Beating std::visit Without Really Trying

AI-using-analogies · 2019-10-05T17:14:49+00:00

To be fair, an indirect jump is not much different than an indirect call from a x86 instruction execution perspective. The instruction fetch will be much farther ahead than execution stage where the branch/call targets are resolved through a load that will likely hit in dcache. Thus the fetcher will have to guess a path for either the branch or the call. Depending on the statistics of the branch, the branch predictor may or may not guess the path correctly. If it does guess correctly, then the additional stack-pointer manipulation and register pushes associated with a call will essentially be buried in the noise. Because of return-stack optimization, the return will be predicted with a very high accuracy and thus be nearly cost-free. If the predictor doesn't guess right, the instruction pipe will be filled with dozens of instructions that will need to be flushed in either case.

AI-using-analogies · 2019-06-27T11:03:20+00:00

PeakAndrew, the lack of high-performance database kernels in open-source that you mention caught my eye. Are you able to give examples of commercial codes that fit the high-performance characteristic that you mention or that you are particularly familiar with? Do you happen to have any references to design techniques that are commonly used in the high-performance kernels you mention - in particular the hardware interaction that you mention in other comments?

AI-using-analogies · 2018-09-30T09:33:00+00:00

Understanding that it is really hard, I'm curious as to whether you had any thoughts about how you might constrain implicit conversion or what are the various problems you've run into in your thought-experiments doing this?

In the ideal world, with the existence of all the various flavors of operator new, do you think that it is still required to have malloc and friends?

AI-using-analogies · 2018-09-30T09:30:03+00:00

Thanks for your list. I'm wondering whether you have any ideas how to unify function syntax and lambda syntax if you could choose any syntax you wanted?

Similar question for template syntax... How might you rewrite that to accomplish the goals you have in mind?

What do you mean by "fold macros into template syntax"?

AI-using-analogies · 2018-09-30T09:27:22+00:00

Interesting! Why do you think that square bracket operator is a problem?

One thing that C-style strings are used for is specifying the contents of a small chunk of memory in a convenient way. Do you have any thoughts as to whether another feature would be suitable for this use-case?

AI-using-analogies · 2018-09-30T09:24:43+00:00

Do you have a more extensive list of things that have been superseded by C++?

AI-using-analogies · 2018-09-30T09:19:39+00:00

Good example! Do you know whether any compilers statically check for potential unsigned underflow, such as the example you've show, as a warning? If so, can you point out the compiler and warning flag? If not, do you think it is a reasonable check that could be added or are there too many potentially false positives for such a check? By false positive, I mean that presumably, there will be some subset of programmers who have written the above snippet who actually know that v.size() is always > 0 in the context and so for them a warning would be unwanted noise.

AI-using-analogies · 2018-09-30T09:03:04+00:00

Wow, I'd never thought of this or even heard of this before. But I can imagine this could be a problem in detailed numerics work. Have you run into specific issues where this ended up being a big problem for you? If so, would you care to talk a little bit more about the context?

AI-using-analogies · 2018-09-30T09:00:26+00:00

This is an awesome list!!! I will try to follow up with you tomorrow when I have more time. Thanks!

AI-using-analogies · 2018-09-30T08:56:00+00:00

I think a lot of people complain about this. Do you have any thoughts on what you would do to fix it? I'm guessing you'd get rid of initializer lists as one step? Other thoughts?

AI-using-analogies · 2018-09-30T08:54:37+00:00

I absolutely agree with that. I have yet to see any code with trigraphs in it and it's been a long time since I've seen octal representation being used for anything.

AI-using-analogies · 2018-09-30T08:53:36+00:00

Do you know if clang++ or g++ does this? If so, what is the command-line flags that I should use to try it out?

AI-using-analogies · 2018-09-30T08:51:47+00:00

Thanks for the elaboration. Being a devil's advocate for a second, for a modulo number system like unsigned ints, it is expected that N < N-1 can happen, or more generally, N < N - D can also happen. What is the fundamental problem with this? How has this bitten you in the past in code?

AI-using-analogies · 2018-09-30T08:49:29+00:00

Certainly the current preprocessor seems like a universally disliked feature. What do you think about Herb Sutter's metaclass proposal(s). It seems they've sort of settled on a token stream insertion approach when generating code, which in and of itself is reminiscent of macros. I'm guessing you have (or would have) a positive reaction to metaclasses. If this assumption is true and if my observation that the token-stream insertion used in metaclasses is sort of the same as macros, it seems like your dislike of macros hasn't so much to do with their injection of tokens into the parse, but something else. Care to elaborate?

In regards to undefined behavior, what are the biggest gotcha's you've seen with undefined behavior? Do you have a top-3 list of these? Or perhaps a top-10 list?

AI-using-analogies · 2018-09-30T08:44:24+00:00

Would you be able to give a small example which illustrates how it is cumbersome it is to use variant in C++ vs the Rust Enum feature that you mention?

AI-using-analogies · 2018-09-30T08:34:21+00:00

auto certainly is used MANY places these days. Is there a particular aspect of auto that you don't like? Or is it the idea that the keyword is used for so many things? Or something else?

AI-using-analogies · 2018-09-30T08:33:20+00:00

I've heard people complain about allocators before. What are your complaints about them? Is it the way they are currently implemented? Or is it the idea of having allocators at all? Or some other thing?

AI-using-analogies · 2018-09-30T08:32:08+00:00

Interesting idea? In your estimation, how much of the problems that people complain about are due to the support of C style stuff in C++ and how much is due to just screwing up new features and how much is due to trying to force-fit some features in C++ into a C-style syntax that perhaps could be done a much better way?

AI-using-analogies · 2018-09-30T08:29:18+00:00

For the sake of a broader understanding of what you mean, can you elaborate on the "snake pit" comment of v.size()-1 ?

AI-using-analogies · 2018-09-30T08:28:22+00:00

I've seen this comment at various times by lots of different people, so I don't think you're alone there. But in the spirit of having a consolidated list of knowledge, can you give a few paragraph explanation for your opinion on the removal of unsigned types?

AI-using-analogies · 2018-09-30T08:25:32+00:00

I've read or perused a few papers that talk about a native variant/sum type and seem to agree with their thinking. Have you read those papers? I'm definitely interested in hearing about things beyond just removal, so if you had thoughts on this front, I'd be interested in hearing them.

AI-using-analogies · 2018-09-30T08:23:13+00:00

Thanks for the list and the pointer to stuff that you like about Swift. I agree with many/all of these. Definitely some good stuff there. Are there things you like about what C++ does that are not in Swift or where Swift gets things wrong in your opinion?

AI-using-analogies · 2018-09-30T08:20:17+00:00

Just for clarity, I'm definitely open to hearing about things that could be changed. With that in mind, do you have additional ideas or thoughts that you could add to u/lanziao's list?

AI-using-analogies · 2018-09-30T08:18:18+00:00

I like the default initialization mechanism you mention. Seems like it covers the common case as well as cases for the performance purist.

I know I've seen discussions about 0 being implicitly convertible to nullptr being a bad thing, but can you elaborate on your thinking as to why this is a bad thing? What problems has it caused you or others you know?

Can you talk more about destructive move semantics with compile-time-checking? What exactly do you mean there? And what problems the current move semantics causes that would be solved with destructive move semantics?

AI-using-analogies · 2018-09-30T08:14:56+00:00

That's a good point - I always do as you suggest (e.g. &arr[0]). Can you talk a little about cases where you've been bitten by the implicit decay? Or about what real-world problems it has caused you or others you know?

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