[Bug middle-end/35560] Missing CSE/PRE for memory operations involved in virtual call.

[davidxl at gcc dot gnu dot org]

------- Comment #13 from davidxl at gcc dot gnu dot org  2010-02-03 22:05 -------
(In reply to comment #12)
> Btw, a destructor call also changes the vtbl pointer.
> 

ctors, dtors, wrapper function calls etc are all handled. Detailed write up
will be available at some point. To put it a simple way, it is done via live
across analyis: if an poly object is referenced before and after a call
(accesses to any field of it) both available and anticipated from a a call --
it is live across the call -- vptr field won't be modified by the call. 
Partially anticipated case is also handled.  Once vptr is handled, vtbl access
follows automatically -- at vtbls are RO. vptr assignment is treated
conservatively.

I implemented this thing in 4.4 line using special shadow symbols and
VUSE/VDEFS. It works as expected except that SCCVN time went to hell. Simple
fix to collapse varying defs in DFS walk help a lot but still slow. Need to do
this using alias oracle.

David


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35560