Lang. More posts you may like. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. To get undef-like semantics we introduce a new freeze instruction to LLVM. In ACM SIGPLAN Workshop on Intermediate Representations, 1995. UBSan modifies the program at compile-time to catch various kinds of undefined behavior during program execution, for example: Array subscript out of bounds, where the bounds can be statically determined. Over the years some very subtle mistakes have crept into the LLVM optimizer where different developers have made different assumptions about deferred UB, and these assumptions can work together to introduce bugs. C. Hathhorn, C. Ellison, and G. Rosu. In recent years C compiler writers have taken the attitude that working production C programs are buggy if they contain undefined behaviour, and they feel free to compile these programs (except benchmarks) in a way that they no longer work. W. Dietz, P. Li, J. Regehr, and V. Adve. Lets take a language like Swift where integer overflow is defined to trap. I think the design makes more sense if we change to a different example: shift past bitwidth. Theres no problem as long as the poisonous results dont propagate somewhere that matters. B. Steensgaard. Masters thesis, MIT, 1999. No need to have IR operations with undefined behaviour. The problem is that the information that it will not overflow gets lost, dropped by the compiler, and a naked shift with ARM semantics will require an extra instruction or two if were doing codegen for x86. An in-depth analysis of the design space for the semantics of pointers and memory in C as it is used in practice is described, a step towards clear, consistent, and accepted semantics for the various use-cases of C. This work presents the first formal memory model that allows many common optimizations and fully supports operations on the representation of pointers and all arithmetic operations are well-defined for pointers that have been cast to integers. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. The compiler can issue a warning at compile time, but only in case it can statically detect some kind of wrongdoing. The Art, Science, and Engineering of Fuzzing: a Survey, Lenient Execution of C on a JVM How I Learned to Stop Worrying and Execute the Code, In Using the GNU Compiler Collection (GCC), Automatic Program Generation for Detecting Vulnerabilities and Errors in Compilers and Interpreters, Integer Arithmetic and Undefined Behavior in C Brad Karp UCL Computer Science, A Differential Approach to Undefined Behavior. 2. immediate UB is too strong (prevents speculation). Taming Undefined Behavior in LLVM (regehr.org) . More. GCC, Clang/LLVM, and Orwell Dev-C++ are probably your best bets out of the 8 options considered. Address Sanitizer; Undefined Behavior Sanitizer; clanggccUBSan clangUBSan3 The first kind of deferred UB in LLVM is the undef value that acts like an uninitialized register: an undef evaluates to an arbitrary value of its type. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. If we didnt have something like undef, wed be forced to initialize locations like this to specific values, which costs space and time. Victor, I take your points about C, but the example I wanted to highlight here is at the LLVM level. More practically, consider when we have code including, say, a data structure from an OS or library definition. Formal verification of an SSA-based middle-end for CompCert. Formal verification of a realistic compiler. UB in LLVM IR Problem of . The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. Every use of a given freeze instruction will produce the same value, but different freezes may give different values. Towards optimization-safe systems: analyzing the impact of undefined behavior. And I certainly dont want to have x+1>x optimized to true in C. I want to use it to test if x!=MAXLONG in shorter code than what gcc and clang generate when I write x!=MAXLONG. Interesting post, but you lost me at Lets try to do that using undef. Taming Undefined Behavior in LLVM Juneyoung Lee Yoonseung Kim Youngju Song Chung-Kil Hur Seoul National University, Korea {juneyoung.lee, yoonseung.kim, youngju.song, gil.hur}@sf.snu.ac.kr Sanjoy Das Azul Systems, USA [email protected] David Majnemer Google, USA [email protected] We need to reason about the meaning of these slightly undefined programs which can be challenging. Bitwise shifts that are out of bounds for their data type Taming undefined behavior in LLVM @article{Lee2017TamingUB, title={Taming undefined behavior in LLVM}, author={Juneyoung Lee and Yoonseung Kim and Youngju Song and Chung-Kil Hur and Sanjoy Das and David Majnemer and John Regehr and Nuno