) usually outperforms regless-TDFA(1), but in some artificially constructed cases it can arbitrarily slower. These are pathological inputs for TDFA(1); the tags have arbitrarily high degree of non-determinism (increased with the repetition counter), so TDFA(1) has to track arbitrary many tag variables. Regless-TDFA(1) does not have tag variables.

The measurements have been taken on an Intel(R) Core(TM) i7-8750H CPU with 32 KiB L1 Data cache (x6 cores), 32 KiB L1 Instruction cache (x6 cores), 256 KiB L2 Unified cache (x6 cores), 9216 KiB L3 Unified cache, 32 GiB RAM. Software versions: re2c built from Git at commit 814d38f. The results may be out of date!

Compile time and run time is shown relative to the first row.

Submatch (libraries, NFA)

This group contains library algorithms for submatch extraction based on non-deterministic automata. They are described in depth in the paper Efficient POSIX submatch extraction on NFA. The goal is to compare algorithms that support POSIX longest-match semantics. A few leftmost greedy algorithms are provided as a baseline, including the Google RE2 library (which does not support POSIX semantics). There are four different variations of Okui-Suzuki algorithm, an algorithm proposed by Kuklewicz and a backward-matching algorithm proposed by Cox (which is generally incorrect).

As the benchmarks show, the basic Okui-Suzuki algorithm the most robust one among POSIX algorithms. It works in bounded memory that depends only on the regular expression and does not grow with the input size, and it has reasonable performance compared to the leftmost-greedy algorithm (although it too has some pathological cases on regular expressions with high ambiguity level). The lazy variation of Okui-Suzuki algorithm is often faster, but its memory requirement is not bounded (it grows with the size of input). Both Kuklewicz and backward algorithms are much slower on large real-world regular expressions.

The measurements have been taken on an Intel(R) Core(TM) i7-8750H CPU with 32 KiB L1 Data cache (x6 cores), 32 KiB L1 Instruction cache (x6 cores), 256 KiB L2 Unified cache (x6 cores), 9216 KiB L3 Unified cache, 32 GiB RAM. Software versions: re2c built from Git at commit b55ab37, re2-2020-11-01. The results may be out of date!

Simulation time is shown relative to the first row.