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.