If the program was previously "fine" on version x.y.z of some compiler, then it is most likely still fine on it. That's the target that the program was written for.
There's some disagreement on whether you can call a program "fine" that breaks after switching to a newer version, or a different compiler.
I see a lot of programmers out there that unfortunately use the behavior of their code on whatever compiler they're using at the moment as a proxy for what the language actually guarantees.
Well, I can imagine a program having something where new C comments ( // ) makes it not divide, like:
a = 5 //* junk here */ 2
;
I'm sure there are ioccc or underhanded C contest entries doing this to make code work differently on compilers based on if // is starting a comment line or not.
Sure it is an ugly way of writing stuff and you'd be hard pressed to find lots of real world traps like this, but when/if you did have code that "suddenly" miscompiles you might actually think your old code with an old compiler did work, and a new compiler for "the same" language breaks your program. I don't think everyone code base should need full rewrites ever time a new compiler comes out.
There's some disagreement on whether you can call a program "fine" that breaks after switching to a newer version, or a different compiler.
I see a lot of programmers out there that unfortunately use the behavior of their code on whatever compiler they're using at the moment as a proxy for what the language actually guarantees.