Compiling with both -g3 and -O3

Question

Most build environments I've seen have at least two strategies: debug build vs final/optimized/release build. With gcc, this usually means some version of -g vs -O. Now I'm seeing a situation where the optimized build is built with -O3 while the debug version is built with -g3 and -O3. man gcc does indicate that is possible, but this seems counterintuitive to me for real debugging purposes.

Reviewing http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html reminded me of -Og, which allows optimizations that do not interfere with debugging. That makes sense to me, but what compelling reason is there to debug with -O3 -g3 unless you are basically trying to debug gcc's own optimization abilities?

Answer 1

Sometimes people write bad code - maybe code that causes undefined behaviour, for example. Now let's say that that undefined behaviour appears to work "correctly" in the case of low- or no optimizations, but that it causes a disastrous crash at -O3. You're going to want to debug this problem at -O3, right? So then you have no choice but to add a -g flag and go to town, even though the debugging experience might be somewhat compromised by optimizations.

There is a big problem in general with build systems conflating the "debug/release" axis with the "optimized/unoptimized" axis. Really, they should be orthogonal - it's often desirable to have a "debug" build with logging, for example, but still have it run fast with optimizations enabled. Similarly, it could potentially be very difficult to track down an optimizer-related bug without having debug symbols available in your optimized build.

                   +--------------------------------+
                   |           Optimizations        |
                   +-----------------+--------------+
                   |        On       |     Off      |
 +----------+------+-----------------+--------------+
 |          |  On  | Debug optimized |  Best debug  |
 |  Debug   |      |    code         |  experience  |
 | Logging/ +------+-----------------+--------------+
 | Symbols  |  Off |   Release build | Probably not |
 |          |      |  for customers  |    useful    |
 +----------+------+-----------------+--------------+