Perhaps the source language has a leaky specification, but why would you want to replicate that mistake inside the compiler? This is exactly what clang's UndefinedBehaviorSanitizer does. J. Zhao, S. Nagarakatte, M. M. Martin, and S. Zdancewic. 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 . 2. Our solutions do not degrade compile time or performance of generated code. Firma graph-based intermediate representation. In CGO, 2017. This work developed a novel memory model for LLVM IR and formalized it, which requires a handful of problematic IR-level optimizations to be removed, but it also supports the addition of new optimizations that were not previously legal. Lets try to do that using undef. Taming Undefined Behavior in LLVM - Embedded in Academia Taming Undefined Behavior in LLVM 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. GCC, Clang/LLVM, and Orwell Dev-C++ are probably your best bets out of the 8 options considered. Undefined Behavior: What Happened to My Code? 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. Computer Science Theory and Application. 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. This behavior enables certain classes of optimizations that are important for some code. The current semantics of LLVM's IR fails to justify some cases of loop unswitching, global value numbering, and other important "textbook" optimizations, causing long-standing bugs. The change is somewhat fundamental and so this is going to take some time. ACM Trans. UB in LLVM IR Problem of . 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). Signed integer overflow: If arithmetic on an 'int' type (for example) overflows, the result is undefined. The compiler can issue a warning at compile time, but only in case it can statically detect some kind of wrongdoing. Undefined Behavior Sanitizer; Thread Sanitizer; gcc. In PLDI, 2015. Syst., 13(4):451490, Oct. 1991. Program. PLDI '17, Barcelona, Spain 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. Done. 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. 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? 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. 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 Nuno P. Lopes PLDI 2017 Barcelona Seoul National Univ. 1. an uninitialized variable which was zero initialized by luck "before", and now it shares some other register that isn't zero. M. A. Ertl. Defining the undefinedness of C. In PLDI, 2015. If we didnt have something like undef, wed be forced to initialize locations like this to specific values, which costs space and time. How does that affect the case in question? 1.5m members in the compsci community. Every use of a given freeze instruction will produce the same value, but different freezes may give different values. Most instructions, taking a poison value on either input, evaluate to poison. virtual bool isAssumedToCauseUB (Instruction *I) const =0 Return true if "undefined behavior" is assumed for a specific instruction. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. A naked shift with UB for shift-past-bitwidth can be codegened to a single shift instruction on any architecture. 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. N. P. Lopes, D. Menendez, S. Nagarakatte, and J. Regehr. In Proc. In LCPC, 2016. Just to be clear: we can make the semantics of an IR anything we like. You can view the source on github and inspect the compiler output on Travis-CI. Bitwise shifts that are out of bounds for their data type 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. 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. Address Sanitizer; Undefined Behavior Sanitizer; clanggccUBSan clangUBSan3 In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. The most important optimizations enabled by deferred undefined behavior are those involving speculative execution such as hoisting loop-invariant code out of a loop. 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 Atmel Inc. AVR32 architecture document, Apr. Undefined Behavior! In KPS, 2015. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. In PACT, 2013. Victor, I take your points about C, but the example I wanted to highlight here is at the LLVM level. Freezing a normal value is a nop and freezing a poison value evaluates to an arbitrary value of the type. Theory and Practice of Logic Programming, 15(4-5):526542, July 2015. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. The IR should make it easy to perform . G. Gange, J. of the GCC Developers Summit, 2007. virtual bool isKnownToCauseUB Program. The IR shouldmake it easy to perform transformations, and should also afford efficient and precise static analysis. What is Undefined Behavior ? 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 should priate number of values to such a continuation is unde- make it easy to perform . Modeling Undefined Behaviour Semantics for Checking Equivalence, Tell Programmers About Signed Integer Overflow Behavior, Programming in C and C++ Lecture 5: Tooling. Juneyoung Lee Yoonseung Kim Youngju Song Chung-Kil Hur Azul Systems Sanjoy Das Google University of Utah John Regehr David Majnemer Microsoft Research / 21 C. Hathhorn, C. Ellison, and G. Rosu. The front-ends correctly refine C, C++, etc. Specifically, we focus on the security of encoding and decoding the BSM. 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. The static single information form. 2011. 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. 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 In Workshop on the LLVM Compiler Infrastructure in HPC, 2015. If the input is INT_MAX, the addition results in UB. 