4 ms·

That could literally produce no output program?

• rjeli 6y ago
  c.f. “nasal demons”
• Spivak 6y ago
  Yep! Dumb example. main() x = get_from_some_external_data_source() if x: print("Hello World") trigger_ub() You might expect this code to always print if x is true but the optimizer can look at this and say "welp, if x is true then it would trigger ub, therefore it must be false, and since x must always be false we can just remove that entire branch."
  • kbenson 6y ago
    My favorite example along these lines (in C) is "Cap'n'Proto remote vuln: pointer overflow check optimized away by compiler"[1] which was covered here a few years back and shows all of these "theoretical" compiler behaviors coming to a head in a real bug which is thoroughly explained. 1: https://news.ycombinator.com/item?id=14163111 https://news.ycombinator.com/item?id=14163111
• cesarb 6y ago
  > That could literally produce no output program? Way worse than even that (you might be noticing a theme here...). Once the optimizer has removed as dead code any branch that, if taken, would provably lead to UB at some arbitrary future point of execution, it can conclude that the other branch is now the only possible execution, and call it unconditionally, even if that leads to removing all your files (the classic example is https://kristerw.blogspot.com/2017/09/why-undefined-behavior-may-call-never.html https://kristerw.blogspot.com/2017/09/why-undefined-behavior...).