amos@dysfun right but couldn't you do the cheap thing when _capturing_ the backtrace and keep the expensive stuff for later? I guess not?
bjorn3@fasterthanlime Symbol resolution should be the most expensive part, but that is already done lazily. Capturing backtraces requires for each frame iterating through the .eh_frame_hdr of every dynamic library to find the right unwind info and then interpret the unwinding byte code from the start of the function until the location where the current instruction pointer is to find where each register is saved. There is a paper about compiling this byte code to executable code which is much faster.
@bjorn3 Is there a fast way to do this without debuginfo? What does `perf` do when you tell it to not use dwarf?
Noratrieb@fasterthanlime @bjorn3 frame pointers, which are disabled by default because you know, back in the old days of 32 bit x86, you didn't have many registers, so you'd abuse the frame pointer to get one, which made your program a bit faster, but also way harder to profile efficiently, making it effectively slower again, and nowadays the tradeoff doesn't really make sense, but it still sticks around.
-Cforce-frame-pointers=true enables them
-Cforce-frame-pointers=true enables them
@nilstrieb @bjorn3 (hi nora!!! 
) I remember reading about major Linux restrictions switching to enabling them by default. I'm surprised that it's not the default in Rust yet!!
) I remember reading about major Linux restrictions switching to enabling them by default. I'm surprised that it's not the default in Rust yet!!@nilstrieb @bjorn3 But yeah, I would be interested in a solution based on frame pointers!
@fasterthanlime @nilstrieb Frame pointers are now on for the standard library to allow people to use them everywhere without having to recompile the standard library. We tried enabling it by default, but the perf loss for running rustc compiled with frame pointers was just a bit too high unfortunately.
niko
Ted Mielczarek@fasterthanlime @bjorn3 point of clarification: .eh_frame is not debug info, it's part of the ABI. It does use almost exactly the same format as DWARF .debug_frame, though. Other platforms have alternate solutions here, like Apple's compact unwind tables.
@bjorn3 @fasterthanlime would it be possible to capture the backtrace for almost free if one pinky promised frame pointers were in use? Then possibly one shouldn’t have to completely unwind and execute the bytecode, or am I missing something?
@nrab @fasterthanlime Yes, but you did still have to write your own code for this. backtrace-rs doesn't support it out of the box. Also make sure all system libraries use frame pointers too.
@bjorn3 @fasterthanlime I know what I’m doing this weekend then