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> says who? Well, the modern software world. Performance is one of C's great advantages. Poorly optimising C compilers do exist, such as obscure compilers targ

> says who? Well, the modern software world. Performance is one of C's great advantages. Poorly optimising C compilers do exist, such as obscure compilers targeting obscure architectures, but much C code is written for speed. Kernels and game-engines are written in C and C++ partly for performance reasons. (Although, again, it's not as if we have to keep using C. Much safer alternatives do exist that don't have significant performance penalties, and it's a pity they don't get more traction.) > Optimizations like "this loop must be infinite because i is incremented from 0 and going past INT_MAX would be undefined behavior; and therefore the code after is unreachable and can be deleted" are not in "vital" in any shape or form. I believe the assumption of no UB can be important in auto-vectorisation, a very significant optimisation. There's a quantifiable performance advantage to any optimisation. The compiler vendors found it worth pursuing, so there must be something to it. These optimisations can be disabled, but this will harm performance. As to whether it's 'worth it', that's a matter of opinion. I still think that if you want a safe language, you should just use a safe language. C is not a safe language, and isn't trying to be. Even without these specific optimisations, there are still plenty of other ways buggy C code can still cause havoc. The only way to properly tame C is with projects like ZZ and Armada, as I previously linked. > That kind of thing points to the work of an ego that took the shortest possible career path into becoming a compiler committer, long before acquiring the required maturity. It's not reasonable to diagnose an engineering decision as a personality disorder. Also, I believe most of these decisions were made by committee, but I'm not well read on C's early days. They decided to make C the kind of language that places a great burden on the programmer, regarding subtle errors. We agree that this is a real downside of the language, but it's not without its advantages. C is the gold standard for performance, and has been for decades. The presence of UB has probably been somewhat helpful to that end, and has also helped C's portability. (Again though, modern compilers can optimise Ada very effectively. With modern compilers, I don't think C's UB is all that advantageous for performance.) > Modern C compilers are crossing from the "not holding your hand" territory into "knocking a tool out of your hand" territory. That's a rather good metaphor for aggressive optimisers. I'm reminded of a comment on programming languages: C treats you like an adult. Pascal treats you like a child. Ada treats you like a criminal. > Removing code because of "if the behavior of prior code was written by a perfect programmer who follows ISO C to the letter, this line cannot be reached" is definitely not a good thing. > Removing code because of some constant expression, or expression that is always true doe to type (like x < 0, x being unsigned) is certainly a good thing. This strikes me as a distinction without a difference. It's the same optimisation: elimination of unreachable code. The compiler is always permitted to assume absence of undefined behaviour, that's how undefined behaviour is, well, defined. If there's any UB present in an execution at all, all bets are off. I think you're suggesting a different approach would be better, but I'm not sure what you're proposing. We could try to define a language similar to C but devoid of undefined behaviour. Part of the spec could be adjusted to use indeterminate values instead, such as for signed integer overflow. We could define divide-by-zero as instantly terminating the process. (The C++ language takes this approach very occasionally, [0] but defining things this way is troublesome for embedded systems compilers.) This language wouldn't be as easy to compile, though. Features like C's unsafe pointer arithmetic, and unsafe function pointers, seem hard to sanitize. I wish I knew Ada better for comparison. [0] https://stackoverflow.com/a/43675980/ https://stackoverflow.com/a/43675980/

kazinator 6y ago
> It's the same optimisation: elimination of unreachable code. No it isn't, because in the one case, if you don't eliminate the code, it turns out to be reachable. The reason is that the logical predicate by which it was deduced to be unreachable ("the program is perfectly written and doesn't invoke undefined behavior") is not actually true. So that is to say, this code will actually terminate due to wraparound past INT_MAX "for (i = 0; i >= 0; i++);" and statements which follow it are reached. That same compiler has wraparound behavior for int. It's not the same as "if (1) return;" where the basis for removing the subsequent code is simply true.
- MaxBarraclough 6y ago
  So you're suggesting restricting the optimisation to trivial cases only, greatly hampering what it can do. Again, your suggestion is imprecise. Would you want undefined behaviour eliminated from the language entirely? If yes, deep changes would need to be made. If no, there's little point, as it's still a highly unsafe language.

kazinator 6y ago

> It's the same optimisation: elimination of unreachable code. No it isn't, because in the one case, if you don't eliminate the code, it turns out to be reachable. The reason is that the logical predicate by which it was deduced to be unreachable ("the program is perfectly written and doesn't invoke undefined behavior") is not actually true. So that is to say, this code will actually terminate due to wraparound past INT_MAX "for (i = 0; i >= 0; i++);" and statements which follow it are reached. That same compiler has wraparound behavior for int. It's not the same as "if (1) return;" where the basis for removing the subsequent code is simply true.

MaxBarraclough 6y ago
So you're suggesting restricting the optimisation to trivial cases only, greatly hampering what it can do. Again, your suggestion is imprecise. Would you want undefined behaviour eliminated from the language entirely? If yes, deep changes would need to be made. If no, there's little point, as it's still a highly unsafe language.