http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50346
--- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> 2012-03-07 13:39:19 UTC --- 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. Richard.
