libcamera: base: log: Do not instantiate disabled LogMessages

At the moment every `LOG()` macro invocation results in a `LogMessage` being
created, the message serialized into an `std::stringstream`. Only in the
destructor is it actually checked whether the given `LogCategory` enables
the given log level.

This is not too efficient, it would be better to skip the log message
construction and all the `operator<<()` invocations if the message will
just be discarded.

This could be easily done if the `LOG()` macro accepted its arguments like a
traditional function as in that case an appropriate `if` statement can be
injected in a do-while loop. However, that is not the case, the `LOG()` macro
should effectively "return" a stream.

It is not possible inject an `if` statement directly as that would
lead to issues:

  if (...)
    LOG(...)
  else
   ...

The `else` would bind the to the `if` in the `LOG()` macro. This is
diagnosed by `-Wdangling-else`.

An alternative approach would be to use a `for` loop and force a single
iteration using a boolean flag or similar. This is entirely doable but
I think the implemented approach is easier to understand.

This change implements the early log level checking using a `switch` statement
as this avoids the dangling else related issues. One small issue arises
because having a boolean controlling expression is diagnosed by clang
(`-Wswitch-bool`); the result is cast to `int` to avoid the warning.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/base/log.h

@@ -108,10 +108,16 @@ LogMessage _log(const LogCategory &category, LogSeverity severity,
 #ifndef __DOXYGEN__
 #define _LOG_CATEGORY(name) logCategory##name
 
+#define _LOG(cat, sev)                                                 \
+	switch (const auto &_logCategory = (cat);                      \
+		static_cast<int>(_logCategory.severity() <= Log##sev)) \
+	case 1:                                                        \
+		_log(_logCategory, Log##sev).stream()
+
 #define _LOG1(severity) \
-	_log(LogCategory::defaultCategory(), Log##severity).stream()
+	_LOG(LogCategory::defaultCategory(), severity)
 #define _LOG2(category, severity) \
-	_log(_LOG_CATEGORY(category)(), Log##severity).stream()
+	_LOG(_LOG_CATEGORY(category)(), severity)
 
 /*
  * Expand the LOG() macro to _LOG1() or _LOG2() based on the number of