[PATCH v1 3/3] libcamera: base: log: Do not instantiate disabled `LogMessage`s

[Barnabás Pőcze]

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 is actually checked whether the corresponding
`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 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 at ideasonboard.com>
---

One remaining questions is the handling of "Fatal" log messages. I think
it would make sense to handle them separately because that way the compiler
could be told that it is actually a `[[noreturn]]` function call.

---
 include/libcamera/base/log.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/libcamera/base/log.h b/include/libcamera/base/log.h
index 958cb488d..1a14b8dbc 100644
--- a/include/libcamera/base/log.h
+++ b/include/libcamera/base/log.h
@@ -108,10 +108,15 @@ LogMessage _log(const LogCategory &category, LogSeverity severity,
 #ifndef __DOXYGEN__
 #define _LOG_CATEGORY(name) logCategory##name

+#define _LOG(cat, sev) \
+	switch (const auto &_logCategory = (cat); 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
--
2.48.1