I think you're looking for an inter-procedural post dominator analysis. I don't think there is one in LLVM already, but it should be relatively straightforward. This gives a sound approximation (i.e. no false positives) of something sort-of equivalent to the halting problem: if the program terminates, then block Y was executed. Cheers, Scott On Tue, Oct 2, 2012 at 7:43 PM, Jim Grosbach <grosbach at apple.com> wrote:> Isn't this effectively the halting problem? Consider the case where block > Y is the exit block of main() and block X is the entry block of main(). > > Jim > > > On Oct 2, 2012, at 4:29 PM, Stephen Schiffli <sschiffli at gmail.com> wrote: > > Is there any inter-procedural analysis that could tell me if some > BasicBlock Y is guaranteed to execute based on my knowledge that BasicBlock > X will execute? For example: > > > extern int x; > > void foo() { } > > int main() { > > if (x) { > > foo(); > > } else { > > foo(); > > } > > } > > > I want to be told that the entry block of foo is guaranteed to be executed > since I know the entry block of main is guaranteed to be executed. > Basically that all paths from X to program termination go through Y at > some point. Please ignore the folding of identical branches and > function in-lining, I want to use this type of analysis in the general case. > > > Thanks, > > -Stephen > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20121002/fb708f90/attachment.html>
Okay thanks for the info. The term program termination was probably a poor choice of words. I'm really just trying to build an inter-procedural BasicBlock graph, and then look for postdominance as Scott suggested. I'll go about making my own since it doesn't sound like there is one out there already. Thanks, -Stephen On Tue, Oct 2, 2012 at 5:06 PM, Scott Moore <sdmoore at fas.harvard.edu> wrote:> I think you're looking for an inter-procedural post dominator analysis. I > don't think there is one in LLVM already, but it should be relatively > straightforward. > > This gives a sound approximation (i.e. no false positives) of something > sort-of equivalent to the halting problem: if the program terminates, then > block Y was executed. > > Cheers, > Scott > > > > On Tue, Oct 2, 2012 at 7:43 PM, Jim Grosbach <grosbach at apple.com> wrote: > >> Isn't this effectively the halting problem? Consider the case where block >> Y is the exit block of main() and block X is the entry block of main(). >> >> Jim >> >> >> On Oct 2, 2012, at 4:29 PM, Stephen Schiffli <sschiffli at gmail.com> wrote: >> >> Is there any inter-procedural analysis that could tell me if some >> BasicBlock Y is guaranteed to execute based on my knowledge that BasicBlock >> X will execute? For example: >> >> >> extern int x; >> >> void foo() { } >> >> int main() { >> >> if (x) { >> >> foo(); >> >> } else { >> >> foo(); >> >> } >> >> } >> >> >> I want to be told that the entry block of foo is guaranteed to be >> executed since I know the entry block of main is guaranteed to be executed. >> Basically that all paths from X to program termination go through Y at >> some point. Please ignore the folding of identical branches and >> function in-lining, I want to use this type of analysis in the general case. >> >> >> Thanks, >> >> -Stephen >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20121002/0ab14c4a/attachment.html>
Hi Stephen, I investigated interprocedural dominators some years ago. One important thing to consider if you want to implement this is that interprocedural (post)dominators do not form a (post)dominator tree. Consider main(){ X; if (...) { f(); g(); } else { g(); f(); } Y; } In this program, Y postdominates the entry and exit blocks of procedures g and f, which in turn postdominate X. But the blocks in f and g do not postdominate each other. So the postdominance relation is a graph, not a tree. We have published an efficient algorithm to compute interprocedural dominators in ACM TOPLAS some years ago. You can find it in the ACM Digital Library or on my homepage: http://users.elis.ugent.be/~brdsutte/research/publications/selected.html#whole-program An implementation of this algorithm can be obtained from the Diablo link-time rewriter, which is available through diablo.elis.ugent.be I wish you a lot of success if you want to re-implement it in LLVM. That would be great! Best, Bjorn De Sutter Computer Systems Lab Ghent University On 03 Oct 2012, at 02:18, Stephen Schiffli wrote:> Okay thanks for the info. The term program termination was probably a poor choice of words. I'm really just trying to build an inter-procedural BasicBlock graph, and then look for postdominance as Scott suggested. I'll go about making my own since it doesn't sound like there is one out there already. > > Thanks, > -Stephen > > On Tue, Oct 2, 2012 at 5:06 PM, Scott Moore <sdmoore at fas.harvard.edu> wrote: > I think you're looking for an inter-procedural post dominator analysis. I don't think there is one in LLVM already, but it should be relatively straightforward. > > This gives a sound approximation (i.e. no false positives) of something sort-of equivalent to the halting problem: if the program terminates, then block Y was executed. > > Cheers, > Scott > > > > On Tue, Oct 2, 2012 at 7:43 PM, Jim Grosbach <grosbach at apple.com> wrote: > Isn't this effectively the halting problem? Consider the case where block Y is the exit block of main() and block X is the entry block of main(). > > Jim > > > On Oct 2, 2012, at 4:29 PM, Stephen Schiffli <sschiffli at gmail.com> wrote: > >> Is there any inter-procedural analysis that could tell me if some BasicBlock Y is guaranteed to execute based on my knowledge that BasicBlock X will execute? For example: >> >> >> >> extern int x; >> >> void foo() { } >> >> int main() { >> >> if (x) { >> >> foo(); >> >> } else { >> >> foo(); >> >> } >> >> } >> >> >> I want to be told that the entry block of foo is guaranteed to be executed since I know the entry block of main is guaranteed to be executed. Basically that all paths from X to program termination go through Y at some point. Please ignore the folding of identical branches and function in-lining, I want to use this type of analysis in the general case. >> >> >> >> Thanks, >> >> -Stephen >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20121003/0f96f5c4/attachment.html>