4 ms·

Whether they get the question exactly right and can pinpoint the specific compiler passes or algebraic properties responsible for reductions like this is totall

Whether they get the question exactly right and can pinpoint the specific compiler passes or algebraic properties responsible for reductions like this is totally irrelevant and not what you’re actually looking for or asking about. It’s a very good jumping point for a conversation about optimization and testing whether they’re the type of developer who has ever looked at the assembly produced in their hotpath or not. Anyone who dumbly suggests that loops in source code will always result in loops in assembly doesn’t have a clue. Anyone who throws their hands up and says, “I have no idea, but I wonder if there’s some loop invariant or algebraic trick that can be used to optimize this, let’s think about it out loud for a bit” has taken a compiler class and gets full marks. Anyone who says, “I dunno, let’s see what godbolt does and look through the llvm-opt pane” gets an explicit, “hire this one” in the feedback to the hiring manager. It’s less about what they know and more about if they can find out.

scuff3d 10mo ago
So in other words, it isn't "basic and essential optimizations" that you would expect even a junior engineer to know (as your comment implies), but a mechanism to trigger a conversation to see how they think about problems. In fact, it sounds like something you wouldn't expect them to know.
- nickysielicki 10mo ago
  I didn’t write the GP comment. I wouldn’t call this basic and essential, but I would say that compilers have been doing similar loop simplifications for quite some time. I’d expect any mid to senior developer with C/C++ on their resume to at least consider the possibility that the compiler can entirely optimize away a loop. > In fact, it sounds like something you wouldn't expect them to know. I’d go a step further, I don’t think anyone, no matter how experienced they are, can confidently claim that optimized assembly will or won’t be produced for a given loop. That’s why the best answer above is, “I dunno”. If performance really matters, you have to investigate and confirm that you’re getting good code. You can have an intuition for what you think might happen, and that’s a useful skill to have on its own, but it’s totally useless if you don’t also know how to confirm your suspicions.
  - mgaunard 10mo ago
    My question is in the context of doing those optimizations yourself, understanding what can be done to make the code more efficient and how to code it up, not the compiler engineering to make that happen.
    - nickysielicki 10mo ago
      Yikes, gross. That’s like an option of last resort IMO. I’d rather maintain the clean loop-based code unless I had evidence that the compiler was doing the wrong thing and it was in my critical path.
      - mgaunard 10mo ago
        The compiler is only able to perform certain optimizations that have no observable behaviour. For example it can only parallelize code which is inherently parallelizable to begin with, and unless you design your algorithm with that in mind, it's unlikely to be. My belief is that it's better to be explicit, be it with low-level or high-level abstractions.

scuff3d 10mo ago

So in other words, it isn't "basic and essential optimizations" that you would expect even a junior engineer to know (as your comment implies), but a mechanism to trigger a conversation to see how they think about problems. In fact, it sounds like something you wouldn't expect them to know.

nickysielicki 10mo ago
I didn’t write the GP comment. I wouldn’t call this basic and essential, but I would say that compilers have been doing similar loop simplifications for quite some time. I’d expect any mid to senior developer with C/C++ on their resume to at least consider the possibility that the compiler can entirely optimize away a loop. > In fact, it sounds like something you wouldn't expect them to know. I’d go a step further, I don’t think anyone, no matter how experienced they are, can confidently claim that optimized assembly will or won’t be produced for a given loop. That’s why the best answer above is, “I dunno”. If performance really matters, you have to investigate and confirm that you’re getting good code. You can have an intuition for what you think might happen, and that’s a useful skill to have on its own, but it’s totally useless if you don’t also know how to confirm your suspicions.
- mgaunard 10mo ago
  My question is in the context of doing those optimizations yourself, understanding what can be done to make the code more efficient and how to code it up, not the compiler engineering to make that happen.
  - nickysielicki 10mo ago
    Yikes, gross. That’s like an option of last resort IMO. I’d rather maintain the clean loop-based code unless I had evidence that the compiler was doing the wrong thing and it was in my critical path.
    - mgaunard 10mo ago
      The compiler is only able to perform certain optimizations that have no observable behaviour. For example it can only parallelize code which is inherently parallelizable to begin with, and unless you design your algorithm with that in mind, it's unlikely to be. My belief is that it's better to be explicit, be it with low-level or high-level abstractions.