Document that LLDB_LOG macros use the format_providers.

Differential Revision: https://reviews.llvm.org/D65293

llvm-svn: 367132

lldb/include/lldb/Utility/Log.h

@@ -206,6 +206,26 @@ private:
 
 } // namespace lldb_private
 
+/// The LLDB_LOG* macros defined below are the way to emit log messages.
+///
+/// Note that the macros surround the arguments in a check for the log
+/// being on, so you can freely call methods in arguments without affecting
+/// the non-log execution flow.
+///
+/// If you need to do more complex computations to prepare the log message
+/// be sure to add your own if (log) check, since we don't want logging to
+/// have any effect when not on.
+///
+/// However, the LLDB_LOG macro uses the llvm::formatv system (see the
+/// ProgrammersManual page in the llvm docs for more details).  This allows
+/// the use of "format_providers" to auto-format datatypes, and there are
+/// already formatters for some of the llvm and lldb datatypes.
+///
+/// So if you need to do non-trivial formatting of one of these types, be
+/// sure to grep the lldb and llvm sources for "format_provider" to see if
+/// there is already a formatter before doing in situ formatting, and if
+/// possible add a provider if one does not already exist.
+
 #define LLDB_LOG(log, ...)                                                     \
   do {                                                                         \
     ::lldb_private::Log *log_private = (log);                                  \