QRegularExpression: improve JIT memory handling

PCRE's JIT uses by default 32K on the pcre_exec caller's stack. This
is fine for most situations, but in some cases (esp. patterns with
lot of recursion) more memory is required.

Therefore, if a match execution fails due to exhausting JIT memory,
we let PCRE allocate up to 512KB to be used for the JIT's stack.

The pointer to the allocated memory is put in thread local storage
(so it can be reused from the same thread, if needed, and automatically
goes away when the thread dies).

Change-Id: Ica5fb7d517068befff88ebb198a603a26ec5d8a7
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 84 insertions, 12 deletions

diff --git a/src/corelib/tools/qregularexpression.cpp b/src/corelib/tools/qregularexpression.cpp
index 0fa7d6459e..5deb485d3e 100644
--- a/src/corelib/tools/qregularexpression.cpp
+++ b/src/corelib/tools/qregularexpression.cpp
@@ -47,6 +47,8 @@
 #include <QtCore/qvector.h>
 #include <QtCore/qstringlist.h>
 #include <QtCore/qdebug.h>
+#include <QtCore/qthreadstorage.h>
+#include <QtCore/qglobal.h>
 
 #include <pcre.h>
 
@@ -989,6 +991,47 @@ void QRegularExpressionPrivate::getPatternInfo()
             (patternNewlineSetting == PCRE_NEWLINE_ANYCRLF);
 }
 
+
+/*!
+    \class QPcreJitStackPointer
+    \internal
+
+    Simple "smartpointer" wrapper around a pcre_jit_stack, to be used with
+    QThreadStorage.
+*/
+class QPcreJitStackPointer
+{
+    Q_DISABLE_COPY(QPcreJitStackPointer);
+
+public:
+    QPcreJitStackPointer()
+    {
+        // The default JIT stack size in PCRE is 32K,
+        // we allocate from 32K up to 512K.
+        stack = pcre16_jit_stack_alloc(32*1024, 512*1024);
+    }
+    ~QPcreJitStackPointer()
+    {
+        if (stack)
+            pcre16_jit_stack_free(stack);
+    }
+
+    pcre16_jit_stack *stack;
+};
+
+Q_GLOBAL_STATIC(QThreadStorage<QPcreJitStackPointer *>, jitStacks)
+
+/*!
+    \internal
+*/
+static pcre16_jit_stack *qtPcreCallback(void *)
+{
+    if (jitStacks()->hasLocalData())
+        return jitStacks()->localData()->stack;
+
+    return 0;
+}
+
 /*!
     \internal
 */
@@ -1044,6 +1087,9 @@ pcre16_extra *QRegularExpressionPrivate::optimizePattern()
     const char *err;
     studyData = pcre16_study(compiledPattern, studyOptions, &err);
 
+    if (studyData && studyData->flags & PCRE_EXTRA_EXECUTABLE_JIT)
+        pcre16_assign_jit_stack(studyData, qtPcreCallback, 0);
+
     if (!studyData && err)
         qWarning("QRegularExpressionPrivate::optimizePattern(): pcre_study failed: %s", err);
 
@@ -1070,6 +1116,32 @@ int QRegularExpressionPrivate::captureIndexForName(const QString &name) const
 /*!
     \internal
 
+    This is a simple wrapper for pcre16_exec for handling the case in which the
+    JIT runs out of memory. In that case, we allocate a thread-local JIT stack
+    and re-run pcre16_exec.
+*/
+static int pcre16SafeExec(const pcre16 *code, const pcre16_extra *extra,
+                          const unsigned short *subject, int length,
+                          int startOffset, int options,
+                          int *ovector, int ovecsize)
+{
+    int result = pcre16_exec(code, extra, subject, length,
+                             startOffset, options, ovector, ovecsize);
+
+    if (result == PCRE_ERROR_JIT_STACKLIMIT && !jitStacks()->hasLocalData()) {
+        QPcreJitStackPointer *p = new QPcreJitStackPointer;
+        jitStacks()->setLocalData(p);
+
+        result = pcre16_exec(code, extra, subject, length,
+                             startOffset, options, ovector, ovecsize);
+    }
+
+    return result;
+}
+
+/*!
+    \internal
+
     Performs a match of type \a matchType on the given \a subject string with
     options \a matchOptions and returns the QRegularExpressionMatchPrivate of
     the result. It also advances a match if a previous result is given as \a
@@ -1134,15 +1206,15 @@ QRegularExpressionMatchPrivate *QRegularExpressionPrivate::doMatch(const QString
     int result;
 
     if (!previousMatchWasEmpty) {
-        result = pcre16_exec(compiledPattern, currentStudyData,
-                             subjectUtf16, subjectLength,
-                             offset, pcreOptions,
-                             captureOffsets, captureOffsetsCount);
+        result = pcre16SafeExec(compiledPattern, currentStudyData,
+                                subjectUtf16, subjectLength,
+                                offset, pcreOptions,
+                                captureOffsets, captureOffsetsCount);
     } else {
-        result = pcre16_exec(compiledPattern, currentStudyData,
-                             subjectUtf16, subjectLength,
-                             offset, pcreOptions | PCRE_NOTEMPTY_ATSTART | PCRE_ANCHORED,
-                             captureOffsets, captureOffsetsCount);
+        result = pcre16SafeExec(compiledPattern, currentStudyData,
+                                subjectUtf16, subjectLength,
+                                offset, pcreOptions | PCRE_NOTEMPTY_ATSTART | PCRE_ANCHORED,
+                                captureOffsets, captureOffsetsCount);
 
         if (result == PCRE_ERROR_NOMATCH) {
             ++offset;
@@ -1157,10 +1229,10 @@ QRegularExpressionMatchPrivate *QRegularExpressionPrivate::doMatch(const QString
                 ++offset;
             }
 
-            result = pcre16_exec(compiledPattern, currentStudyData,
-                                 subjectUtf16, subjectLength,
-                                 offset, pcreOptions,
-                                 captureOffsets, captureOffsetsCount);
+            result = pcre16SafeExec(compiledPattern, currentStudyData,
+                                    subjectUtf16, subjectLength,
+                                    offset, pcreOptions,
+                                    captureOffsets, captureOffsetsCount);
         }
     }