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
-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
-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