----- Original Message -----> From: "Chandler Carruth" <chandlerc at google.com> > To: "Hal Finkel" <hfinkel at anl.gov> > Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu> > Sent: Thursday, July 16, 2015 2:33:21 AM > Subject: Re: [LLVMdev] [RFC] Defining Infinite Loops > > > > > On Thu, Jul 16, 2015 at 12:27 AM Hal Finkel < hfinkel at anl.gov > > wrote: > > > ----- Original Message ----- > > From: "Chandler Carruth" < chandlerc at google.com > > > To: "Hal Finkel" < hfinkel at anl.gov >, "LLVM Dev" < > > llvmdev at cs.uiuc.edu > > > Sent: Thursday, July 16, 2015 1:00:05 AM > > Subject: Re: [LLVMdev] [RFC] Defining Infinite Loops > > > > > > FWIW, I'm very much in favor of having a firm and clear answer to > > these questions. > > > > I also agree that it is an absolute requirement that LLVM have > > *some* > > mechanism for supporting both languages with defined behavior for > > infinite loops and a language requirement that all loops terminate. > > > > > > However, I'd like to float an alternative approach. I've not spent > > a > > lot of time thinking about it, so I'm not sure its actually better. > > I'm wondering if you've already thought about it. > > > > > > What if we have an @llvm.noop.sideeffect() or some such which > > doesn't > > read or write memory in any way, but which a frontend can place > > inside a loop body to mark that its execution (perhaps infinitely) > > is in-and-of-itself a side effect of the program. We could then > > teach loop unrolling or the few other things that would care to > > collapse multiple ones into a single one, and not count them > > towards > > anything. > > > > > > I know an intrinsic is kind of awkward for this, but it seems like > > the least bad way we have to annotate a loop in a fully generic > > way. > > I'd somewhat like this to be a property of the *loop* and not of > > the > > function. And it really needs to be truly generic, handling > > unnatural CFGs and even recursion-loops. My only idea for how to > > accomplish that is an intrinsic to mark the dynamic path which if > > executed infinitely can't be eliminated. > > My largest concern is that the frontend would need to add these > things all over the place, not just before the loop backedges. For > one thing, if the language has gotos, where should they be inserted? > > > The target of every goto. > > > For computed goto, very label whose address is taken. > > > This at least doesn't seem that bad to me. > > > Before every branch will be conservatively correct, but I'm worried > that will unnecessarily block optimizations. They'd also be needed > at the entry to every function. > > > Only external, address taken, or internal-and-recursively-called > functions. All of which we already have some blockers to > optimization, so this part i'm not worried about. > > > On the other hand, maybe if we added an optimization that removed > these things along any control-flow path that already had any other > side effect, it might not be too bad? > > > > Absolutely, much like lower-expect, we'd need to make sure that easy > cases were folded quickly in the optimizer so this didn't get out of > hand. > > > > > > > > > As for why I'm particularly interested in this being a property of > > the loop, consider if you were to have a mixture of Java and C++ > > code, all compiled to LLVM. How do you inline between them? > > > > You add the attribute to the caller. > > > This has the really unfortunate side effect of pessimizing code > during cross language optimizations. > > > FWIW, I suspect I might care a lot about this particular case > (because I believe that Fortran has defined behavior for infinite > loops). > > > > Yea, you could argue that C does too, which is one reason why I'm so > interested in this being done really well even in an LTO situation. > > > I think it would be really useful to not have this cross between > adjacent loops after inlining when they come from different source > languages, and it would be nice for it to not apply to nested loops > when those nested loops were inlined from a language without this > guarantee. > > > But I'm still not convinced that the noise of the intrinsic is > *definitely* worth it. I come from the background of the C++ rules' > rationale, and so I naturally see the languages that define this as > giving up optimizations and so wanting to localize the impact of > that... Not sure that's actually the right perspective though. ;] >I'm leaning toward agreeing with you, primarily because I think it will more-naturally fit into the optimizer than the attribute. We need to check loops for side effects anyway (if we otherwise default to C++-like rules), and so this intrinsic will do the right thing without any special logic. -Hal> > > -Hal > > > > > Anyways, I've not spent