Re: What is exactly in O2?

[Phil Edwards <pedwards at disaster dot jaj dot com>]

Answering your questions in reverse order:

> A subsidiary question if this is the right place to ask this kind of questions.

Sure.

> I would like to learn how can I determine exactly  the set of compile flags who
> are triggered by O2.

If you want an /exact/ answer you have to go to the source.  In this case,
a good place to start is gcc/toplev.c, around line 4600.  There you will
find the logic that eventually includes (among other things):

      flag_optimize_sibling_calls = 1;
      flag_cse_follow_jumps = 1;
      flag_cse_skip_blocks = 1;
      flag_gcse = 1;
      flag_expensive_optimizations = 1;
      flag_strength_reduce = 1;
      flag_rerun_cse_after_loop = 1;
      flag_rerun_loop_opt = 1;
      flag_caller_saves = 1;
      flag_force_mem = 1;
      flag_peephole2 = 1;
   #ifdef INSN_SCHEDULING
      flag_schedule_insns = 1;
      flag_schedule_insns_after_reload = 1;
   #endif
      flag_regmove = 1;
      flag_strict_aliasing = 1;
      flag_delete_null_pointer_checks = 1;
      flag_reorder_blocks = 1;

There are some others as well from the -O1 setting, plus any platform-
dependant tweaks.

Somewhere I have a patch to allow a -print-flags switch to the compiler,
which will parse all the other command-line switches and then print the
flag_* variables which get changed.  It's useful for this kind of thing,
e.g., "-O2 -print-flags".  If you would this useful I'll mail it to you.


Phil

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