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. into IR. A formal C memory model supporting integer-pointer casts. Semantic Scholar is a free, AI-powered research tool for scientific literature, based at the Allen Institute for AI. Formal verification of an SSA-based middle-end for CompCert. Undefined behavior is notorious for code-breaking optimizations. But the compiler does not actually guarantee that x+1 is greater than x and the C standard doesn't guarantee it either. 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. S. Grebenshchikov, N. P. Lopes, C. Popeea, and A. Rybalchenko. UndefinedBehaviorSanitizer (UBSan) is a fast undefined behavior detector. Im an author of this paper, but not the main one. You can easily verify this by compiling the following C program with -fsanitize=undefined and looking at the generated LLVM code: bla.c: #include <stdio.h> int main (void) { int x; scanf ("%d . The input is not INT_MAX, in which case the addition returns input + 1. Lets take a language like Swift where integer overflow is defined to trap. For example, knowing that INT_MAX+1 is undefined allows optimizing "X+1 > X" to "true". Provably correct peephole optimizations with Alive. . Of course nobody said that a binary program that traps is meaningless. Our problems start because LLVM also contains two kinds of deferred UB which dont explode, but rather have a contained effect on the program. The choice made by the LLVM developers, however, was to introduce a second, stronger, form of deferred UB called poison. 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. 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. In PLDI, 2012. A program that traps is not meaningless. In PLDI, 1990. 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. 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. Victor, I dont know of a design document that covers this stuff, it sort of evolved over a period of time. 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. The next part of the paper (Sections 4, 5, 6) introduces and evaluates our proposed fix, which is to remove undef, leaving only poison. W. Dietz, P. Li, J. Regehr, and V. Adve. However, if this naked shift has no UB then it must commit to some particular semantics for the shift-past-bitwidth case. Formalizing the concurrency semantics of an LLVM fragment. Authors: Juneyoung Lee, Yoonseung Kim, Youngju Song, Chung-Kil Hur, Sanjoy Das, David Majnemer, John Regehr, Nuno P. LopesTitle: Taming Undefined Behavior in. 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. Can we make the comparison true for case 2, giving us the overall result that we want? Taming Undefined Behavior in LLVM. 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. The input is INT_MAX, in which case the addition returns undef. I think the design makes more sense if we change to a different example: shift past bitwidth. 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. This is the difference between a coverage and a non-coverage build B. MacCabe. Technical Report 35, Karlsruhe Institute of Technology, 2011. What the UndefinedBehaviorSanitizer does is to generate calls to llvm.sadd.with.overflow. Assume a language that throws an exception when you do that. A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. 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. Taming Undefined Behavior in LLVM. 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 PLDI, 2016. In this article I will avoid explaining undefined behavior and I'll give . 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. Taming Undefined Behavior in LLVM Abstract: A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. I dont particularly want C/C++ compilers to optimize x+1>x to true either. 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. Unfortunately, this is not reliably possible with gcc, and therefore this optimization leads to bigger code. Why bother with notions of an undef value (or deferred UB), when plain UB is all you need? The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. If poison propagates to a programs output, the result is immediate UB. In DAMP, 2011. 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. 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. I've set up a fresh project to try to isolate this problem. 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. To manage your alert preferences, click on the button below. Keq is presented, the first program equivalence checker that is parametric to the input and output language semantics and has no dependence on the transformation between theinput and output programs. 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). 2017 IEEE/ACM International Symposium on Code Generation and Optimization (CGO). To justify the desired optimization we can observe that returning 1 is a refinement of returning poison. During code generation, undef usually gets turned into whatever was already in the register.. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. The ACM Digital Library is published by the Association for Computing Machinery. Tapir: Embedding fork-join parallelism into LLVMs intermediate representation. M. Braun, S. Buchwald, and A. Zwinkau. Software Foundations Lab ISO/IEC 9899:2011 Programming languages -C / 45 H. Jordan, S. Pellegrini, P. Thoman, K. Kofler, and T. Fahringer. Verified peephole optimizations for CompCert. For example, you can almost never move loop-invariant code out of loops. More posts you may like. 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;. We need to reason about the meaning of these slightly undefined programs which can be challenging. 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. ACM, 52(7):107115, July 2009. 