Nested unwind_protect() optimization isn't working correctly

[DavisVaughan]

I believe that the unwind_protect() optimization originally added in #207 isn't quite right. I think it was broken after that PR was merged in 9a62c3a with the move to a local static object. I can prove that nested calls to unwind_protect() aren't being skipped as they should be.

First, here is a benchmark to show that the nested unwind_protect() optimization isn't working, and the performance loss is pretty brutal. The unwind_protect() calls happen on every iteration of the loop, because assigning an r_string to a writable::strings proxy element is wrapped in unwind_protect(), even though I wrapped the whole loop in unwind_protect() too.

cpp11/inst/include/cpp11/strings.hpp

Line 58 in 2aba46e

unwind_protect([&] { SET_STRING_ELT(data_, index_, rhs); });

set.seed(123)
x <- sample(letters, 1e6, replace = TRUE)

cpp11::cpp_function("

cpp11::writable::strings test(cpp11::strings x) {
  R_xlen_t size = x.size();
  
  cpp11::writable::strings out(size);
  
  cpp11::unwind_protect([&] {
    for (R_xlen_t i = 0; i < size; ++i) {
      out[i] = x[i];
    }
  });
  
  return out;
}

")

cpp11::cpp_function("

SEXP test2(SEXP x) {
  R_xlen_t size = Rf_xlength(x);
  
  SEXP out = PROTECT(Rf_allocVector(STRSXP, size));
  
  for (R_xlen_t i = 0; i < size; ++i) {
    SET_STRING_ELT(out, i, STRING_ELT(x, i));
  }
  
  UNPROTECT(1);
  return out;
}

")

bench::mark(test(x), test2(x), iterations = 20)
#> Warning: Some expressions had a GC in every iteration; so filtering is disabled.
#> # A tibble: 2 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 test(x)    715.35ms 751.55ms      1.31    7.63MB     1.96
#> 2 test2(x)     7.65ms   9.11ms    100.      7.63MB    35.1

Next, here is a video that shows how inner calls to unwind_protect() aren't exiting early. It uses VS Code to step through the example from above. The sequence of steps are:

• We step through the outer unwind_protect() call to see how it sets up the static should_unwind_protect local object
• Then we step through the first iteration of the out[i] = x[i] copy. That puts us back in unwind_protect(), but that static local object is not yet initialized (weird right?), so we end up going back through the process of setting it up. That means unwind_protect() doesn't early exit here when it should.
• Then on the second iteration of out[i] = x[i], the static local should_unwind_protect is set up, but is set to TRUE so we again can't early exit. This continues on for the rest of the loop.

https://vimeo.com/769942052

I'm fairly certain the main problem here is that SEXP unwind_protect(Fun&& code) is a template function, meaning that each new instantiation of it gets its own copy of should_unwind_protect, and it has to be set up every time.

I've fixed this in a branch I'll send in, and I'll explain the fix there.

[jimhester]

Made a comment in 9a62c3a#r90124412