[Lldb-commits] [PATCH] D149096: [lldb] Speed up looking for dSYM next to executable

[Alex Langford via Phabricator via lldb-commits]

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The goal of this patch is to speed up the code that searches for a dSYM
next to an executable.

I generated a project with 10,000 c source files and accompanying header
files using a script that I wrote. I generated objects for each of them
(with debug info in it) but intentionally did not create a dSYM for
anything. I also built a driver, and then linked everything into one
binary. The project itself is very simple, but the number of object files
involved is very large. I then measured lldb's performance debugging
this binary. (NB: I plan on putting this script into lldb's script
directory at some point after some cleanups)

Using the command `lldb -x -b -o 'b main' $binary` (launch lldb with no
lldbinit, place a breakpoint on main, and then quit), I measured how
long this takes and measured where time was being spent.

On my machine, I used hyperfine to get some statistics about how long
this actually took:

  alex at alangford many-objects % hyperfine -w 3 -- "$lldb -x -b -o 'b main' $binary"
  Benchmark 1: $lldb -x -b -o 'b main' $binary
    Time (mean ± σ):      4.395 s ±  0.046 s    [User: 3.239 s, System: 1.245 s]
    Range (min … max):    4.343 s …  4.471 s    10 runs

Out of the ~4.4 seconds of work, it looks like ~630ms were being spent
on `LocateDSYMInVincinityOfExecutable`. After digging in further, we
were spending the majority of our time manipulating paths in `FileSpec`
with `RemoveLastPathComponent` and `AppendPathComponent`. There were a
lot of FileSystem operations as well, so I made an attempt to minimize
the amount of calls to `Exists` and `Resolve` as well.

Given how widely used the code in FileSpec is, I opted to improve this
codepath specifically rather than attempt to refactor the internals of
FileSpec. I believe that improving FileSpec is also worth doing, but as
it is a larger change with far reaching implications, I opted to make
this change instead.

With this change, we shave off approximately 600ms:

  alex at alangford many-objects % hyperfine -w 3 -- "$lldb -x -b -o 'b main' $binary"
  Benchmark 1: $lldb -x -b -o 'b main' $binary
    Time (mean ± σ):      3.822 s ±  0.056 s    [User: 2.749 s, System: 1.167 s]
    Range (min … max):    3.750 s …  3.920 s    10 runs


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149096

Files:
  lldb/source/Symbol/LocateSymbolFile.cpp

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