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[1/3] Use manual regex algorithm switching

Message ID CAPrifDm8MwcHY8XSnW_z6Z_hLVS+Lw4huqWEF3_eNT=fw3mkRg@mail.gmail.com
State New
Headers show

Commit Message

Tim Shen April 25, 2014, 8:34 p.m. UTC 
We used _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT to control which
algorithm we should use; so when compiling a regex, the number of
quantifiers(*, +, ?) will be counted and will be used for the
algorithm switching.

However, I do think give user the manual switch may make things
simpler: _GLIBCXX_REGEX_USE_THOMPSON_NFA. By default we'll use the DFS
executor, which Boost and libc++ have (only, I believe).

Users who know what they are doing and willing to pay for what they
get may switch to the Thompson NFA (we call it BFS approach) by
defining this macro.

A regex that contains back references will never be executed by a Thompson NFA.

The whole patch series are booted and tested, but this one is not
seperatly tested.

Thanks!

Comments

Jonathan Wakely April 25, 2014, 9:14 p.m. UTC | #1 
>--- a/libstdc++-v3/include/bits/regex.tcc
>+++ b/libstdc++-v3/include/bits/regex.tcc
>@@ -28,12 +28,12 @@
>  *  Do not attempt to use it directly. @headername{regex}
>  */
>
>-// See below __regex_algo_impl to get what this is talking about. The default
>-// value 1 indicated a conservative optimization without giving up worst case
>-// performance.
>-#ifndef _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT
>-#define _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT 1
>-#endif
>+// A non-standard switch to let the user pick the matching algorithm.
>+// If _GLIBCXX_REGEX_USE_THOMPSON_NFA is defined, the thompson NFA
>+// algorithm will be used. This algorithm is not by default, and cannot

This should say "not enabled by default" or "not used by default".

>+// be used if the regex contains back-references, but has better
>+// (polynomial instead of exponential) worst case performace.
>+// See __regex_algo_impl below.
>
> namespace std _GLIBCXX_VISIBILITY(default)
> {
>@@ -68,7 +68,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>       // This function decide which executor to use under given circumstances.
>       // The _S_auto policy now is the following: if a NFA has no
>-      // back-references and has more than _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT
>+      // back-references and has more than _GLIBCXX_REGEX_USE_THOMPSON_NFA

This comment needs more than just a simple replacement, as the way the
macro is used changed, not just its name.

>       // quantifiers (*, +, ?), the BFS executor will be used, other wise
>       // DFS executor. This is because DFS executor has a exponential upper
>       // bound, but better best-case performace. Meanwhile, BFS executor can
>@@ -82,8 +82,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       bool __ret;
>       if (!__re._M_automaton->_M_has_backref
> 	  && (__policy == _RegexExecutorPolicy::_S_alternate
>-	      || __re._M_automaton->_M_quant_count
>-		> _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT))
>+#ifdef _GLIBCXX_REGEX_USE_THOMPSON_NFA
>+	      || true
>+#endif
>+	      ))

I wonder if this would be simpler to read like this:

        if (!__re._M_automaton->_M_has_backref
#ifndef _GLIBCXX_REGEX_USE_THOMPSON_NFA
  	   && __policy == _RegexExecutorPolicy::_S_alternate
#endif
           )

What do you think?
diff mbox

Patch

diff --git a/libstdc++-v3/include/bits/regex.tcc b/libstdc++-v3/include/bits/regex.tcc
index 5fa1f01..d4d16a6 100644
--- a/libstdc++-v3/include/bits/regex.tcc
+++ b/libstdc++-v3/include/bits/regex.tcc
@@ -28,12 +28,12 @@ 
  *  Do not attempt to use it directly. @headername{regex}
  */
 
