Where does GCC pick passes for different opt. levels

[Steve Ellcey]

I have a basic question about optimization selection in GCC.  There used to
be some code in GCC (passes.c?) that would set various optimize pass flags
depending on if the 'optimize' flag was > 0, > 1, or > 2; later I think
there may have been a table.  This code seems gone now and I can't figure
out how GCC is selecting what optimization passes to run at what optimization
levels (-O1 vs. -O2 vs. -O3).  How is this handled in the top-of-tree GCC code?

I see passes.def but there doesn't seem to be anything in there to tie 
specific passes to specific optimization levels.  Likewise in common.opt
I see flags for various optimization passes but nothing to tie them to
-O1 or -O2, etc.

I'm probably missing something obvious, but a pointer would be much
appreciated.

Steve Ellcey