Can compiler optimization introduce bugs?

Question

Today I had a discussion with a friend of mine and we debated for a couple of hours about \"compiler optimization\".

I defended the point that sometimes

Answer 1

Compiler (and runtime) optimization can certainly introduce undesired behaviour - but it at least should only happen if you're relying on unspecified behaviour (or indeed making incorrect assumptions about well-specified behaviour).

Now beyond that, of course compilers can have bugs in them. Some of those may be around optimisations, and the implications could be very subtle - indeed they're likely to be, as obvious bugs are more likely to be fixed.

Assuming you include JITs as compilers, I've seen bugs in released versions of both the .NET JIT and the Hotspot JVM (I don't have details at the moment, unfortunately) which were reproducible in particularly odd situations. Whether they were due to particular optimisations or not, I don't know.