We use cookies to ensure that we give you the best experience on our website. More. 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. Efficiently computing static single assignment form and the control dependence graph. The choice made by the LLVM developers, however, was to introduce a second, stronger, form of deferred UB called poison. 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. virtual bool isKnownToCauseUB PLDI '17, Barcelona, Spain The correctness-security gap in compiler optimization. 2014. B. Steensgaard. For example, if we wanted to find bugs in a PDF reader, we could grab a . The IR shouldmake it easy to perform transformations, and should also afford efficient and precise static analysis. LLVM parallel intermediate representation: design and evaluation using OpenSHMEM communications. The front-ends correctly refine C, C++, etc. Parallel computing with the pi-calculus. 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. USA, PLDI '23: ACM SIGPLAN Conference on Programming Language Design and Implementation, All Holdings within the ACM Digital Library. For example, knowing that INT_MAX+1 is undefined allows optimizing "X+1 > X" to "true". M. Sperber, R. K. Dybvig, M. Flatt, A. van Straaten, R. Kelsey, W. Clinger, J. Rees, R. B. Findler, and J. Matthews. Our translation validation project is codenamed utcTV (short for translation validation for UTC - Microsoft's C++ compiler). We share and discuss any content that computer scientists find Freezing a normal value is a nop and freezing a poison value evaluates to an arbitrary value of the type. 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. Synthesizing software verifiers from proof rules. 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. 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. In Proc. a new paper about undefined behavior in LLVM, the details are all there in Section 3 of the paper. 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. 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. One choice we could make is to give up on performing this optimization (and others like it) at the LLVM level. Hi Eelis, if you only have immediate UB, speculation becomes impossible. bool isKnownToCauseUB const Return true if "undefined behavior" is known. which is to replace x+1 < x" with FALSE. Most compilers rely on undefined behavior for aggressive optimizations. Deferred UB lets us go ahead and speculatively execute the code without triggering immediate UB. Orlando , 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. Like nan in floating point. In KPS, 2015. 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. Firma graph-based intermediate representation. 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. Taming Undefined Behavior in LLVM (regehr.org) . 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. Abstract At the programming language level, Scheme R6RS [21, A central concern for an optimizing compiler is the design of p. 54] mentions that "The effect of passing an inappro- its intermediate representation (IR) for code. Taming Undened Behavior in LLVM. If the input is INT_MAX, the addition results in UB. Every use of a given freeze instruction will produce the same value, but different freezes may give different values. . For example, you can almost never move loop-invariant code out of loops. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. Lets try to do that using undef. 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. The next part of the paper (Sections 4, 5, 6) introduces and evaluates our proposed fix, which is to remove undef, leaving only poison. 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. Eelis, what I was going for is a two-part argument to explain why poison has to exist: Tapir: Embedding fork-join parallelism into LLVMs intermediate representation. Return true if "undefined behavior" is assumed. In ICSE, 2012. Hard to imagine how greater(x+1,x) and less(x,MAX_INT) both can be true theorems. 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. Undefined behavior is notorious for code-breaking optimizations. Of course nobody said that a binary program that traps is meaningless. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. F. Peschanski. Authors: Juneyoung Lee, Yoonseung Kim, Youngju Song, Chung-Kil Hur, Sanjoy Das, David Majnemer, John Regehr, Nuno P. LopesTitle: Taming Undefined Behavior in. 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. Formal verification of a realistic compiler. In Workshop on the LLVM Compiler Infrastructure in HPC, 2015. This page is powered by a knowledgeable community that helps you make an informed decision. Passing --coverage to gcc while also linking LLVM causes an undefined reference to `__gcov_exit' error from the linker. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. 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. 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. 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. In LCPC, 2016. 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. 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. 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. 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. To justify the desired optimization we can observe that returning 1 is a refinement of returning poison. One example is that "INT_MAX+1" is not guaranteed to be INT_MIN. Luckily, there are ways to detect at least some of the undefined behavior in a program. The current semantics of LLVMs IR fails to justify some cases of loop unswitching, global value numbering, and other important textbook optimizations, causing long-standing bugs. 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. Program. A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. The IR should make it easy to perform . In fact the C standard, by defining a maximum value for a signed int clearly implies otherwise. Technical Report 35, Karlsruhe Institute of Technology, 2011. 