P. Lopes}, journal={Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and . Taming Undefined Behavior in LLVM. In PLDI, 2012. Software Foundations Lab ISO/IEC 9899:2011 Programming languages -C / 45 The compiler is going to use the rule x+1 > x when x is signed to validate an optimization Facts About UB in LLVM It exists to support generaon of good code It is independent of undened behavior in source or target languages - You can compile an UB-free language to LLVM It comes in several avors Reasoning about opLmizaons in the presence of UB is very dicult PLDI '17, Barcelona, Spain M. Braun, S. Buchwald, and A. Zwinkau. This is commonly exposed by register allocation changes. Crellvm is presented: a verified credible compilation framework for LLVM, which can be used as a systematic way of providing a high level of reliability for major optimizations in LLVM. However, if this naked shift has no UB then it must commit to some particular semantics for the shift-past-bitwidth case. Parallel computing with the pi-calculus. So this example has two cases: In case 1 the comparison returns true. Taming undefined behavior in LLVM Pages 633-647 ABSTRACT References Index Terms ABSTRACT A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. ACM Trans. The static single information form. LLVM parallel intermediate representation: design and evaluation using OpenSHMEM communications. The IR for every optimizing compiler we have looked at, including GCC, LLVM, Intel's, and Microsoft's, supports one or more forms of undefined behavior (UB View on ACM cs.utah.edu Save to Library Create Alert Figures from this paper And if Swift is defined to trap on integer overflow, then optimizing x+1>x to true is miscompilation, not optimization, because it will not behave correctly when x=maxint. The IR for every optimizing compiler we have looked at, including GCC, LLVM, Intel's, and Microsoft's, supports one or more forms of undefined behavior (UB), not only to reflect the semantics of UB-heavy programming languages such as C and C++, but also to model inherently . Program. Undefined Behavior! R. Cytron, J. Ferrante, B. K. Rosen, M. N. Wegman, and F. K. Zadeck. The IR for every optimizing compiler we have looked at, includingGCC, LLVM, Intels, and Microsofts, supports one or more forms of undefined behavior (UB), not only to reflect the semantics of UB-heavy programming languages such as C and C++, but also to model inherently unsafe low-level operations such as memory stores and to avoid over-constraining IR semantics to the point that desirable transformations become illegal. E. Mullen, D. Zuniga, Z. Tatlock, and D. Grossman. There will be no problem as long as: The problem is that #2 is hard. Theory and Practice of Logic Programming, 15(4-5):526542, July 2015. If the optimizer can prove that a particular shift doesnt do the wrong thing, the checking code goes away and were left with a naked shift instruction in the IR. While this is a discussion on its own, in this article we'll explore the flip side of the coin; a less known artifact of undefined behavior. Thus, this optimization is not justified by the semantics of undef. Just to be clear: we can make the semantics of an IR anything we like. The ACM Digital Library is published by the Association for Computing Machinery. There is no undefined overflow. This is the difference between a coverage and a non-coverage build Return true if "undefined behavior" is assumed. Sparse functional stores for imperative programs. This paper presents initial steps in ensuring that V2V communications are implemented securely. 1. an uninitialized variable which was zero initialized by luck "before", and now it shares some other register that isn't zero. Finally, we noticed that there has been an interesting bit of convergent evolution in compiler IRs: basically all heavily optimizing AOT compilers (including GCC, MSVC, and Intel CC) have their own versions of deferred UB. The IR shouldmake it easy to perform transformations, and should also afford efficient and precise static analysis. I dont particularly want C/C++ compilers to optimize x+1>x to true either. the ir for every optimizing compiler we have looked at, including gcc, llvm, intel's, and microsoft's, supports one or more forms of undefined behavior (ub), not only to reflect the semantics of ub-heavy programming languages such as c and c++, but also to model inherently unsafe low-level operations such as memory stores and to avoid The IR for every optimizing compiler we have looked at, including GCC, LLVM, Intel's, and Microsoft's, supports one or more forms of undefined behavior (UB), not only to reflect the semantics of UB-heavy programming languages such as C and C++, but also to model inherently unsafe low-level