This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [PATCH] Fix cond_exec_find_if_block (PR rtl-optimization/56745)


On Tue, 2 Apr 2013, Jakub Jelinek wrote:

> On Tue, Apr 02, 2013 at 02:57:13PM +0200, Richard Biener wrote:
> > On Tue, 2 Apr 2013, Jakub Jelinek wrote:
> > > On the (undefined behavior) testcase below, we end up with
> > > then_bb ending with __builtin_unreachable () at the tree level, therefore
> > > no successor at the RTL level, and else_bb being EXIT_BLOCK_PTR (i.e.
> > > conditional return before a bb with undefined behavior at the end).
> > > Trying to optimize that into a conditional execution of the then_bb insns
> > > doesn't work, we can't merge the else_bb with then_bb and test_bb in this
> > > case, plus it doesn't look like something that would be desirable to do
> > > (conditional return is surely better).
> > > 
> > > Fixed thusly, ok for trunk/4.8?
> > 
> > I wonder if
> > 
> >   /* Make sure IF, THEN, and ELSE, blocks are adjacent.  Actually, we get 
> > the
> >      first condition for free, since we've already asserted that there's a
> >      fallthru edge from IF to THEN.  Likewise for the && and || blocks, 
> > since
> >      we checked the FALLTHRU flag, those are already adjacent to the last 
> > IF
> >      block.  */
> >   /* ??? As an enhancement, move the ELSE block.  Have to deal with
> >      BLOCK notes, if by no other means than backing out the merge if they
> >      exist.  Sticky enough I don't want to think about it now.  */
> >   next = then_bb;
> >   if (else_bb && (next = next->next_bb) != else_bb)
> >     return FALSE;
> >   if ((next = next->next_bb) != join_bb && join_bb != EXIT_BLOCK_PTR)
> >     {
> >       if (else_bb)
> >         join_bb = NULL;
> >       else
> >         return FALSE;
> >     }
> > 
> > somehow tries to guard against join_bb == EXIT_BLOCK_PTR but fails.
> > Thus, why not do that explicitely here instead of just in the
> > single case you cover?  (I can't see why join_bb could not be
> > set to EXIT_BLOCK_PTR in some weird case)
> 
> From my reading of the code, it can handle the normal case where
> join_bb is EXIT_BLOCK_PTR just fine, provided that single_succ (then_bb)
> == join_bb and !else_bb || single_succ (else_bb) == join_bb.
> The ICE is there only because of the extra optimization I've tweaked,
> the problem is there that then_bb has no successors and join_bb is
> EXIT_BLOCK_PTR, so while then_bb can be successfully merged together with
> test_bb, it has no successor and as join_bb is EXIT_BLOCK_PTR, we just give
> up.  If then_bb has no successor and join_bb isn't EXIT_BLOCK_PTR, we'd
> normally do:
>   else if (EDGE_COUNT (join_bb->preds) < 2
>            && join_bb != EXIT_BLOCK_PTR)
>     {
>       /* We can merge the JOIN cleanly and update the dataflow try
>          again on this pass.*/
>       merge_blocks (combo_bb, join_bb);
>       num_true_changes++;
>     }
> and all is fine, and if then_bb (and else_bb if it exists) has a single
> successor of join_bb, all is fine too.

Ah, I see.  The patch is ok then.

Thanks,
Richard.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]