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. Thus, this optimization is not justified by the semantics of undef. The engineering tradeoffs being made inside the compiler enable some extra optimization power and have a cost in terms of complexity (UB is hard to think about) but this doesnt necessarily have anything to do with UB in C, C++, or in any source language. I dont particularly want C/C++ compilers to optimize x+1>x to true either. 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. This is exactly what clang's UndefinedBehaviorSanitizer does. Authors: Juneyoung Lee, Yoonseung Kim, Youngju Song, Chung-Kil Hur, Sanjoy Das, David Majnemer, John Regehr, Nuno P. LopesTitle: Taming Undefined Behavior in. Undefined Behavior: What Happened to My Code? Programming languages & software engineering, Programming languages and software engineering. Our solutions do not degrade compile time or performance of generated code. So undef is basically a note to the compiler that it can choose whatever value it likes. 1. an uninitialized variable which was zero initialized by luck "before", and now it shares some other register that isn't zero. G. Gange, J. High-Throughput, Formal-Methods-Assisted Fuzzing for LLVM. A naked shift with UB for shift-past-bitwidth can be codegened to a single shift instruction on any architecture. Bitwise shifts that are out of bounds for their data type 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] INSPIRE: The Insieme parallel intermediate representation. 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. In CGO, 2017. B. MacCabe. It's still a subset of LLVM 3.7 - adding new HLSL intrinsic functions or supporting additional LLVM data types wouldn't really break bytecode compatibility. In Workshop on the LLVM Compiler Infrastructure in HPC, 2015. The most important optimizations enabled by deferred undefined behavior are those involving speculative execution such as hoisting loop-invariant code out of a loop. Recall that undef resolves as an arbitrary value of its type. 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. Lets try to do that using undef. 2017 IEEE/ACM International Symposium on Code Generation and Optimization (CGO). Taming Undefined Behavior in LLVM. Another way to say the same thing is that were always allowed to make code more defined than it was, though of course were never allowed to make it less defined. Unfortunately, this is not reliably possible with gcc, and therefore this optimization leads to bigger code. The change is somewhat fundamental and so this is going to take some time. In PACT, 2013. For example, you can almost never move loop-invariant code out of loops. In POPL, 2012. | June 2017. 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. No need to have IR operations with undefined behaviour. The IR should make it easy to perform transformations, and should also afford efficient. To justify the desired optimization we can observe that returning 1 is a refinement of returning poison. Formalizing the concurrency semantics of an LLVM fragment. Earlier I wrote that Undefined Behavior != Unsafe Programming, a piece intended to convince you that there's nothing inherently wrong with undefined behavior as long as it isn't in developer-facing parts of the system.. Today I want to talk about a new paper about undefined behavior in LLVM that's going to be presented in June at PLDI 2017.I'm an author of this paper, but not the main one. 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 . Translation validation is a powerful ally for verification: first we can support older code that may be out of reach for current automated verification techniques, and second, it provides an extra safety net. Eelis, what I was going for is a two-part argument to explain why poison has to exist: It seems kinda hacky, but hard to improve on. We spent a lot of time trying to explain this clearly in the paper and Im unlikely to do better here! K. Memarian, J. Matthiesen, J. Lingard, K. Nienhuis, D. Chisnall, R. N. M. Watson, and P. Sewell. ACM Trans. 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. V. DSilva, M. Payer, and D. Song. Taming Undened Behavior in LLVM. 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 Returning to the x + 1 > x example above, making add nsw INT_MAX, 1 evaluate to poison allows the desired optimization: the resulting poison value makes the icmp also return poison. Specifically, we focus on the security of encoding and decoding the BSM. Synthesizing software verifiers from proof rules. One choice we could make is to give up on performing this optimization (and others like it) at the LLVM level. What every compiler writer should know about programmers. Taming Undefined Behavior in LLVM Abstract: A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. 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. However, if this naked shift has no UB then it must commit to some particular semantics for the shift-past-bitwidth case. 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. PLDI 2017: Proceedings of the 38th ACM SIGPLAN Conference on Programming Language Design and Implementation. which is to replace x+1 < x" with FALSE. bool isKnownToCauseUB const Return true if "undefined behavior" is known. 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). Very few of these bugs can result in end-to-end miscompilation (where a well-formed source-level program is compiled to machine code that does the wrong thing) but even this can happen. 