a lot of time thinking about what this > > might > > break for languages that allow infinite loops. Maybe it doesn't > > work > > as well as I'd hope. > > > > > > -Chandler > > > > > > On Wed, Jul 15, 2015 at 9:16 PM Hal Finkel < hfinkel at anl.gov > > > wrote: > > > > > > Hello everyone, > > > > The topic of whether or not LLVM allows for infinite loops has come > > up a lot recently (several times this week already). Regarding > > motivation, there are two important facts: > > > > 1. Some languages, such as Java, have well-defined infinite loops. > > See: > > > > http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.9 > > > > and: > > > > http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.2 > > > > and, as a community, it seems to be important for us to support > > such > > languages. That means that we must have a way, at the IR level, to > > support and model infinite loops. > > > > 2. Other languages, such a C and C++, allow us to assume that > > otherwise-side-effect-free loops terminate, specifically, for C++, > > 1.10p27 says: > > > > The implementation may assume that any thread will eventually do > > one > > of the following: > > - terminate > > - make a call to a library I/O function > > - access or modify a volatile object, or > > - perform a synchronization operation or an atomic operation > > > > [Note: This is intended to allow compiler transformations such as > > removal of empty loops, even > > when termination cannot be proven. — end note ] > > > > and taking advantage of these guarantees is part of providing a > > high-quality optimizer for C/C++ programs. > > > > And this leaves us somewhat in a bind. To provide a high-quality > > C/C++ optimizer, we want to take advantage of this guarantee, but > > we > > can't do so in a generic sense without harming our ability to serve > > as a compiler for other languages. > > > > In 2010, Nick proposed to add a 'halting' attribute that could be > > added to functions to indicate that they would not execute > > indefinitely ( > > http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20100705/103670.html > > ). At the time that the patch was proposed, there were > > infrastructure problems with inferring the attribute for functions > > with loops (related to using function-level analysis passes from a > > CGSCC pass), but hopefully those will be fixed with the new pass > > manager. Regardless, however, such inference is much more powerful > > if it can take advantage of the guarantees that C/C++ provide. > > > > Thus, while I would eventually like a 'halting' attribute, or some > > variant of that (including, for example, the lack of calls to > > longjmp), I think that a first step is to provide an attribute that > > Clang, and other frontends, can add when producing IR from sources > > where the language provides C/C++-like guarantees on loop > > termination. This attribute would indicate that the function will > > not execute indefinitely without producing some > > externally-observable side effect (calling an external function or > > executing a volatile/atomic memory access). I could name this > > attribute 'finite', but bikeshedding is welcome. > > > > With such an attribute in place, we would be able to clarify our > > overall position on infinite loops, be in a stronger position to > > infer more specific function properties (like halting), and can put > > in place generally-correct fixes to outstanding bugs (PR24078, for > > example). I know there are some Clang users who want it to optimize > > while honoring infinite loops, and I think adding this attribute > > helps them as well (assuming we'd provide some non-default option > > to > > prevent Clang from adding it). Thoughts? > > > > Thanks again, > > Hal > > > > -- > > Hal Finkel > > Assistant Computational Scientist > > Leadership Computing Facility > > Argonne National Laboratory > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > -- > Hal Finkel > Assistant Computational Scientist > Leadership Computing Facility > Argonne National Laboratory >-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory
On Thu, Jul 16, 2015 at 11:08 AM Hal Finkel <hfinkel at anl.gov> wrote:> ----- Original Message ----- > > From: "Chandler Carruth" <chandlerc at google.com> > > To: "Hal Finkel" <hfinkel at anl.gov> > > Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu> > > Sent: Thursday, July 16, 2015 2:33:21 AM > > Subject: Re: [LLVMdev] [RFC] Defining Infinite Loops > > > > > > > > > > On Thu, Jul 16, 2015 at 12:27 AM Hal Finkel < hfinkel at anl.gov > > > wrote: > > > > > > ----- Original Message ----- > > > From: "Chandler Carruth" < chandlerc at google.com > > > > To: "Hal Finkel" < hfinkel at anl.gov >, "LLVM Dev" < > > > llvmdev at cs.uiuc.edu > > > > Sent: Thursday, July 16, 2015 1:00:05 AM > > > Subject: Re: [LLVMdev] [RFC] Defining Infinite Loops > > > > > > > > > FWIW, I'm very much in favor of having a firm and clear answer to > > > these questions. > > > > > > I also agree that it is an absolute requirement that LLVM have > > > *some* > > > mechanism for supporting both languages with defined behavior for > > > infinite loops and a language requirement that all loops terminate. > > > > > > > > > However, I'd like to float an alternative approach. I've not spent > > > a > > > lot of time thinking about it, so I'm not sure its actually better. > > > I'm wondering if you've already thought about it. > > > > > > > > > What if we have an @llvm.noop.sideeffect() or some such which > > > doesn't > > > read or write memory in any way, but which a frontend can place > > > inside a loop body to mark that its execution (perhaps infinitely) > > > is in-and-of-itself a side effect of the program. We could then > > > teach loop unrolling or the few other things that would care to > > > collapse multiple ones into a single one, and not count them > > > towards > > > anything. > > > > > > > > > I know an intrinsic is kind of awkward for this, but it seems like > > > the least bad way we have to annotate a loop in a fully generic > > > way. > > > I'd somewhat like this to be a property of the *loop* and not of > > > the > > > function. And it really needs to be truly generic, handling > > > unnatural CFGs and even recursion-loops. My only idea for how to > > > accomplish that is an intrinsic to mark the dynamic path which if > > > executed infinitely can't be eliminated. > > > > My largest concern is that the frontend would need to add these > > things all over the place, not just before the loop backedges. For > > one thing, if the language has gotos, where should they be inserted? > > > > > > The target of every goto. > > > > > > For computed goto, very label whose address is taken. > > > > > > This at least doesn't seem that bad to me. > > > > > > Before every branch will be conservatively correct, but I'm worried > > that will unnecessarily block optimizations. They'd also be needed > > at the entry to every function. > > > > > > Only external, address taken, or internal-and-recursively-called > > functions. All of which we already have some blockers to > > optimization, so this part i'm not worried about. > > > > > > On the other hand, maybe if we added an optimization that removed > > these things along any control-flow path that already had any other > > side effect, it might not be too bad? > > > > > > > > Absolutely, much like lower-expect, we'd need to make sure that easy > > cases were folded quickly in the optimizer so this didn't get out of > > hand. > > > > > > > > > > > > > > > As for why I'm particularly interested in this being a property of > > > the loop, consider if you were to have a mixture of Java and C++ > > > code, all compiled to LLVM. How do you inline between them? > > > > > > > You add the attribute to the caller. > > > > > > This has the really unfortunate side effect of pessimizing code > > during cross language optimizations. > > > > > > FWIW, I suspect I might care a lot about this particular case > > (because I believe that Fortran has defined behavior for infinite > > loops). > > > > > > > > Yea, you could argue that C does too, which is one reason why I'm so > > interested in this being done really well even in an LTO situation. > > > > > > I think it would be really useful to not have this cross between > > adjacent loops after inlining when they come from different source > > languages, and it would be nice for it to not apply to nested loops > > when those nested loops were inlined from a language without this > > guarantee. > > > > > > But I'm still not convinced that the noise of the intrinsic is > > *definitely* worth it. I come from the background of the C++ rules' > > rationale, and so I naturally see the languages that define this as > > giving up optimizations and so wanting to localize the impact of > > that... Not sure that's actually the right perspective though. ;] > > > > I'm leaning toward agreeing with you, primarily because I think it will > more-naturally fit into the optimizer than the attribute. We need to check > loops for side effects anyway (if we otherwise default to C++-like rules), > and so this intrinsic will do the right thing without any special logic. >FWIW, this is why I first started thinking along these lines. I'm increasingly thinking that if this approach works it will make the implementation of testing for this more natural in the optimizer. Simple things like instruction predicates will "just work", etc. I'm really interested in the downsides. You mentioned a few potential ones, but seem to be happy with my responses. I wonder, are there others? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150717/ef2c56c5/attachment.html>
