Hacker News Reader

Comment by vyodaiken | Hacker News Reader

I'm puzzled how I could have given the impression that I am looking for safety. I just don't want the compiler to defeat programmer intentions. I don't see how your example can depend on overflow semantics. The vectorizer is free to assume there is no overflow and let the programmer suffer if she screwed up. What I don't want the optimizer to do is to prevent the programmer from checking for overflow because it can't happen in theory while it can happen in practice.That is, I am interested in prioritizing programmer control over the safety net.

View on HN

Well, now I'm confused. When does the compiler defeat programmer intentions? If you specify -fwrapv and insert manual overflow checks wherever you wish, the compiler will never optimize them away and you will have as many runtime checks as you want. Programmer control is not compromised, which is good.

One of the motivating cases for this proposal is that gcc/clang, without fwrapv, will both implement wrapping on most architectures (since that is what the processor implemements) and silently remove manual overflow checks for overflow because it is "impossible" under a stupid interpretation of what UB means. Similarly, those compilers will remove manual checks for null pointers if there is a prior dereference to a pointer on the theory that dereferences of null pointers are "impossible" even when they happen.

Which is a very reasonable assumption for an optimizer. Every major compiler has switches to turn that behavior off. I'd be sympathetic to an argument that perhaps those switches should be on by default, but if you change the language standard, you're essentially saying entire classes of optimizations should be outright prohibited by a conformant compiler, with no programmer control. That's a classic case of throwing out the baby with the bathwater.

I do not see how FALSE is ever a reasonable assumption for an optimizer: " a+1 > a" is not a true statement for code emitted by gcc/clang on most architectures (with or without frwapv) Can you put one of your fwrapv deoptimizations on godbolt or otherwise show it?

Reply on news.ycombinator.com