[Bug rtl-optimization/38740] [4.4 Regression] Incorrect delayed branch optimization

["jakub at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #16 from jakub at gcc dot gnu dot org  2009-01-28 15:20 -------
The problem is -O0 together with -fdelayed-branch.  Shouldn't we just reject it
or silently not do dbr at !optimize?  If it is really important to support (I
don't see any rationale for it), then the problem is that ira.c only calls
df_analyze after RA if -O1 and above:
  df_finish_pass (true);
  if (optimize > 1)
    df_live_add_problem ();
  df_scan_alloc (NULL);
  df_scan_blocks ();

  if (optimize)
    df_analyze ();

  timevar_pop (TV_IRA);
}

No other -O0 pass in between ira and dbr calls df_analyze and dbr itself can't
call it, as the cfg is gone.  So, if we really want to support this, either
ira.c could
  /* -O0 -fdelayed-branch uses dataflow info during dbr pass, but uses it after
     the cfg is gone.  Ensure it is updated even in that case.  */
  if (optimize || flag_delayed_branch)
    df_analyze ();
or we could have some pass right before pass_free_cfg that would be gated on
!optimize && flag_delayed_branch and would call df_analyze ().


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38740