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posted by cmn32480 on Thursday February 16 2017, @03:36PM   Printer-friendly
from the for-all-you-code-writing-types-out-there dept.

John Regehr, Professor of Computer Science, University of Utah, writes:

Undefined behavior (UB) in C and C++ is a clear and present danger to developers, especially when they are writing code that will execute near a trust boundary. A less well-known kind of undefined behavior exists in the intermediate representation (IR) for most optimizing, ahead-of-time compilers. For example, LLVM IR has undef and poison in addition to true explodes-in-your-face C-style UB. When people become aware of this, a typical reaction is: "Ugh, why? LLVM IR is just as bad as C!" This piece explains why that is not the correct reaction.

Undefined behavior is the result of a design decision: the refusal to systematically trap program errors at one particular level of a system. The responsibility for avoiding these errors is delegated to a higher level of abstraction. For example, it is obvious that a safe programming language can be compiled to machine code, and it is also obvious that the unsafety of machine code in no way compromises the high-level guarantees made by the language implementation. Swift and Rust are compiled to LLVM IR; some of their safety guarantees are enforced by dynamic checks in the emitted code, other guarantees are made through type checking and have no representation at the LLVM level. Either way, UB at the LLVM level is not a problem for, and cannot be detected by, code in the safe subsets of Swift and Rust. Even C can be used safely if some tool in the development environment ensures that it will not execute UB. The L4.verified project does exactly this.


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  • (Score: 2) by lgw on Thursday February 16 2017, @08:59PM

    by lgw (2836) on Thursday February 16 2017, @08:59PM (#467958)

    Sure, it's undefined, but I think all the compiler vendors actually do the same thing - check for reallocation in debug, and let it blow up with debug off.

    It's odd though, and always bugged me, that there's not a "slow but safe" choice in this case. Offering both the safe way and the fast way makes sense for fundamental library actions, as with index-based element access.

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  • (Score: 2) by c0lo on Thursday February 16 2017, @11:07PM

    by c0lo (156) Subscriber Badge on Thursday February 16 2017, @11:07PM (#468005) Journal

    It's odd though, and always bugged me, that there's not a "slow but safe" choice in this case. Offering both the safe way and the fast way makes sense for fundamental library actions, as with index-based element access.

    C++/STL does have the safe but slow - at(size_type ) [cppreference.com] .

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