[Bug tree-optimization/18046] Missed jump threading optimization

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #13 from law at redhat dot com  2008-09-23 21:55 -------
Subject: Re:  Missed jump threading optimization

steven at gcc dot gnu dot org wrote:
> ------- Comment #12 from steven at gcc dot gnu dot org  2008-09-21 13:58 -------
> tree PRE now *does* handle the partially redundant global variable load. This
> is the .final_cleanup dump:
>
>
> ;; Function bar (bar)
>
> bar ()
> {
>   int prephitmp.13;
>
> <bb 2>:
>   prephitmp.13 = i;
>   switch (prephitmp.13) <default: <L1>, case 0: <L0>>
>
> <L0>:
>   foo ();
>   prephitmp.13 = i;
>
> <L1>:
>   switch (prephitmp.13) <default: <L5>, case 0: <L4>>
>
> <L4>:
>   foo (); [tail call]
>
> <L5>:
>   return;
>
> }
>
>
> But we still miss the jump threading opportunity.
>   
Thanks for the update.  One blocking issue out of the way....

Things have changed a lot since that original bug report.  I believe the 
best solution for this particular case is to lower the switch statements 
early enough to expose the conditionals to DOM & VRP.

The 2nd best approach would be to extend VRP to create a range for the 
default case of a SWITCH and extend the jump threading code in 
tree-vrp.c to handle switch statements (they're currently ignored).  The 
biggest difficulty here would be to avoid dropping to varying too 
quickly.  I think you'd want to sort the cases, then build up a range 
containing all the cases.  If you get a gap in the range, you drop to 
varying.  If after extracting all the ranges you haven't dropped to 
varying, then you invert the range and create the appropriate ASSERT_EXPRs.

I'm not currently working on either solution.

jeff


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18046