[Bug tree-optimization/50346] Function call foils VRP/jump-threading of redundant predicate on struct member

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50346

--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> 2011-10-12 12:44:15 UTC ---
On Wed, 12 Oct 2011, scovich at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50346
> 
> --- Comment #4 from Ryan Johnson <scovich at gmail dot com> 2011-10-12 12:40:25 UTC ---
> (In reply to comment #3)
> > Well, it's a tree optimization issue.  It's simple - the local aggregate f
> > escapes the function via the member function call to baz:
> > 
> > <bb 5>:
> >   foo::baz (&f);
> > 
> > and as our points-to analysis is not flow-sensitive for memory/calls this
> > causes f to be clobbered by the call to bar
> 
> Is flow-sensitive analysis within single functions prohibitively expensive? All
> the papers I can find talk about whole-program analysis, where it's very
> expensive in both time and space; the best I could find (CGO'11 best paper)
> gets it down to 20-30ms and 2-3MB per kLoC for up to ~300kLoC. 

It would need a complete rewrite, it isn't integratable into the current
solver (which happens to be shared between IPA and non-IPA modes).

> > as neither the bodies of baz nor bar are visible there is nothing we can do
> 
> Would knowing the body of bar() help if the latter cannot be inlined?

Not at present, but it's possible to improve mod-ref analysis on an
IPA level then.

Richard.