operations such as memory stores and to avoid over-constraining IR semantics to the point that desirable transformations become illegal. virtual bool isAssumedToCauseUB (Instruction *I) const =0 Return true if "undefined behavior" is assumed for a specific instruction. | June 2017. This is exactly what clang's UndefinedBehaviorSanitizer does. K. Memarian, J. Matthiesen, J. Lingard, K. Nienhuis, D. Chisnall, R. N. M. Watson, and P. Sewell. Often this is not the case and the checking has to take place at run time. . Undefined behavior in C and C++ is sort of like a bomb: either it explodes or it doesnt. Sometimes the result is "Undefined behavior". For example, knowing that INT_MAX+1 is undefined allows optimizing "X+1 > X" to "true". This page is powered by a knowledgeable community that helps you make an informed decision. Atmel Inc. AVR32 architecture document, Apr. Technical Report 35, Karlsruhe Institute of Technology, 2011. J. Zhao, S. Nagarakatte, M. M. Martin, and S. Zdancewic. Done. A. Navas, P. Schachte, H. Sndergaard, and P. J. Stuckey. Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and Implementation, A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. Computer Science Theory and Application. So one solution would be to define the IR 32-bit shift operation to mean x<31 may result in either 0 or in x<<(n&31) (i.e., what the various architectures do, and what leads to the intended behaviour), but not undefined behaviour; that operation would be appropriate for a C 32-bit << operation. Most instructions, taking a poison value on either input, evaluate to poison. The choice made by the LLVM developers, however, was to introduce a second, stronger, form of deferred UB called poison. Our problems start because LLVM also contains two kinds of deferred UB which dont explode, but rather have a contained effect on the program. Enter ubsan. ACM, 52(7):107115, July 2009. Today I want to talk about a new paper about undefined behavior in LLVM thats going to be presented in June at PLDI 2017. Our solutions do not degrade compile time or performance of generated code. This work isnt about debating the merits of undefined behavior, its goal is to describe and try to fix some unintended consequences of the design of undefined behavior at the level of LLVM IR. 1.5m members in the compsci community. But the compiler does not actually guarantee that x+1 is greater than x and the C standard doesn't guarantee it either. Freezing a normal value is a nop and freezing a poison value evaluates to an arbitrary value of the type. B. MacCabe. Taming Undefined Behavior in LLVM (from /r/compsci) blog.regehr.org comments sorted by Best Top New Controversial Q&A Add a Comment . mediate undened behavior, this transformation would be illegal because it makes the domain of denedness smaller: the code would execute UB when x was INT_MAX, even if n was zero. Understanding integer overflow in C/C++. The spec no longer applies in this case and therefore provides no guidance on the behaviour of the program, either before or after the occurrence of the triggering behaviour. Into the depths of C: Elaborating the de facto standards. During code generation, undef usually gets turned into whatever was already in the register.. Unfortunately, this is not reliably possible with gcc, and therefore this optimization leads to bigger code. If the input is INT_MAX, the addition results in UB. In Proc. Deferred UB lets us go ahead and speculatively execute the code without triggering immediate UB. * Example: In C, integer division by zero yields undefined behaviour. This alert has been successfully added and will be sent to: You will be notified whenever a record that you have chosen has been cited. The details differ from those described here, but the effect is the same: deferred UB gives the compiler freedom to perform useful transformations that would otherwise be illegal. Like nan in floating point. C is not a language that is designed for complex transformations and I believe compiler developers would do better to consider programmers as colleagues who would benefit from better diagnostics and analysis. In DAMP, 2011. Syst., 36(1):4:14:35, Mar. Im an author of this paper, but not the main one. The backends correctly refine IR into machine code. Request PDF | Taming undefined behavior in LLVM | A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. Formalizing the concurrency semantics of an LLVM fragment. PLDI 2017: Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and Implementation. Most compilers rely on undefined behavior for aggressive optimizations. Earlier I wrote that Undefined Behavior != Unsafe Programming, a piece intended to convince you that theres nothing inherently wrong