On 07/17/2015 02:03 AM, Chandler Carruth wrote:> On Thu, Jul 16, 2015 at 11:08 AM Hal Finkel <hfinkel at anl.gov > <mailto:hfinkel at anl.gov>> wrote: > > ----- Original Message ----- > > From: "Chandler Carruth" <chandlerc at google.com > <mailto:chandlerc at google.com>> > > To: "Hal Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov>> > > Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>> > > Sent: Thursday, July 16, 2015 2:33:21 AM > > Subject: Re: [LLVMdev] [RFC] Defining Infinite Loops > > > > > > > > > > On Thu, Jul 16, 2015 at 12:27 AM Hal Finkel < hfinkel at anl.gov > <mailto:hfinkel at anl.gov> > > > wrote: > > > > > > ----- Original Message ----- > > > From: "Chandler Carruth" < chandlerc at google.com > <mailto:chandlerc at google.com> > > > > To: "Hal Finkel" < hfinkel at anl.gov <mailto:hfinkel at anl.gov> >, > "LLVM Dev" < > > > llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu> > > > > Sent: Thursday, July 16, 2015 1:00:05 AM > > > Subject: Re: [LLVMdev] [RFC] Defining Infinite Loops > > > > > > > > > FWIW, I'm very much in favor of having a firm and clear answer to > > > these questions. > > > > > > I also agree that it is an absolute requirement that LLVM have > > > *some* > > > mechanism for supporting both languages with defined behavior for > > > infinite loops and a language requirement that all loops > terminate. > > > > > > > > > However, I'd like to float an alternative approach. I've not spent > > > a > > > lot of time thinking about it, so I'm not sure its actually > better. > > > I'm wondering if you've already thought about it. > > > > > > > > > What if we have an @llvm.noop.sideeffect() or some such which > > > doesn't > > > read or write memory in any way, but which a frontend can place > > > inside a loop body to mark that its execution (perhaps infinitely) > > > is in-and-of-itself a side effect of the program. We could then > > > teach loop unrolling or the few other things that would care to > > > collapse multiple ones into a single one, and not count them > > > towards > > > anything. > > > > > > > > > I know an intrinsic is kind of awkward for this, but it seems like > > > the least bad way we have to annotate a loop in a fully generic > > > way. > > > I'd somewhat like this to be a property of the *loop* and not of > > > the > > > function. And it really needs to be truly generic, handling > > > unnatural CFGs and even recursion-loops. My only idea for how to > > > accomplish that is an intrinsic to mark the dynamic path which if > > > executed infinitely can't be eliminated. > > > > My largest concern is that the frontend would need to add these > > things all over the place, not just before the loop backedges. For > > one thing, if the language has gotos, where should they be inserted? > > > > > > The target of every goto. > > > > > > For computed goto, very label whose address is taken. > > > > > > This at least doesn't seem that bad to me. > > > > > > Before every branch will be conservatively correct, but I'm worried > > that will unnecessarily block optimizations. They'd also be needed > > at the entry to every function. > > > > > > Only external, address taken, or internal-and-recursively-called > > functions. All of which we already have some blockers to > > optimization, so this part i'm not worried about. > > > > > > On the other hand, maybe if we added an optimization that removed > > these things along any control-flow path that already had any other > > side effect, it might not be too bad? > > > > > > > > Absolutely, much like lower-expect, we'd need to make sure that easy > > cases were folded quickly in the optimizer so this didn't get out of > > hand. > > > > > > > > > > > > > > > As for why I'm particularly interested in this being a property of > > > the loop, consider if you were to have a mixture of Java and C++ > > > code, all compiled to LLVM. How do you inline between them? > > > > > > > You add the attribute to the caller. > > > > > > This has the really unfortunate side effect of pessimizing code > > during cross language optimizations. > > > > > > FWIW, I suspect I might care a lot about this particular case > > (because I believe that Fortran has defined behavior for infinite > > loops). > > > > > > > > Yea, you could argue that C does too, which is one reason why I'm so > > interested in this being done really well even in an LTO situation. > > > > > > I think it would be really useful to not have this cross between > > adjacent loops after inlining when they come from different source > > languages, and it would be nice for it to not apply to nested loops > > when those nested loops were inlined from a language without this > > guarantee. > > > > > > But I'm still not convinced that the noise of the intrinsic is > > *definitely* worth it. I come from the background of the C++ rules' > > rationale, and so I naturally see the languages that define this as > > giving up optimizations and so wanting to localize the impact of > > that... Not sure that's actually the right perspective though. ;] > > > > I'm leaning toward agreeing with you, primarily because I think it > will more-naturally fit into the optimizer than the attribute. We > need to check loops for side effects anyway (if we otherwise > default to C++-like rules), and so this intrinsic will do the > right thing without any special logic. > > > FWIW, this is why I first started thinking along these lines. I'm > increasingly thinking that if this approach works it will make the > implementation of testing for this more natural in the optimizer. > Simple things like instruction predicates will "just work", etc. > > I'm really interested in the downsides. You mentioned a few potential > ones, but seem to be happy with my responses. I wonder, are there others?I'm really not a fan of this approach. I think it could be made to work, but I suspect we'd run into a lot of quality of implementation issues if we went down this route. We'd have to teach many places in the optimizer to merge or split such calls. Consider simple things like tail commoning, if-then-else hoisting, or the like. In each of these, we'd need code to recognize having the intrinsic on one path, but not the other, and then still perform the optimization. Think the various debug intrinsics, but worse. I worry about the interactions with memory aliasing and hoisting rules. We don't currently have the notion of a non-memory side effect in LLVM. To prevent this intrinsic from being DCE'd we'd likely need to mark it as writing to some known location. Given the number of places in the optimizer which give up when encountering any write (EarlyCSE for one), that would be problematic for optimization effectiveness. The other approach would be to teach LLVM about non-memory side effects. I think this is solvable, but a large investment. In practice, most of the contributors to LLVM care about C++. I worry we'd end up in a situation where languages which need infinite loops would become second class citizens and that a large number of optimizations wouldn't apply to them in practice. This is by far my biggest worry. Now, I'm certainly biased, but I'd much rather see a scheme where a quality of implementation issue effects the performance of C++. These are far more likely to be actually fixed. :) Earlier in this thread, the idea of using metadata on loops was mentioned. Chandler's point about generic handling for recursive loops is a good one, but in general, a metadata annotation of finiteness seems like a better starting point. What if we introduced a piece of branch (really, control transfer instruction) metadata (not loop) called "productive" (to steal Sanjoy's term) whose semantics are such that it can be assumed to only execute a finite number of times between productive actions (volatile, synchronization, io, etc..). We then tag *all* branches emitted by Clang with this metadata. This gives us the benefit of the loop metadata in that a single tagged backedge branch implies a productive loop, but allows productive recursive functions to be converted into productive loops in a natural way. The function attribute "productive" now has an easy inference rule in that if all branches in the function are productive and all callees are productive, so is the function. This would seem to allow us to perform DSE, LICM, and related optimizations without trouble. Inlining now has a reasonable definition where you can inline between languages w/the semantics of each language preserved. One cool thing here is that the set of operations which are "productive" could actually be encoded in the metadata. This could potentially support other languages than C++ w.r.t. the "no infinite loops except when" type rules. Thoughts? Philip p.s. The current implementation of readonly is correct w.r.t. C++ rules only because we designate all atomic, volatile, or fencing operations as read/write. One thing we might end up with out of this discussion is the possibility of talking about functions which are readonly, but involved in synchronization. That seems like it might be useful for optimizing concurrent C++ programs in interesting ways. It would require a separate notion of a synchronization side effect independent of read/write though. This seems related, but slightly orthogonal to the assumptions about finiteness of loops. -------------- next part -------------- An HTML attachment was scrubbed... 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