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. Efficiently computing static single assignment form and the control dependence graph. Since it is often difficult to prove that a loop executes at least once, loop-invariant code motion threatens to take a defined program where UB sits inside a loop that executes zero times and turn into into an undefined program. More. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. 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. Sparse functional stores for imperative programs. More. D. Khaldi, P. Jouvelot, F. Irigoin, C. Ancourt, and B. Chapman. In SOSP, 2013. The program dependence web: A representation supporting control-, data-, and demand-driven interpretation of imperative languages. Perhaps it was a mistake for me, in this post, to connect this LLVM example to a fragment of C code. There have been long threads on the LLVM developers mailing list going back and forth about this. Every IR-level optimization implements a refinement. 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. D. Novillo. In both languages the undefinedness of signed overflow needs to be recognized to make the optimization go. K. Memarian, J. Matthiesen, J. Lingard, K. Nienhuis, D. Chisnall, R. N. M. Watson, and P. Sewell. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. We spent a lot of time trying to explain this clearly in the paper and Im unlikely to do better here! Masters thesis, MIT, 1999. X. Leroy. The key is to put freezes in the right places. Interesting post, but you lost me at Lets try to do that using undef. For example, if we wanted to find bugs in a PDF reader, we could grab a . If the input is not INT_MAX, the addition returns input + 1. 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. Unfortunately, the semantics of undef dont justify all of the optimizations that wed like to perform on LLVM code. Recall that undef resolves as an arbitrary value of its type. F. Peschanski. G. Barthe, D. Demange, and D. Pichardie. The backends correctly refine IR into machine code. 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. Memory SSA a unified approach for sparsely representing memory operations. There is no undefined overflow. 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. 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. 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. There will be no problem as long as: The problem is that #2 is hard. Is there any design document or paper you can recommend on this topic? 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. 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] 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. No need to have IR operations with undefined behaviour. T. B. Schardl, W. S. Moses, and C. E. Leiserson. Ok, good point. Since assuming something false allows you to prove anything, the compiler can perform an unsound transformation. In ACM SIGPLAN Workshop on Intermediate Representations, 1995. Ok that makes sense. M. Sperber, R. K. Dybvig, M. Flatt, A. van Straaten, R. Kelsey, W. Clinger, J. Rees, R. B. Findler, and J. Matthews. Undefined behavior in C and C++ is sort of like a bomb: either it explodes or it doesnt. Syst., 36(1):4:14:35, Mar. We share and discuss any content that computer scientists find USA, PLDI '23: ACM SIGPLAN Conference on Programming Language Design and Implementation, All Holdings within the ACM Digital Library. 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. Often this is not the case and the checking has to take place at run time. Formalizing the LLVM intermediate representation for verified program transformations. 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. FL , Into the depths of C: Elaborating the de facto standards. LLVM parallel intermediate representation: design and evaluation using OpenSHMEM communications. This paper presents initial steps in ensuring that V2V communications are implemented securely. Hi Eelis, if you only have immediate UB, speculation becomes impossible. I wonder what the cutting-edge is on opposite end for type-safe languages and type-directed optimizations. The IR should make it easy to perform transformations, and should also afford efficient. J. Kang, C.-K. Hur, W. Mansky, D. Garbuzov, S. Zdancewic, and V. Vafeiadis. LLVM works around this problem by adding the concept of deferred undened behavior: the undened addition is allowed, but the resulting value cannot be . Or are you saying that it was a bad example? Revised 6 report on the algorithmic language Scheme, Sept. 2007. Request PDF | Taming undefined behavior in LLVM | A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. 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. DeepCode, Switzerland / ETH Zurich, Switzerland, https://dl.acm.org/doi/10.1145/3062341.3062343. So far so good! 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. But the details are all there in Section 3 of the paper. 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. R. Cytron, J. Ferrante, B. K. Rosen, M. N. Wegman, and F. K. Zadeck. Understanding integer overflow in C/C++. So undef is basically a note to the compiler that it can choose whatever value it likes. 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. Taming Undened Behavior in LLVM. One example is that "INT_MAX+1" is not guaranteed to be INT_MIN. 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. It seems kinda hacky, but hard to improve on. 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. 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. 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. Check if you have access through your login credentials or your institution to get full access on this article.
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