-// See below __regex_algo_impl to get what this is talking about. The default
-// value 1 indicated a conservative optimization without giving up worst case
-// performance.
-#ifndef _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT
-#define _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT 1
-#endif
+// A non-standard switch to let the user pick the matching algorithm.
+// If _GLIBCXX_REGEX_USE_THOMPSON_NFA is defined, the thompson NFA
+// algorithm will be used. This algorithm is not by default, and cannot
+// be used if the regex contains back-references, but has better
+// (polynomial instead of exponential) worst case performace.
+// See __regex_algo_impl below.
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -68,7 +68,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // This function decide which executor to use under given circumstances.
       // The _S_auto policy now is the following: if a NFA has no
-      // back-references and has more than _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT
+      // back-references and has more than _GLIBCXX_REGEX_USE_THOMPSON_NFA
       // quantifiers (*, +, ?), the BFS executor will be used, other wise
       // DFS executor. This is because DFS executor has a exponential upper
       // bound, but better best-case performace. Meanwhile, BFS executor can
@@ -82,8 +82,10 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       bool __ret;
       if (!__re._M_automaton->_M_has_backref
 	  && (__policy == _RegexExecutorPolicy::_S_alternate
-	      || __re._M_automaton->_M_quant_count
-		> _GLIBCXX_REGEX_DFS_QUANTIFIERS_LIMIT))
+#ifdef _GLIBCXX_REGEX_USE_THOMPSON_NFA
+	      || true
+#endif
+	      ))
 	{
 	  _Executor<_BiIter, _Alloc, _TraitsT, false>
 	    __executor(__s, __e, __m, __re, __flags);
diff --git a/libstdc++-v3/include/bits/regex_automaton.h b/libstdc++-v3/include/bits/regex_automaton.h
index a442cfe..64ecd6d 100644
--- a/libstdc++-v3/include/bits/regex_automaton.h
+++ b/libstdc++-v3/include/bits/regex_automaton.h
@@ -74,8 +74,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       size_t _M_backref_index;  // for _S_opcode_backref
       struct
       {
-	// for _S_opcode_alternative.
-	_StateIdT  _M_quant_index;
 	// for _S_opcode_alternative or _S_opcode_subexpr_lookahead
 	_StateIdT  _M_alt;
 	// for _S_opcode_word_boundary or _S_opcode_subexpr_lookahead or
@@ -120,7 +118,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     explicit
     _NFA_base(_FlagT __f)
     : _M_flags(__f), _M_start_state(0), _M_subexpr_count(0),
-    _M_quant_count(0), _M_has_backref(false)
+    _M_has_backref(false)
     { }
 
     _NFA_base(_NFA_base&&) = default;
@@ -145,7 +143,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _FlagT                    _M_flags;
     _StateIdT                 _M_start_state;
     _SizeT                    _M_subexpr_count;
-    _SizeT                    _M_quant_count;
     bool                      _M_has_backref;
   };
 
@@ -175,7 +172,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	_StateT __tmp(_S_opcode_alternative);
 	// It labels every quantifier to make greedy comparison easier in BFS
 	// approach.
-	__tmp._M_quant_index = this->_M_quant_count++;
 	__tmp._M_next = __next;
 	__tmp._M_alt = __alt;
 	__tmp._M_neg = __neg;
diff --git a/libstdc++-v3/include/bits/regex_automaton.tcc b/libstdc++-v3/include/bits/regex_automaton.tcc
index 1476ae2..38787fa 100644
--- a/libstdc++-v3/include/bits/regex_automaton.tcc
+++ b/libstdc++-v3/include/bits/regex_automaton.tcc
@@ -74,9 +74,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       case _S_opcode_alternative:
 	__ostr << __id << " [label=\"" << __id << "\\nALT\"];\n"
 	       << __id << " -> " << _M_next
-	       << " [label=\"epsilon\", tailport=\"s\"];\n"
+	       << " [label=\"next\", tailport=\"s\"];\n"
 	       << __id << " -> " << _M_alt
-	       << " [label=\"epsilon\", tailport=\"n\"];\n";
+	       << " [label=\"alt\", tailport=\"n\"];\n";
 	break;
       case _S_opcode_backref:
 	__ostr << __id << " [label=\"" << __id << "\\nBACKREF "