[Bug c++/58377] spurious "may be used uninitialized" warning with -Og

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58377

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to davidxl from comment #10)
> When an incoming edge to a phi is a critical edge, the 'use BB' for the phi
> arg should be in the split BB of the edge. Pushing the use into either the
> Source BB  or the dest BB will result in extending the 'use' falsely in more
> BBs.  In this case, simply use the PHI's BB won't solve the problem, as
> there is an incoming path introduced not guarded by if (iftmp.1_3 != 0)
> 
> I don't see a good way to fix it unless splitting the edge.

I wondered if we can make is_use_properly_guarded take the case of a use
on an edge into account by simply injecting the predicate at the end of
the predecessor into the list of predicates found by find_predicates.

Hmm, but I see it's not a simple list of predicates ...

I wonder what effect splitting critical edges somewhere before late
uninit has on following optimization passes and RTL expansion and its
out-of-SSA process (though the next cfgcleanup pass will unsplit the
edges again, which is at latest pass_cleanup_cfg_post_optimizing)

But I can confirm that the testcase is fixed with

Index: gcc/passes.c
===================================================================
--- gcc/passes.c        (revision 202445)
+++ gcc/passes.c        (working copy)
@@ -1543,6 +1543,7 @@ init_optimization_passes (void)
       /* ???  We do want some kind of loop invariant motion, but we possibly
          need to adjust LIM to be more friendly towards preserving accurate
         debug information here.  */
+      NEXT_PASS (pass_split_crit_edges);
       NEXT_PASS (pass_late_warn_uninitialized);
       NEXT_PASS (pass_uncprop);
       NEXT_PASS (pass_local_pure_const);

which does this just for -Og.

I'm seriously considering this, splitting critical edges is cheap.