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Logging: capture the backtrace closer to the user's entry point

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Instead of deep inside the processing of the message pattern tokens.
This reduces the number of uninlineable functions in release builds to
2. Experimentation shows that even in debug mode our overhead to
backtrace() is just 2, despite the debugger's backtrace showing 3 frames
-- the debugger is using QtCore's debug symbols and does know about the
inlined function.

As a nice side-effect, we capture the backtrace only once per message,
not once per %{backtrace} token in the QT_MESSAGE_PATTERN.

Change-Id: I01ec3c774d9943adb903fffd17b7d6de09167620
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
This commit is contained in:
Thiago Macieira 2024-02-27 13:53:52 -08:00
parent d9f7104166
commit 710276ed85
2 changed files with 92 additions and 33 deletions
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112
src/corelib/global/qlogging.cpp
View File

@ -1143,6 +1143,7 @@ struct QMessagePattern
int backtraceDepth;
};
QList<BacktraceParams> backtraceArgs; // backtrace arguments in sequence of %{backtrace
int maxBacktraceDepth = 0;
#endif
bool fromEnvironment;
@ -1176,6 +1177,7 @@ void QMessagePattern::setPattern(const QString &pattern)
timeArgs.clear();
#ifdef QLOGGING_HAVE_BACKTRACE
backtraceArgs.clear();
maxBacktraceDepth = 0;
#endif
// scanner
@ -1270,6 +1272,7 @@ void QMessagePattern::setPattern(const QString &pattern)
backtraceParams.backtraceDepth = backtraceDepth;
backtraceParams.backtraceSeparator = backtraceSeparator;
backtraceArgs.append(backtraceParams);
maxBacktraceDepth = qMax(maxBacktraceDepth, backtraceDepth);
#else
error += "QT_MESSAGE_PATTERN: %{backtrace} is not supported by this Qt build\n"_L1;
tokens[i] = "";
@ -1326,23 +1329,31 @@ void QMessagePattern::setPattern(const QString &pattern)
// make sure the function has "Message" in the name so the function is removed
/*
A typical backtrace in debug mode looks like:
#0 backtraceFramesForLogMessage (frameCount=0) at qlogging.cpp:1398
#1 formatBacktraceForLogMessage (backtraceParams=..., function=0x5555555580b0 "virtual void MyClass::myFunction(int)") at qlogging.cpp:1525
#2 formatLogMessage (type=QtDebugMsg, context=..., str=...) at qlogging.cpp:1642
#3 qDefaultMessageHandler (type=QtDebugMsg, context=..., message=...) at qlogging.cpp:1997
#4 qt_message_print (msgType=QtDebugMsg, context=..., message=...) at qlogging.cpp:2043
#5 qt_message_output (msgType=QtDebugMsg, context=..., message=...) at qlogging.cpp:2082
#6 QDebug::~QDebug (this=0x7fffffffd9b8, __in_chrg=<optimized out>) at qdebug.cpp:166
#0 QInternalMessageLogContext::populateBacktrace (this=0x7fffffffd660, frameCount=5) at qlogging.cpp:1342
#1 QInternalMessageLogContext::QInternalMessageLogContext (logContext=..., this=<optimized out>) at qlogging_p.h:42
#2 QDebug::~QDebug (this=0x7fffffffdac8, __in_chrg=<optimized out>) at qdebug.cpp:160
In release mode, the QInternalMessageLogContext constructor will be usually
inlined. Empirical testing with GCC 13 and Clang 17 suggest they do obey the
Q_ALWAYS_INLINE in that constructor even in debug mode and do inline it.
Unfortunately, we can't know for sure if it has been.
*/
static constexpr int TypicalBacktraceFrameCount = 7;
static constexpr int TypicalBacktraceFrameCount = 3;
# if defined(Q_CC_GNU) && !defined(Q_CC_CLANG)
// force skipping the frame pointer, to save the backtrace() function some work
# pragma GCC push_options
# pragma GCC optimize ("omit-frame-pointer")
# endif
Q_NEVER_INLINE void QInternalMessageLogContext::populateBacktrace(int frameCount)
{
assert(frameCount >= 0);
BacktraceStorage &result = backtrace.emplace(TypicalBacktraceFrameCount + frameCount);
int n = ::backtrace(result.data(), result.size());
if (n <= 0)
result.clear();
else
result.resize(n);
}
static QStringList backtraceFramesForLogMessage(int frameCount)
static QStringList
backtraceFramesForLogMessage(int frameCount,
const QInternalMessageLogContext::BacktraceStorage &buffer)
{
struct DecodedFrame {
QString library;
@ -1353,12 +1364,6 @@ static QStringList backtraceFramesForLogMessage(int frameCount)
if (frameCount == 0)
return result;
QVarLengthArray<void *, 32> buffer(TypicalBacktraceFrameCount + frameCount);
int n = backtrace(buffer.data(), buffer.size());
if (n <= 0)
return result;
buffer.resize(n);
auto shouldSkipFrame = [&result](const auto &library, const auto &function) {
if (!result.isEmpty() || !library.contains("Qt6Core"_L1))
return false;
@ -1366,7 +1371,9 @@ static QStringList backtraceFramesForLogMessage(int frameCount)
return true;
if (function.contains("6QDebug"_L1))
return true;
if (function.contains("Message"_L1) || function.contains("_message"_L1))
if (function.contains("14QMessageLogger"_L1))
return true;
if (function.contains("17qt_message_output"_L1))
return true;
return false;
};
@ -1445,7 +1452,7 @@ static QStringList backtraceFramesForLogMessage(int frameCount)
};
# endif
for (void *&addr : buffer) {
for (void *const &addr : buffer) {
DecodedFrame frame = decodeFrame(addr);
if (!frame.library.isEmpty()) {
if (frame.function.isEmpty())
@ -1465,25 +1472,29 @@ static QStringList backtraceFramesForLogMessage(int frameCount)
}
static QString formatBacktraceForLogMessage(const QMessagePattern::BacktraceParams backtraceParams,
const char *function)
const QMessageLogContext &ctx)
{
// do we have a backtrace stored?
if (ctx.version <= QMessageLogContext::CurrentVersion)
return QString();
auto &fullctx = static_cast<const QInternalMessageLogContext &>(ctx);
if (!fullctx.backtrace.has_value())
return QString();
QString backtraceSeparator = backtraceParams.backtraceSeparator;
int backtraceDepth = backtraceParams.backtraceDepth;
QStringList frames = backtraceFramesForLogMessage(backtraceDepth);
QStringList frames = backtraceFramesForLogMessage(backtraceDepth, *fullctx.backtrace);
if (frames.isEmpty())
return QString();
// if the first frame is unknown, replace it with the context function
if (function && frames.at(0).startsWith(u'?'))
frames[0] = QString::fromUtf8(qCleanupFuncinfo(function));
if (ctx.function && frames.at(0).startsWith(u'?'))
frames[0] = QString::fromUtf8(qCleanupFuncinfo(ctx.function));
return frames.join(backtraceSeparator);
}
# if defined(Q_CC_GNU) && !defined(Q_CC_CLANG)
# pragma GCC pop_options
# endif
#endif // QLOGGING_HAVE_BACKTRACE && !QT_BOOTSTRAPPED
Q_GLOBAL_STATIC(QMessagePattern, qMessagePattern)
@ -1585,7 +1596,7 @@ static QString formatLogMessage(QtMsgType type, const QMessageLogContext &contex
} else if (token == backtraceTokenC) {
QMessagePattern::BacktraceParams backtraceParams = pattern->backtraceArgs.at(backtraceArgsIdx);
backtraceArgsIdx++;
message.append(formatBacktraceForLogMessage(backtraceParams, context.function));
message.append(formatBacktraceForLogMessage(backtraceParams, context));
#endif
} else if (token == timeTokenC) {
QString timeFormat = pattern->timeArgs.at(timeArgsIdx);
@ -1631,6 +1642,12 @@ static QString formatLogMessage(QtMsgType type, const QMessageLogContext &contex
return str;
}
#endif
#ifndef QLOGGING_HAVE_BACKTRACE
void QInternalMessageLogContext::populateBacktrace(int)
{
Q_UNREACHABLE();
}
#endif
static void qDefaultMessageHandler(QtMsgType type, const QMessageLogContext &context, const QString &buf);
@ -2246,6 +2263,37 @@ void qSetMessagePattern(const QString &pattern)
}
#endif
static void copyInternalContext(QInternalMessageLogContext *self,
const QMessageLogContext &logContext) noexcept
{
if (logContext.version == self->version) {
auto other = static_cast<const QInternalMessageLogContext *>(&logContext);
self->backtrace = other->backtrace;
}
}
/*!
\internal
Copies context information from \a logContext into this QMessageLogContext.
Returns the number of backtrace frames that are desired.
*/
int QInternalMessageLogContext::initFrom(const QMessageLogContext &logContext)
{
version = CurrentVersion + 1;
copyContextFrom(logContext);
#ifdef QLOGGING_HAVE_BACKTRACE
if (backtrace.has_value())
return 0; // we have a stored backtrace, no need to get it again
// initializes the message pattern, if needed
if (auto pattern = qMessagePattern())
return pattern->maxBacktraceDepth;
#endif
return 0;
}
/*!
Copies context information from \a logContext into this QMessageLogContext.
Returns a reference to this object.
@ -2260,6 +2308,8 @@ QMessageLogContext &QMessageLogContext::copyContextFrom(const QMessageLogContext
this->file = logContext.file;
this->line = logContext.line;
this->function = logContext.function;
if (Q_UNLIKELY(version == CurrentVersion + 1))
copyInternalContext(static_cast<QInternalMessageLogContext *>(this), logContext);
return *this;
}

