Which -O is the default for clang?

Clang Frontend
#1

Hi,

I don't see the manpage mentions which -O is the default for clang.
Does anybody know? Thanks.

   Code Generation Options
       -O0, -O1, -O2, -O3, -Ofast, -Os, -Oz, -Og, -O, -O4
              Specify which optimization level to use:
                 -O0 Means "no optimization": this level compiles the
fastest and generates the most debuggable code.

                 -O1 Somewhere between -O0 and -O2.

                 -O2 Moderate level of optimization which enables most
optimizations.

                 -O3 Like -O2, except that it enables
optimizations that take longer to perform or that may generate larger
code (in an attempt to make the program run
                 faster).

                 -Ofast Enables all the optimizations from -O3 along
with other aggressive optimizations that may violate strict compliance
with language standards.

                 -Os Like -O2 with extra optimizations to reduce code size.

                 -Oz Like -Os (and thus -O2), but reduces code size further.

                 -Og Like -O1. In future versions, this option might
disable different optimizations in order to improve debuggability.

                 -O Equivalent to -O2.

                 -O4 and higher
                     Currently equivalent to -O3

#2

If you don't set it, neither does the default clang driver. You can tell this by running a the driver with the parameter -###, which will print the output.

The top of CompilerInvocation.cpp has the function that determines the optimization level based on the parameter to the CC1 invocation. That should answer your questions about the different versions. Note a handful of language options appear in the function so it isn't that easy of a question to just answer.