Is it possible to use this with C/C++ code compiled to dll/so and called by Python?
rr – record and replay debugger for C/C++
111–120 of 134 posts
Re: rr – record and replay debugger for C/C++
#112Earlier quoted context omitted.
It's probably mean for me to say "empty type" to C++ people because of course just as std::move doesn't move likewise std::is_empty doesn't detect empty types. It can't because C++ doesn't have any. You may need to sit down. An empty type has no values. Not one value, like the unit type which C++ makes a poor job of as you explain, but no values. None at all. Because it has no values we will never be called upon to s…
void is an empty type in C++. It's less useful than it could be, but it does exist.
void isn't a type. If you try to use it as a type you'll be told "incomplete type".
People who want void to be a type in C++ (proponents of "regular void") mostly want it to be a unit type. If they're really ambitious they want it to have zero size. Generally a few committee meetings will knock that out of them.
Re: rr – record and replay debugger for C/C++
#113GDBs built-in reverse debugging: https://www.sourceware.org/gdb/wiki/ProcessRecord/Tutorial I assume rr provides more features and flexibility. Anyway I want to mention that GDB itself can already reverse debug for some time now.
If you want to mention this, then you very clearly haven't actually tried it. The implementation in GDB is more convenient than rr (you can start/stop recording at will), but it is also orders of magnitude less efficient. It's only usable for very small code snippets. Otherwise it takes effectively forever and/or runs out of resources.
> runs out of resources
Still, thank you to share your first-hand experience. Did you ask the GDB Dev team for any feedback on the slow performance?
Re: rr – record and replay debugger for C/C++
#114Earlier quoted context omitted.
If you want to mention this, then you very clearly haven't actually tried it. The implementation in GDB is more convenient than rr (you can start/stop recording at will), but it is also orders of magnitude less efficient. It's only usable for very small code snippets. Otherwise it takes effectively forever and/or runs out of resources.
> runs out of resources RAM? What kind of dev box runs out of RAM in 2024? I built a 64GB RAM dev box during COVID-19 crisis. I have never once come close to using all that RAM, even with a squillion Chrome tabs open. Still, thank you to share your first-hand experience. Did you ask the GDB Dev team for any feedback on the slow performance?
Uuh, any kind of dev box that requires more RAM than available? I promise you that storing that much data about the runtime can be really memory consumptive.
Re: rr – record and replay debugger for C/C++
#115GDBs built-in reverse debugging: https://www.sourceware.org/gdb/wiki/ProcessRecord/Tutorial I assume rr provides more features and flexibility. Anyway I want to mention that GDB itself can already reverse debug for some time now.
I have successfully used GDB's build in reverse debugging once, on a platform that rr didn't yet support at the time.
It worked, it helped me track down the bug, but it was painfully slow, I had to do things to limit the size of the input to make it possible to use at all (and thankfully was luckily able to still repro the problem after doing so).
Re: rr – record and replay debugger for C/C++
#116Earlier quoted context omitted.
If you want to mention this, then you very clearly haven't actually tried it. The implementation in GDB is more convenient than rr (you can start/stop recording at will), but it is also orders of magnitude less efficient. It's only usable for very small code snippets. Otherwise it takes effectively forever and/or runs out of resources.
> runs out of resources RAM? What kind of dev box runs out of RAM in 2024? I built a 64GB RAM dev box during COVID-19 crisis. I have never once come close to using all that RAM, even with a squillion Chrome tabs open. Still, thank you to share your first-hand experience. Did you ask the GDB Dev team for any feedback on the slow performance?
umm, embedded devices without active cooling f.e. those node-b’s sitting on cellphone towers come to mind here, there can be quite a few other similar examples i can think of.
Re: rr – record and replay debugger for C/C++
#117Can someone explain how it works?
Re: rr – record and replay debugger for C/C++
#118Earlier quoted context omitted.
> There's a bit of a higher abstraction ceiling in Rust Compared to C, yes, but not compared to C++.
This isn't really true. Rust has a much better type system. When writing generic code the impact is enormous. C++ doesn't have a real Empty Type, and it thinks Units have non-zero size. In practical terms this makes it incredibly wasteful and in terms of a clear abstraction it encourages you to come up with a hack that's unclear but efficient.
Can you instantiate an empty type? If yes, are all instances unique? Years ago, I was surprised to learn how C++ handles the (essentially) empty type (no data): A single byte to differentiate each instance.
Re: rr – record and replay debugger for C/C++
#119Earlier quoted context omitted.
C++20 added `[[no_unique_address]]`, which lets a `std::is_empty` field alias another field, so long as there is only 1 field of that `is_empty` type. https://godbolt.org/z/soczz4c76 That is, example 0 shows 8 bytes, for an `int` plus an empty field. Example 1 shows two empty fields with the `int`, but only 4 bytes thanks to `[[no_unique_address]]`. Example 2 unfortunately is back up to 8 bytes because we have two em…
It's probably mean for me to say "empty type" to C++ people because of course just as std::move doesn't move likewise std::is_empty doesn't detect empty types. It can't because C++ doesn't have any. You may need to sit down. An empty type has no values. Not one value, like the unit type which C++ makes a poor job of as you explain, but no values. None at all. Because it has no values we will never be called upon to s…
Thanks for the clarification.
Re: rr – record and replay debugger for C/C++
#120GDBs built-in reverse debugging: https://www.sourceware.org/gdb/wiki/ProcessRecord/Tutorial I assume rr provides more features and flexibility. Anyway I want to mention that GDB itself can already reverse debug for some time now.
gdb's built-in replay implementation imposes a slowdown of about 10000× on your program, so if you can binary-search to the desired program state in less than 10000 restarts of the program, that will take less machine time than using reverse execution. in fact, the slowdown is large enough that even interactively navigating the debugger close to the right state repeatedly and then restarting the program is often enough
i have been able to use gdb's replay functionality usefully because i had an input file which crashed the program within a fraction of a second after startup. this meant that i could navigate backward from "this variable is wrong" to "how did this variable get set to that wrong value?" in only several minutes of waiting on the computer