Major performance regression in certain files starting with 1.9.4 -- 100% CPU for about 1 minute

[sean-mcmanus]

Certain .cpp files make VS Code unusuably slow due to high 100% CPU usage on one core (extension host process) starting with 1.9.4 -- the issue doesn't repro with 1.9.3. It repros without our C/C++ extension. Could it be caused by https://github.com/jeff-hykin/cpp-textmate-grammar/pull/199/files ?

The repro we have has 2349 lines in it. Let me know if you guys need help getting a better repro.

I'm using the latest VS Code Insiders.

[sean-mcmanus]

I ran the profiler -- it's showing vscode-textmate matchRule taking all the time.

[sean-mcmanus]

I got the repro down to 91 lines (the CPU usage lasts only around 10 sec though). Invoking Developer: Inspect TM Scope causes major blockage "forever" until I reload.

I don't see anything unusual in the code.

[matter123]

The reproduction case would certainly be helpful. The only significant difference between v1.9.3 and v1.9.4 is support for default arguments in function definitions a048985, and all those patterns were preexisting.

[sean-mcmanus]

Here's a repro -- I've replaced all the identifiers with "a". Freeze is 10-15 seconds -- more with Developer: Inspect TM Scope enabled. 1.9.3 has 0 seconds freezing. Repros with VS Code non-Insiders too.

static void a(
                           a    a,
                           a a,
                           a a,
                           a a,
                           a a,
                           bool                       a,
                           bool                       a,
                           bool                       a,
                           bool                       a)

{
  a a;


  a(a, a,
                                 a,
                                 a,
                                 a,
                                 a,
                                 a,
                                 a,
                                 a);
  a->a(&a);
  for (auto a : a) {
    auto &a = a.a;
    auto a = a.a;

    if (a && a) {
      switch (a) {
        case a:
          a->a(a->a());
          break;
        case a:
          a->a(a->a());
          break;
        case a:
          a->a(a->a());
          break;
        case a:
          a->a(a->a());
          break;
      }  
    }
    a(a, a,
                                   a,
                                   a,
                                   a,
                                   a,
                                   a,
                                   a,
                                   a);
  } 
}  

static void a(
                      a                    a,
                      a           a,
                      a a,
                      a                                 a)

{
  a a;
  a a =0;
                               a->a()->a();

  a(a->a());
  a->a(a);
  a->a(a);
  a(a, a, a,
                                 a,
                                 a);
  a->a(&a);
  for (auto a: a) {
    a(a->a(), a, a,
                                                                   a);
  }
}

[matter123]

@jeff-hykin The issue seems to be that function_definition is allowed inside of the function parameters. vscode-textmate spends about half of its time (4700ms) to determine that :function_definition is not a match. Do you know why its there?

Edit: :function_definition isn't slow because backtracking. wrapping the entire start_pattern in an atomic group causes a 1ms (4%) performance gain.

[matter123]

It looks like the performance issue is with generateScopeResolutionFinder. when the pattern is forced to be atomic the total time becomes .91s. This causes other issues with matching so it's not a solution, but hopefully, the scope resolution generator can be made non-backtracking.

[jeff-hykin]

vscode-textmate spends about half of its time (4700ms) to determine that :function_definition is not a match. Do you know why its there?

No I don't, and I can't think of a reason why it would intentionally be there. If removing it solves the issue, then we should definitely remove it.

It's still unclear to me why :function_definition would cause this, especially with backtracking disabled. It could be related to the look-arounds. I'm going to be without a computer tomorrow so I'm going to try to look into the issue now.

[jeff-hykin]

Thanks @sean-mcmanus for posting the example code

[jeff-hykin]

@matter123

This is strange
This code is not a problem:

static void a(
    bool a){
  a(a, a, a, a, a);
}

However this is near unusably slow:

static void a(
    bool                                                                     a){
  a(a, a, a, a, a);
}

Something is going on with the matching of whitespace.