4 ms·

what's the history of having undefined behavior? why would a language designer not specify?

• gpderetta 5y ago
  Sanitizers attempt to do define the behaviour of undefined constructs, but expect an integer multiple slowdown compared even with unoptimized builds. It is very hard to specify the behaviour of, for example, use-after-free, or accessing an automatic variable after it goes out of scope, or data races, in a C-like language without significant runtime cost.
• rocqua 5y ago
  Because the code should be somewhat portable, and not all processors act the same. This is why left and right shift are weird with overflow. Not consistent between x86 and some other instruction set. Similar for accessing the null address. Besides that, how would you specify the behavior of reading from an address the user randomly generated himself? What is stored at an address not generated by the compiler is out of scope for the compiler. If you try to define something like 'always trap' or 'always return 0', you incur massive overhead. Besides that. The fact that signed integer over/underflow is undefined behavior actually unlocks a lot of reasonable optimizations that treat integers like actual integers. Things like (a+1 > a) always being true.
  • deleted 5y ago
    [deleted]