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. Victor, I take your points about C, but the example I wanted to highlight here is at the LLVM level. You can view the source on github and inspect the compiler output on Travis-CI. 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. Unfortunately, the semantics of undef dont justify all of the optimizations that wed like to perform on LLVM code. Into the depths of C: Elaborating the de facto standards. Specifically, we focus on the security of encoding and decoding the BSM. 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 How does that affect the case in question? Signed integer overflow: If arithmetic on an 'int' type (for example) overflows, the result is undefined. of the GCC Developers Summit, 2007. 2. immediate UB is too strong (prevents speculation). 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). 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. I wonder what the cutting-edge is on opposite end for type-safe languages and type-directed optimizations. 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. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. 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. 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. GCC, Clang/LLVM, and Orwell Dev-C++ are probably your best bets out of the 8 options considered. T. B. Schardl, W. S. Moses, and C. E. Leiserson. 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. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. The most important optimizations enabled by deferred undefined behavior are those involving speculative execution such as hoisting loop-invariant code out of a loop. The IR should make it easy to perform transformations, and should also afford efficient and precise static analysis. In both languages the undefinedness of signed overflow needs to be recognized to make the optimization go. 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. A formal C memory model supporting integer-pointer casts. K. Memarian, J. Matthiesen, J. Lingard, K. Nienhuis, D. Chisnall, R. N. M. Watson, and P. Sewell. Program. Enter ubsan. Bitwise shifts that are out of bounds for their data type UB in LLVM IR Problem of . 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. In CGO, 2017. 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 . So this example has two cases: In case 1 the comparison returns true. ACM Trans. Taming Undefined Behavior in LLVM Abstract: A central concern for an optimizing compiler is the design of its intermediate representation (IR) for code. To manage your alert preferences, click on the button below. The IR should priate number of values to such a continuation is unde- make it easy to perform . Memory SSA a unified approach for sparsely representing memory operations. This is commonly exposed by register allocation changes. Or are you saying that it was a bad example? A. Navas, P. Schachte, H. Sndergaard, and P. J. Stuckey. In SPW, 2015. D. Khaldi, P. Jouvelot, F. Irigoin, C. Ancourt, and B. Chapman. | June 2017. What is Undefined Behavior ? Modeling Undefined Behaviour Semantics for Checking Equivalence, Tell Programmers About Signed Integer Overflow Behavior, Programming in C and C++ Lecture 5: Tooling. 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 Every IR-level optimization implements a refinement. To get undef-like semantics we introduce a new freeze instruction to LLVM. The program dependence web: A representation supporting control-, data-, and demand-driven interpretation of imperative languages. 2017 IEEE/ACM International Symposium on Code Generation and Optimization (CGO). 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. Done. In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. S. Grebenshchikov, N. P. Lopes, C. Popeea, and A. Rybalchenko. Taming Undened Behavior in LLVM. Often this is not the case and the checking has to take place at run time. This could be ARM, x86, or one of the other ones that is out there. Thus, this optimization is not justified by the semantics of undef. Assume a language that throws an exception when you do that. 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. Lets take a language like Swift where integer overflow is defined to trap. 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] In this paper we study an aspect of IR design that has received little attention: the role of undefined behavior. Taming Undefined Behavior in LLVM. So undef is basically a note to the compiler that it can choose whatever value it likes. Since assuming something false allows you to prove anything, the compiler can perform an unsound transformation. This is exactly what clang's UndefinedBehaviorSanitizer does. If the input is not INT_MAX, the addition returns input + 1. 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. 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;. 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. In PLDI, 2015. The static single information form. Recall that undef resolves as an arbitrary value of its type. C. Hathhorn, C. Ellison, and G. Rosu. We need to reason about the meaning of these slightly undefined programs which can be challenging. M. A. Ertl. Theres no problem as long as the poisonous results dont propagate somewhere that matters. I think the design makes more sense if we change to a different example: shift past bitwidth. The backends correctly refine IR into machine code. Why bother with notions of an undef value (or deferred UB), when plain UB is all you need? Undefined Behavior: What Happened to My Code? Masters thesis, MIT, 1999. Interesting post, but you lost me at Lets try to do that using undef. 1.5m members in the compsci community. 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. Sparse functional stores for imperative programs. Our solutions do not degrade compile time or performance of generated code. 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 . C++ Undefined Behavior What Is It, and Why Should I Care?

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