Wan Zhiyuan
2015-May-05 21:33 UTC
[LLVMdev] llvm DSA - reproduce the result in PLDI 07 paper
Dear John, I intend to implement the improvements on DSA. After running DSA on SPEC, I found DSA gives low precision for mcf and bzip2. I have checked the most imprecise c files in mcf an found that the code seems to be a mixture of "PHI" and "GEP" instructions. Could you please give me some hints about what the big picture of the improvement should be and how to start? Thank you! Regards, Zhiyuan On Mon, Apr 6, 2015 at 5:22 PM, John Criswell <jtcriswel at gmail.com> wrote:> Dear Zhiyuan, > > In order to reproduce the results from the paper, you'll need to replicate > a system from that era. You'll need to use the same version of LLVM and > DSA that the paper used. I think that was LLVM 1.9 (the release_19 branch > of LLVM and poolalloc), but I'm not sure. You should check to see if the > paper specifies the version. > > As you'll be using a very old version of LLVM, it may be worth setting up > a VM with a corresponding old version of Linux. I suspect newer compilers > will not compile a version of LLVM that is that old, so using an old > version of Linux with an old version of GCC may be needed. I think Fedora > Core 2 is the OS we were using at the time. > > To answer the question of why you can't use a modern version of LLVM and > poolalloc, it's because LLVM has changed significantly. DSA relies upon > the type annotations provided in the LLVM IR to "bootstrap" its type > inference (bootstrap is not quite the right word, but it's the closest one > of which I could think). As LLVM matured, transformations would ditch the > type information (e.g., transforming typed GEPs into untyped GEPs into a > byte array), making DSA's ability to do type-inference (and thereby > improving field sensitivity) more difficult. Throw into the mix the fact > that DSA is maintained by an academic research group, and the result is > that modern DSA doesn't have the accuracy that the original DSA did. > > The good news is that I think DSA can be fixed by making its > type-inferencing code smarter. The bad news is that it'd be a fair amount > of work to do. So far, no one has had sufficient desire/motivation to > design and implement the improvements. > > Regards, > > John Criswell > > > > On 4/6/15 4:56 PM, Wan Zhiyuan wrote: > > Dear all, > I am trying to reproduce the "Percent May Alias" result described in PLDI > 07's paper "Making Context-Sensitive Points-to Analysis with Heap Cloning > Practical For The Real World" ( > http://llvm.org/pubs/2007-06-10-PLDI-DSA.html). > > However, my "Percent May Alias" for all the benchmarks is much greater, > especially "bzip2". > > The DSA code I use is from "https://github.com/smackers/smack". I have > diff the code between smack and poolalloc_32. They are almost the same > except the "#include" statements. > > I was wondering whether I need to do some configuration to make DSA work > properly. > > Thank you! > > Zhiyuan > > > > > > _______________________________________________ > LLVM Developers mailing listLLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > -- > John Criswell > Assistant Professor > Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150505/813b03f8/attachment.html>
John Criswell
2015-May-06 14:35 UTC
[LLVMdev] llvm DSA - reproduce the result in PLDI 07 paper
On 5/5/15 5:33 PM, Wan Zhiyuan wrote:> Dear John, > I intend to implement the improvements on DSA.There be nasty dragons in DSA. Don't say I didn't warn you. :) On a side note, I recently discovered that someone (I think Will Dietz) updated the code to work with LLVM mainline, so you should be able to update to a newer version of LLVM and use DSA. I've created a snapshot of the LLVM source tree and a modified version of the poolalloc tree in https://github.com/jtcriswell/llvm-dsa that you may find useful. I think Will is also maintaining a github tree that he updates regularly.> After running DSA on SPEC, I found DSA gives low precision for mcf and > bzip2. > I have checked the most imprecise c files in mcf an found that the > code seems to be a mixture of "PHI" and "GEP" instructions. > > Could you please give me some hints about what the big picture of the > improvement should be and how to start?There may be many reasons why DSA's precision is low. You'll need to figure out what the reason is for each program. In some cases, it may be type inference. In other cases, it may be that DSA is cognizant of the influence of external code (which causes it to correctly give pessimistic results). For programs using function pointers, there's a bug in the algorithm in which DSNodes that should be marked complete are not. It could also be that you're using the wrong DSA pass (e.g., using EQTD when TD will do). Looking at type inference specifically, the problem, in a nutshell, is that DSA's type inference should not rely upon LLVM's type annotations. It should just create a map from offsets to types. Some work on this has already been done (e.g., DSA can figure out that casting a pointer to a structure into a pointer to the type of the structure's first field is still offset 0). However, there are still places in which DSA is relying upon LLVM's type annotations. One thing that I would like to look at is changing how DSA analyzes arrays of structures. Right now, DSA tries to infer a structure type for a memory object and then tries to infer whether the memory object is a singleton structure or an array of structures (you can see this in DSA's interface; you can see a map between offsets and types and an 'A' flag indicating that the object is an array). I think this makes DSA needlessly complicated. I think it would be better if DSA did what Value Set Analysis (VSA) does. VSA was designed to analyze untyped binary code. For each abstract memory object, it creates a map of 4-tuples to types. Each 4-tuple represents a formula ax+b as (a,b,c,d) in which b if offset, a is stride, and x is constrained between values c and d (c and d can be constants or +-infinity). For example, if you have an array of struct {int a ; char * b} on a 32-bit machine, DSA currently tries to figure out that there's an array of structure elements in which there's an int at offset 0 and a char * at offset 4 within the structure. VSA would say that there's a memory object with an int at every multiple of 4x+0 offset and a char * at every 4x+4 offset. The VSA approach should be agnostic to all sorts of weird casting, embedded arrays, and embedded unions, though this is an educated guess at present. Regards, John Criswell> > Thank you! > > Regards, > Zhiyuan > > On Mon, Apr 6, 2015 at 5:22 PM, John Criswell <jtcriswel at gmail.com > <mailto:jtcriswel at gmail.com>> wrote: > > Dear Zhiyuan, > > In order to reproduce the results from the paper, you'll need to > replicate a system from that era. You'll need to use the same > version of LLVM and DSA that the paper used. I think that was > LLVM 1.9 (the release_19 branch of LLVM and poolalloc), but I'm > not sure. You should check to see if the paper specifies the version. > > As you'll be using a very old version of LLVM, it may be worth > setting up a VM with a corresponding old version of Linux. I > suspect newer compilers will not compile a version of LLVM that is > that old, so using an old version of Linux with an old version of > GCC may be needed. I think Fedora Core 2 is the OS we were using > at the time. > > To answer the question of why you can't use a modern version of > LLVM and poolalloc, it's because LLVM has changed significantly. > DSA relies upon the type annotations provided in the LLVM IR to > "bootstrap" its type inference (bootstrap is not quite the right > word, but it's the closest one of which I could think). As LLVM > matured, transformations would ditch the type information (e.g., > transforming typed GEPs into untyped GEPs into a byte array), > making DSA's ability to do type-inference (and thereby improving > field sensitivity) more difficult. Throw into the mix the fact > that DSA is maintained by an academic research group, and the > result is that modern DSA doesn't have the accuracy that the > original DSA did. > > The good news is that I think DSA can be fixed by making its > type-inferencing code smarter. The bad news is that it'd be a > fair amount of work to do. So far, no one has had sufficient > desire/motivation to design and implement the improvements. > > Regards, > > John Criswell > > > > On 4/6/15 4:56 PM, Wan Zhiyuan wrote: >> Dear all, >> I am trying to reproduce the "Percent May Alias" result described >> in PLDI 07's paper "Making Context-Sensitive Points-to Analysis >> with Heap Cloning Practical For The Real World" >> (http://llvm.org/pubs/2007-06-10-PLDI-DSA.html). >> >> However, my "Percent May Alias" for all the benchmarks is much >> greater, especially "bzip2". >> >> The DSA code I use is from "https://github.com/smackers/smack". I >> have diff the code between smack and poolalloc_32. They are >> almost the same except the "#include" statements. >> >> I was wondering whether I need to do some configuration to make >> DSA work properly. >> >> Thank you! >> >> Zhiyuan >> >> >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > -- > John Criswell > Assistant Professor > Department of Computer Science, University of Rochester > http://www.cs.rochester.edu/u/criswell > >-- John Criswell Assistant Professor Department of Computer Science, University of Rochester http://www.cs.rochester.edu/u/criswell -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150506/982426e3/attachment.html>
Wan Zhiyuan
2015-May-07 03:07 UTC
[LLVMdev] llvm DSA - reproduce the result in PLDI 07 paper
Dear John, Thank you so much for your reply! Please kindly correct me if my understanding is wrong.>From your reply, I learn that there could be mainly four reasons why DSA'sprecision is low: 1. type inference 2. influence of external code 3. bug in handling function pointers 4. using the wrong DSA pass As you mentioned in your previous mail, LLVM changes a lot. So I was wondering if DSA's algorithm (presented in PLDI '07) can achieve a comparable precision on top of LLVM 3.5+ after implementing the improvements. Also, I have diff the DSA code in poolalloc release_19 and release_32. It seems to me that the changes in DSA mainly comes from the aspects as below: 1. release_32 introduces the DataLayout to do some type inference; 2. release_32 replaces some data structures with LLVM built-in data structures; 3. release_32 handles different types of LLVM IR instructions, and in a different way. 4. the inheritance of Analysis Passes has changed a bit. Therefore, I was wondering if the introduction of DataLayout and changes in LLVM IR cause the imprecision as well. Thank you! Regards, Zhiyuan Wan On Wed, May 6, 2015 at 10:35 AM, John Criswell <jtcriswel at gmail.com> wrote:> On 5/5/15 5:33 PM, Wan Zhiyuan wrote: > > Dear John, > I intend to implement the improvements on DSA. > > > There be nasty dragons in DSA. Don't say I didn't warn you. > :) > > On a side note, I recently discovered that someone (I think Will Dietz) > updated the code to work with LLVM mainline, so you should be able to > update to a newer version of LLVM and use DSA. I've created a snapshot of > the LLVM source tree and a modified version of the poolalloc tree in > https://github.com/jtcriswell/llvm-dsa that you may find useful. I think > Will is also maintaining a github tree that he updates