with undefined behavior as long as it isnt in developer-facing parts of the system. Simuliris is the first simulation technique to establish termination preservation for a range of concurrent program transformations that exploit UB in the source language, and is based on the idea of using ownership to reason modularly about the assumptions the compiler makes about programs with well-defined behavior. By clicking accept or continuing to use the site, you agree to the terms outlined in our. We present solutions to the problems we have identified in LLVMs IR and show that most optimizations currently in LLVM remain sound, and that some desirable new transformationsbecome permissible. the refinement check proves that (1) the transformed program only triggers ub when the original program does (i.e., ub can only be removed), (2) the root variable of the transformed program is only poison when it is also poison in the original program, and (3) variables' values in the final states of the two programs are the same when no ub is By running Alive2 over LLVMs unit test suite, this work discovered and reported 47 new bugs, 28 of which have been fixed already and led to eight patches to the LLVM Language Reference. Defining the undefinedness of C. In PLDI, 2015. The IR should make it easy to perform transformations, and should also afford efficient. D. Khaldi, P. Jouvelot, F. Irigoin, C. Ancourt, and B. Chapman. Copyright 2022 ACM, Inc. C. S. Ananian. Indeed, optimizing x+1>x to true in Swift would be wrong, and the LLVM-based Swift toolchain will not do that (its easy to try this out). Check if you have access through your login credentials or your institution to get full access on this article. How does that affect the case in question? C++ Undefined Behavior What Is It, and Why Should I Care? Taming Undefined Behavior in LLVM SIGPL 2017 Summer Juneyoung Lee Software Foundations Lab (Advisor: Chung-Kil Hur) 2 / 45 Undefined Behavior (UB)? Ok that makes sense. Taming Undefined Behavior in LLVM Juneyoung Lee Yoonseung Kim Youngju Song Chung-Kil Hur Seoul National University, Korea {juneyoung.lee, yoonseung.kim, youngju.song, gil.hur}@sf.snu.ac.kr Sanjoy Das Azul Systems, USA sanjoy@azul.com David Majnemer Google, USA majnemer@google.com John Regehr University of Utah, USA regehr@cs.utah.edu Nuno P. Lopes Microsoft Research, UK nlopes@microsoft.com . To justify the desired optimization we can observe that returning 1 is a refinement of returning poison. The IR should priate number of values to such a continuation is unde- make it easy to perform . 2011. The input is not INT_MAX, in which case the addition returns input + 1. T. B. Schardl, W. S. Moses, and C. E. Leiserson. Recall that undef resolves as an arbitrary value of its type. It is shown how OpenSHMEM one-sided communications can be optimized via the implementation of PIR into the LLVM compiler, and two important optimizations for such operations are illustrated using loop tiling and communication vectorization. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. In order to support LLVM's semantics for racy accesses, the formal model does not work on the level of single executions as the hardware and the C/C++ models do, but rather uses more elaborate structures called event structures. S. Grebenshchikov, N. P. Lopes, C. Popeea, and A. Rybalchenko. This work developed a novel memory model for LLVM IR and formalized it, and implemented the new model and shown that it fixes all known memory-model-related miscompilations without impacting the quality of generated code. To manage your alert preferences, click on the button below. High-Throughput, Formal-Methods-Assisted Fuzzing for LLVM. S. Chakraborty and V. Vafeiadis. When LLVM optimizes IR generated from Swift code, optimizations like this (perhaps not this exact one) are still useful and they cannot be observed from the level of the programming language. So UB in IR could be thought of as a code word for we dont care about this situation which has been proved to not occur.. Taming Undefined Behavior in LLVM Nuno P. Lopes PLDI 2017 Barcelona Seoul National Univ. Taming Undefined Behavior in LLVM. A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. Taming Undefined Behavior in LLVM Juneyoung Lee, Yoonseung Kim, Youngju Song, Chung-Kil Hur, Sanjoy Das, David Majnemer, John Regehr, Nuno P. Lopes Pre-print Media Attached Verifying invariants of lock-free data structures with rely-guarantee and refinement type Colin Gordon, Michael D. Ernst, Dan Grossman, Matthew Parkinson The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. Taming Undefined Behavior in LLVM Abstract: A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. Undefined behavior is notorious for