13
src/corelib/global/qlogging_p.h
View File

@ -18,6 +18,7 @@
#include <QtCore/private/qglobal_p.h>
#include "qlogging.h"
#include "qloggingcategory.h"
#include "qvarlengtharray.h"
QT_BEGIN_NAMESPACE
@ -30,9 +31,15 @@ Q_CORE_EXPORT bool shouldLogToStderr();
class QInternalMessageLogContext : public QMessageLogContext
{
public:
QInternalMessageLogContext(const QMessageLogContext &logContext)
static constexpr int DefaultBacktraceDepth = 32;
using BacktraceStorage = QVarLengthArray<void *, DefaultBacktraceDepth>;
std::optional<BacktraceStorage> backtrace;
Q_ALWAYS_INLINE QInternalMessageLogContext(const QMessageLogContext &logContext)
{
copyContextFrom(logContext);
int backtraceFrames = initFrom(logContext);
if (backtraceFrames)
populateBacktrace(backtraceFrames);
}
QInternalMessageLogContext(const QMessageLogContext &logContext,
const QLoggingCategory &categoryOverride)
@ -41,6 +48,8 @@ public:
category = categoryOverride.categoryName();
}
int initFrom(const QMessageLogContext &logContext);
void populateBacktrace(int frameCount);
};
QT_END_NAMESPACE