In the past it was more common to suspect the compiler, as others mention here. On a minicomputer I worked with in the late eighties, early nineties, I occasionally found errors in the compiler output. This was a Pascal compiler and because of that it didn't take too long to figure out that the code was actually correct and something else must be going on. Then firing up the debugger/tracer and scrutinizing and analyzing what happens in the disassembly.. when the problem was found, send a fax (yes!) to the head designer of the compiler, get a fixed test compiler back on a set of floppies.. went through this several times. I still have a printout somewhere with my pen marks pointing out a bug in the generated code.