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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.
(Score: 2) by Pino P on Thursday February 16 2017, @04:56PM
Most of the time I want an addition operation to spectacularly fail with an exception if it overflows. But there may be times where I don't care what happens in the event of overflow because I can guarantee before the addition is done that overflow simply cannot occur. The simple example is that the operands are already restricted to a smaller range making overflow impossible in the data type that the addition will use. (eg, adding two bytes that are widened to ints) And depending on the purpose I may not even care about any overflow bits. Maybe wanting "mod 256" arithmetic widened to ints when the addition is performed.
I just searched for gcc trap add overflow on Google, and the second result [robertelder.org] states that -ftrapv in GCC is supposed to enable behavior similar to what you describe. But it was broken until 2014 when GCC 4.8.4 fixed a serious bug [gnu.org].
aUsing -ftrapv in GCC 4.8.4 or later enables the following rules: