Can compiler optimization introduce bugs?

To combine the other posts:

1. Compilers do occasionally have bugs in their code, like most software. The "smart people" argument is completely irrelevant to this, as NASA satellites and other apps built by smart people also have bugs. The coding that does optimization is different coding from that which doesn't, so if the bug happens to be in the optimizer then indeed your optimized code may contain errors while your non-optimized code will not.

2. As Mr. Shiny and New pointed out, it's possible for code that is naive with regard to concurrency and/or timing issues to run satisfactorily without optimization yet fail with optimization as this may change the timing of execution. You could blame such a problem on the source code, but if it will only manifest when optimized, some people might blame optimization.

I certainly agree that it's silly to say the because compilers are written by "smart people" that they are therefore infallible. Smart people designed the Hindenberg and the Tacoma Narrows Bridge, too. Even if it's true that compiler-writers are among the smartest programmers out there, it's also true that compilers are among the most complex programs out there. Of course they have bugs.

On the other hand, experience tells us that the reliability of commercial compilers is very high. I've had many many times that someone told me that the reason why is program doesn't work MUST be because of a bug in the compiler because he has checked it very carefully and he is sure that it is 100% correct ... and then we find that in fact the program has an error and not the compiler. I'm trying to think of times that I've personally run across something that I was truly sure was an error in the compiler, and I can only recall one example.

So in general: Trust your compiler. But are they ever wrong? Sure.