code-breaking optimizations. There are lots of details that need to be ironed out, and I think people are (rightfully) worried about subtle bugs being introduced during the transition. Perhaps it was a mistake for me, in this post, to connect this LLVM example to a fragment of C code. The semantics of deferred UB in these compilers has not, as far as we know, been rigorously defined and so it is possible that they have issues analogous to those described here. Tapir: Embedding fork-join parallelism into LLVMs intermediate representation. The program dependence web: A representation supporting control-, data-, and demand-driven interpretation of imperative languages. The front-ends correctly refine C, C++, etc. LLVM works around this problem by adding the concept of deferred undened behavior: the undened addition is allowed, but the resulting value cannot be . But shouldnt the compiler then generate LLVM code with an explicit overflow test perhaps one that can be peephole optimized away on an instruction set that traps overflows? In ICSE, 2012. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. Authors: Juneyoung Lee, Yoonseung Kim, Youngju Song, Chung-Kil Hur, Sanjoy Das, David Majnemer, John Regehr, Nuno P. LopesTitle: Taming Undefined Behavior in. Luckily, there are ways to detect at least some of the undefined behavior in a program. G. Gange, J. Undefined Behavior Sanitizer; Thread Sanitizer; gcc. There have been long threads on the LLVM developers mailing list going back and forth about this. Juneyoung Lee Sanjoy Das David Majnemer Yoonseung Kim Azul Systems, USA Google, USA Youngju Song [email protected] [email protected] Chung-Kil Hur Seoul National University, Korea John Regehr Nuno P. Lopes {juneyoung.lee, yoonseung.kim, University of Utah, USA Microsoft Research, UK youngju.song, gil.hur}@sf.snu.ac.kr [email protected] [email protected] What it looks like on the outside is that considerable ingenuity and effort are being expended to defend an optimization that is of limited utility. Semantic Scholar is a free, AI-powered research tool for scientific literature, based at the Allen Institute for AI. A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. Taming Undefined Behavior in LLVM Juneyoung Lee Yoonseung Kim Youngju Song Chung-Kil Hur Seoul National University, Korea {juneyoung.lee, yoonseung.kim, youngju.song, gil.hur}@sf.snu.ac.kr In PLDI, 2015. We mean "secure" in the sense of having no flaws that could be a vulnerability; confidentiality and authentication are provided in other software layers and are not in scope here. The input is INT_MAX, in which case the addition returns undef. 1. Anton, yes, its clear that there are alternative design choices that would avoid UB in shift operators and that it is most likely possible to generate good assembly code from these. Alive is presented, a domain-specific language for writing optimizations and for automatically either proving them correct or else generating counterexamples, and can be automatically translated into C++ code that is suitable for inclusion in an LLVM optimization pass. What is Undefined Behavior ? A program that traps is not meaningless. H. Jordan, S. Pellegrini, P. Thoman, K. Kofler, and T. Fahringer. 2017 IEEE/ACM International Symposium on Code Generation and Optimization (CGO). One example is that "INT_MAX+1" is not guaranteed to be INT_MIN. What every compiler writer should know about programmers. What the UndefinedBehaviorSanitizer does is to generate calls to llvm.sadd.with.overflow. The point is that so far these bugs have resisted fixing: nobody has come up with a way to make everything consistent without giving up optimizations that the LLVM community is unwilling to give up. It seems kinda hacky, but hard to improve on. Since assuming something false allows you to prove anything, the compiler can perform an unsound transformation. In POPL, 2012. So undef is basically a note to the compiler that it can choose whatever value it likes. Taming undefined behavior in LLVM Authors: Juneyoung Lee Pohang University of Science and Technology Yoonseung Kim Youngju Song Chung-Kil Hur Abstract A central concern for an optimizing compiler. Can we make the comparison true for case 2, giving us the overall result that we want? For example, if we wanted to find bugs in a PDF reader, we could grab a . This paper augments an LLVM optimizer to generate translation results together with their correctness proofs, which can then be checked by a proof checker formally verified in Coq, which creates a verified credible compilation framework for LLVM. The compiler is allowed to choose this value. You can view the source on github and inspect the compiler output on Travis-CI. 2. into IR. Juneyoung