You must be interpreting that differently than I meant it. The approach I'm suggesting is "pretend debuggers don't exist when optimizing". It gives you the fast code.
A debugger can break any assumption you make. Even unoptimized code could crash if a debugger messes with it. The fear of a debugger should never make you decide not to do an optimization.
How would you use a guard if you want to deoptimize because a debugger attached? You'd have to have a guard between each instruction, and even then it might not be enough.
> Yes, you're guessing that nobody will attach a debugger and you're guarding that no debugger has been attached. That guard is usually implicit.
> So you deoptimise when someone starts debugging.
If an "implicit" guard means "we'll have a function the debugger calls, telling us to redo the compilation", then that's not something you need to do dynamic analysis for, and it doesn't make your compilation more complicated. You don't "speculate away" that case unless you're using the word "speculate" to include "completely ignore for now, without even a guard", which I didn't think fell under that umbrella. Does it?
> It can be wrong... if someone's using a debugger.
It's not wrong. A debugger can make 2+2 be 5. Debuggers don't follow any rules, but that doesn't mean your compiler should try (and inevitably fail) to make code that works in a world where no rules exist.
