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

[rguenther at suse dot de]

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

--- Comment #9 from rguenther at suse dot de <rguenther at suse dot de> 2012-03-12 08:56:40 UTC ---
On Wed, 7 Mar 2012, scovich at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50346
> 
> --- Comment #8 from Ryan Johnson <scovich at gmail dot com> 2012-03-07 14:28:29 UTC ---
> (In reply to comment #7)
> > On Wed, 7 Mar 2012, scovich at gmail dot com wrote:
> > 
> > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50346
> > > 
> > > --- Comment #6 from Ryan Johnson <scovich at gmail dot com> 2012-03-07 13:31:19 UTC ---
> > > (In reply to comment #5)
> > > > 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).
> > > That makes sense...
> > > 
> > > Wild idea: would it be possible to annotate references as "escaped" or "not
> > > escaped yet" ? Anything global or passed into the function would be marked as
> > > escaped, while anything allocated locally would start out as not escaped;
> > > assigning to an escaped location or passing to a function would mark it as
> > > escaped if it wasn't already. The status could be determined in linear time
> > > using local information only (= scalable), and would benefit strongly as
> > > inlining (IPA or not) eliminates escape points.
> > 
> > Well, you can compute the clobber/use sets of individual function calls,
> > IPA PTA computes a simple mod-ref analysis this way.  You can also
> > annotate functions whether they make arguments escape or whether it
> > reads from them or clobbers them.
> > 
> > The plan is to do some simple analysis and propagate that up the
> > callgraph, similar to pure-const analysis.  The escape part could
> > be integrated there.
> 
> That sounds really slick to have in general... but would it actually catch the
> test case above? What you describe seems to depend on test() having information
> about foo::baz() -- which it does not -- while analyzing the body of test()
> could at least identify the part of f's lifetime where it cannot possibly have
> escaped.
> 
> Or does the local analysis come "for free" once those IPA changes are in place?

No, the local analysis is what makes the IPA changes "free" ;)  Of course
the local analysis would need to be flow sensitive.

Richard.