regularly. > > After running DSA on SPEC, I found DSA gives low precision for mcf and > bzip2. > I have checked the most imprecise c files in mcf an found that the code > seems to be a mixture of "PHI" and "GEP" instructions. > > Could you please give me some hints about what the big picture of the > improvement should be and how to start? > > > There may be many reasons why DSA's precision is low. You'll need to > figure out what the reason is for each program. In some cases, it may be > type inference. In other cases, it may be that DSA is cognizant of the > influence of external code (which causes it to correctly give pessimistic > results). For programs using function pointers, there's a bug in the > algorithm in which DSNodes that should be marked complete are not. It > could also be that you're using the wrong DSA pass (e.g., using EQTD when > TD will do). > > Looking at type inference specifically, the problem, in a nutshell, is > that DSA's type inference should not rely upon LLVM's type annotations. It > should just create a map from offsets to types. Some work on this has > already been done (e.g., DSA can figure out that casting a pointer to a > structure into a pointer to the type of the structure's first field is > still offset 0). However, there are still places in which DSA is relying > upon LLVM's type annotations. > > One thing that I would like to look at is changing how DSA analyzes arrays > of structures. Right now, DSA tries to infer a structure type for a memory > object and then tries to infer whether the memory object is a singleton > structure or an array of structures (you can see this in DSA's interface; > you can see a map between offsets and types and an 'A' flag indicating that > the object is an array). I think this makes DSA needlessly complicated. > > I think it would be better if DSA did what Value Set Analysis (VSA) does. > VSA was designed to analyze untyped binary code. For each abstract memory > object, it creates a map of 4-tuples to types. Each 4-tuple represents a > formula ax+b as (a,b,c,d) in which b if offset, a is stride, and x is > constrained between values c and d (c and d can be constants or > +-infinity). For example, if you have an array of struct {int a ; char * > b} on a 32-bit machine, DSA currently tries to figure out that there's an > array of structure elements in which there's an int at offset 0 and a char > * at offset 4 within the structure. VSA would say that there's a memory > object with an int at every multiple of 4x+0 offset and a char * at every > 4x+4 offset. > > The VSA approach should be agnostic to all sorts of weird casting, > embedded arrays, and embedded unions, though this is an educated guess at > present. > > Regards, > > John Criswell > > > > Thank you! > > Regards, > Zhiyuan > > On Mon, Apr 6, 2015 at 5:22 PM, John Criswell <jtcriswel at gmail.com> wrote: > >> Dear Zhiyuan, >> >> In order to reproduce the results from the paper, you'll need to >> replicate a system from that era. You'll need to use the same version of >> LLVM and DSA that the paper used. I think that was LLVM 1.9 (the >> release_19 branch of LLVM and poolalloc), but I'm not sure. You should >> check to see if the paper specifies the version. >> >> As you'll be using a very old version of LLVM, it may be worth setting up >> a VM with a corresponding old version of Linux. I suspect newer compilers >> will not compile a version of LLVM that is that old, so using an old >> version of Linux with an old version of GCC may be needed. I think Fedora >> Core 2 is the OS we were using at the time. >> >> To answer the question of why you can't use a modern version of LLVM and >> poolalloc, it's because LLVM has changed significantly. DSA relies upon >> the type annotations provided in the LLVM IR to "bootstrap" its type >> inference (bootstrap is not quite the right word, but it's the closest one >> of which I could think). As LLVM matured, transformations would ditch the >> type information (e.g., transforming typed GEPs into untyped GEPs into a >> byte array), making DSA's ability to do type-inference (and thereby >> improving field sensitivity) more difficult. Throw into the mix the fact >> that DSA is maintained by an academic research group, and the result is >> that modern DSA doesn't have the accuracy that the original DSA did. >> >> The good news is that I think DSA can be fixed by making its >> type-inferencing code smarter. The bad news is that it'd be a fair amount >> of work to do. So far, no one has had sufficient desire/motivation to >> design and implement the improvements. >> >> Regards, >> >> John Criswell >> >> >> >> On 4/6/15 4:56 PM, Wan Zhiyuan wrote: >> >> Dear all, >> I am trying to reproduce the "Percent May Alias" result described in PLDI >> 07's paper "Making Context-Sensitive Points-to Analysis with Heap Cloning >> Practical For The Real World" ( >> http://llvm.org/pubs/2007-06-10-PLDI-DSA.html). >> >> However, my "Percent May Alias" for all the benchmarks is much greater, >> especially "bzip2". >> >> The DSA code I use is from "https://github.com/smackers/smack". I have >> diff the code between smack and poolalloc_32. They are almost the same >> except the "#include" statements. >> >> I was wondering whether I need to do some configuration to make DSA >> work properly. >> >> Thank you! >> >> Zhiyuan >> >> >> >> >> >> _______________________________________________ >> LLVM Developers mailing listLLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >> >> -- >> John Criswell >> Assistant Professor >> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell >> >> > > > -- > John Criswell > Assistant Professor > Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150506/585fe91c/attachment.html>