2. immediate UB is too strong (prevents speculation). X. Wang, N. Zeldovich, M. F. Kaashoek, and A. Solar-Lezama. This is the difference between a coverage and a non-coverage build 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? Taming Undefined Behavior in LLVM (regehr.org) . Sometimes the result is "Undefined behavior". Verified peephole optimizations for CompCert. virtual bool isAssumedToCauseUB (Instruction *I) const =0 Return true if "undefined behavior" is assumed for a specific instruction. 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. M. A. Ertl. The IR should priate number of values to such a continuation is unde- make it easy to perform . 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, 2016. This is commonly exposed by register allocation changes. Check if you have access through your login credentials or your institution to get full access on this article. 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. 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. 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. Lang. "Mature" is the primary reason people pick GCC over the competition. G. Barthe, D. Demange, and D. Pichardie. What is Undefined Behavior ? The ACM Digital Library is published by the Association for Computing Machinery. This paper introduces the correctness-security gap, which arises when a compiler optimization preserves the functionality of but violates a security guarantee made by source code, and shows that several standard optimizations, which have been formally proved correct, in-habit this correctness- security gap. Our problems start because LLVM also contains two kinds of deferred UB which dont explode, but rather have a contained effect on the program. This page is powered by a knowledgeable community that helps you make an informed decision. The key is to put freezes in the right places. Revised 6 report on the algorithmic language Scheme, Sept. 2007. In PLDI, 2016. * Example: In C, integer division by zero yields undefined behaviour. The choice made by the LLVM developers, however, was to introduce a second, stronger, form of deferred UB called poison. If the upstream maintainers change member definitions from unsigned to signed, the program code is now silently asking for an unsound transformation that something like Coverity will flag, but the compiler does not. Interesting post, but you lost me at Lets try to do that using undef. 1. undef is too weak (the x+1 > x example) Semantic Scholar is a free, AI-powered research tool for scientific literature, based at the Allen Institute for AI. Juneyoung Lee Yoonseung Kim Youngju Song Chung-Kil Hur Azul Systems Sanjoy Das Google University of Utah John Regehr David Majnemer Microsoft Research / 21 Alas, theres no value of type i32 that is larger than INT_MAX, when we use a signed comparison. 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. 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. 2. an array overflow on the stack which starts clobbering a variable that actually matters, instead of something that was dead. LLVM IR contains this same kind of UB, which well call immediate UB. It is triggered by bad operations such as an out-of-bounds store (which is likely to corrupt RAM) or a division by zero (which may cause the processor to trap). Can we make the comparison true for case 2, giving us the overall result that we want? N. P. Lopes, D. Menendez, S. Nagarakatte, and J. Regehr. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. Efficiently computing static single assignment form and the control dependence graph. GCC, Clang/LLVM, and Orwell Dev-C++ are probably your best bets out of the 8 options considered. Assume a language that throws an exception when you do that. 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. 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 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. In PLDI, 2015. 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;. Alive is a domain-specific language for writing optimizations and for automatically either proving them correct or else generating counterexamples, which can be automatically translated into C++ code that is suitable for inclusion in an LLVM optimization pass. There is no undefined overflow. Ok that makes sense. So this example has two cases: In case 1 the comparison returns true. [This piece is coauthored by Yuyou Fan and John Regehr] Mutation-based fuzzing is based on the idea that new, bug-triggering inputs can often be created by randomly modifying existing, non-bug-triggering inputs. FLINT for SML is the first one I saw like that: We need to reason about the meaning of these slightly undefined programs which can be challenging. 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. Firma graph-based intermediate representation. In ICSE, 2012. Formalizing the LLVM intermediate representation for verified program transformations. Theory and Practice of Logic Programming, 15(4-5):526542, July 2015. it hasn't really lived up to promise of being a portable abstraction for compute kernels Since there are no released implementations, you can't really say anything about portability. Like nan in floating point. Why bother with notions of an undef value (or deferred UB), when plain UB is all you need? In Proc. 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. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. In PLDI, 1990. USA, PLDI '23: ACM SIGPLAN Conference on Programming Language Design and Implementation, All Holdings within the ACM Digital Library. This paper presents initial steps in ensuring that V2V communications are implemented securely. Lang. 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. 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. Taming Undefined Behavior in LLVM SIGPL 2017 Summer Juneyoung Lee Software Foundations Lab (Advisor: Chung-Kil Hur) 2 / 45 Undefined Behavior (UB)? Unfortunately, the semantics of undef dont justify all of the optimizations that wed like to perform on LLVM code. For example, knowing that INT_MAX+1 is undefined allows optimizing "X+1 > X" to "true". If we didnt have something like undef, wed be forced to initialize locations like this to specific values, which costs space and time. But the compiler does not actually guarantee that x+1 is greater than x and the C standard doesn't guarantee it either. 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. You can view the source on github and inspect the compiler output on Travis-CI. 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. R. Cytron, J. Ferrante, B. K. Rosen, M. N. Wegman, and F. K. Zadeck. If the input is INT_MAX, the addition results in UB. I think the design makes more sense if we change to a different example: shift past bitwidth. Since the optimizer may assume that the program wont incur UB, it can assume that case 2 does not happen, and so only case 1 has to be considered, where x+1>x indeed holds. 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. 2. Program. 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 Authors: Juneyoung Lee Pohang University of Science and Technology Yoonseung Kim Youngju Song Chung-Kil Hur Abstract A central concern for an optimizing compiler. To get undef-like semantics we introduce a new freeze instruction to LLVM. LKML Archive on lore.kernel.org help / color / mirror / Atom feed * [PATCH v7 00/25] Unify vDSOs across more architectures @ 2019-06-21 9:52 Vincenzo Frascino 2019-06-21 9:52 ` [PATCH v7 01/25] kernel: Standardize vdso_datapage Vincenzo Frascino ` (26 more replies) 0 siblings, 27 replies; 142+ messages in thread From: Vincenzo Frascino @ 2019-06-21 9:52 UTC (permalink / raw It is argued that this work is the most comprehensive and complete semantic treatment of undefined behavior in C, and thus of the C language itself. M. Braun, S. Buchwald, and A. Zwinkau. 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. 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. D. Khaldi, P. Jouvelot, F. Irigoin, C. Ancourt, and B. Chapman. LLVM parallel intermediate representation: design and evaluation using OpenSHMEM communications. W. Dietz, P. Li, J. Regehr, and V. Adve. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. Into the depths of C: Elaborating the de facto standards. The input is INT_MAX, in which case the addition returns undef. 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. 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. Towards optimization-safe systems: analyzing the impact of undefined behavior. Every IR-level optimization implements a refinement. 2011. Luckily, there are ways to detect at least some of the undefined behavior in a program. 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. Just to be clear: we can make the semantics of an IR anything we like. 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. Understanding integer overflow in C/C++. Address Sanitizer; Undefined Behavior Sanitizer; clanggccUBSan clangUBSan3 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. 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. Taming Undefined Behavior in LLVM Nuno P. Lopes PLDI 2017 Barcelona Seoul National Univ. Every use of a given freeze instruction will produce the same value, but different freezes may give different values. 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. 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] In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. Our solutions do not degrade compile time or performance of generated code. In PLDI, 2012. The front-ends correctly refine C, C++, etc. In both languages the undefinedness of signed overflow needs to be recognized to make the optimization go. The next part of the paper (Sections 4, 5, 6) introduces and evaluates our proposed fix, which is to remove undef, leaving only poison. Signed integer overflow: If arithmetic on an 'int' type (for example) overflows, the result is undefined. In KPS, 2015. If poison propagates to a programs output, the result is immediate UB. 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. In DAMP, 2011. More posts you may like. The compiler can issue a warning at compile time, but only in case it can statically detect some kind of wrongdoing. S. Chakraborty and V. Vafeiadis. One example is that "INT_MAX+1" is not guaranteed to be INT_MIN. Undefined Behavior! X. Leroy. Masters thesis, MIT, 1999. Horn clauses as an intermediate representation for program analysis and transformation. For example, if we wanted to find bugs in a PDF reader, we could grab a . Undef is useful because sometimes we want to say that a value doesnt matter, for example because we know a location is going to be over-written later.
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