Lee Yoonseung Kim Youngju Song Chung-Kil Hur Azul Systems Sanjoy Das Google University of Utah John Regehr David Majnemer Microsoft Research / 21 Yeah, it is hacky, and it makes compiler optimizations quite a bit harder to reason about, but as you say once you go the route of heavily optimizing ahead of time compiler I dont know that there are good alternatives. In PLDI, 2015. M. A. Ertl. Orlando , In KPS, 2015. In PLDI, 1990. A sequential model is introduced and it is formally shown that sequential reasoning is adequate and sufficient for establishing soundness of various compiler optimizations under weakly consistent shared-memory concurrency. Perhaps the source language has a leaky specification, but why would you want to replicate that mistake inside the compiler? The change is somewhat fundamental and so this is going to take some time. We present solutions to the problems we have identified in LLVM's IR and show that most optimizations currently in LLVM remain sound, and that some desirable new transformations become permissible. What would be more useful is a warning of a type error so that the programmer could choose to optimize by hand, which is simple , or to use type coercion to avoid the ambiguity. Firma graph-based intermediate representation. For example, consider this LLVM function: This is equivalent to return x+1 > x; in C and wed like to be able to optimize it to return true;. Provably correct peephole optimizations with Alive. Assume a language that throws an exception when you do that. One secret weapon we have is Alive where Nuno has implemented the new semantics in the newsema branch and we can use this to test a large number of optimizations. Yes, the shift example is certainly a hard one for IR design (because there is no common hardware behaviour), but undefining the too-far shift is a bad solution, and has lead to the undesirable result that clang optimized rol(x,0) into IIRC 0 for a common rol()-using-shifts idiom, while a straightforward translation to the native shift instruction of all architectures I looked at would have lead to the intended behaviour. N. P. Lopes, D. Menendez, S. Nagarakatte, and J. Regehr. Is there any design document or paper you can recommend on this topic? In SPW, 2015. Of course nobody said that a binary program that traps is meaningless. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. M. Sperber, R. K. Dybvig, M. Flatt, A. van Straaten, R. Kelsey, W. Clinger, J. Rees, R. B. Findler, and J. Matthews. Why bother with notions of an undef value (or deferred UB), when plain UB is all you need? 2. an array overflow on the stack which starts clobbering a variable that actually matters, instead of something that was dead. Formalizing the LLVM intermediate representation for verified program transformations. If poison propagates to a programs output, the result is immediate UB. The answer to this may be read up on llvm design, but I dont get why an add LLVM instruction should have an undefined or nondeterministic operation. If the input is not INT_MAX, the addition returns input + 1. Its something that programmers avoid most of the time, but it trapping is fully defined behaviour; indeed, languages that avoid undefined behaviour often do that by defining that performing various operations produces an error, rather than undefining the behaviour. J. Kang, C.-K. Hur, W. Mansky, D. Garbuzov, S. Zdancewic, and V. Vafeiadis. Ah, I guess it makes sense as an example illustrating the unfortunate consequences of using deferred UB analysis, rather than an example motivating the use of deferred UB analysis. Program. "Mature" is the primary reason people pick GCC over the competition. Taming Undened Behavior in LLVM. We spent a lot of time trying to explain this clearly in the paper and Im unlikely to do better here! My colleagues have implemented a fork of LLVM 4.0 that uses freeze; we found that it more or less doesnt affect compile times or the quality of the generated code. In PLDI, 2016. Peek is presented, a framework for expressing, verifying, and running meaning-preserving assembly-level program trans- formations in CompCert, and a set of local properties are proved are sufficient to ensure global transformation correctness. Alive is extended to 1) reduce the SMT formula sizes generated to improve performance and 2) significantly increase analysis coverage with a new loop unrolling algorithm for loops written in Alive IR. Efficiently computing static single assignment form and the control dependence graph. In SOSP, 2013. GCC recently (version 4.9) gained Undefined Behavior . The IR should make it easy to perform . 1. undef is too weak (the x+1 > x example)

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