Move static local variable out of templated function

[DavisVaughan]

Closes#298

@jimhester would you mind reviewing this for me? You are the last one to have touched this code, and probably understand it better than anyone else. The #298 issue provides the backstory.

A rerun of the benchmark in #298 gives me:

# Mainbench::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# This PRbench::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) 69.13ms 79.75ms 11.7 7.63MB 17.5#> 2 test2(x) 7.47ms 9.34ms 98.1 7.63MB 34.3

Which better matches the performance we had in 0.4.0

# cpp11 0.4.0bench::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) 63.56ms 76.4ms 12.1 7.63MB 18.1#> 2 test2(x) 7.19ms 8.3ms 103. 7.63MB 36.1

I also confirmed that I can't reproduce the original issue in #244 that forced us to move away from a global static variable.

FWIW the pattern I've used here is fairly similar to what is done in <date> to determine the tzdb file path
https://github.com/HowardHinnant/date/blob/22ceabf205d8d678710a43154da5a06b701c5830/src/tz.cpp#L394-L429

[DavisVaughan]

This was previously overwriting the global option to TRUE every time a new compilation unit needed to initialize its static local variable. I don't think we want to do that. I think we only want to initialize to TRUE the very first time this is created, i.e. in the if branch above.

[DavisVaughan]

The function holding the static local variable is no longer templated, so each compilation unit will only have to run setup_should_unwind_protect() 1 time

[romainfrancois]

LGTM. Not strictly related to this PR's business, but should we be concerned about the option being set to something that is not a logical ?

[romainfrancois]

I can confirm the performance gain:

main:

> bench::mark(test(x), test2(x), iterations = 20)
# A tibble: 2 × 13
expression min median `itr/sec` mem_alloc `gc/sec` n_itr n_gc total_time result memory time gc <bch:expr> <bch:tm> <bch:tm> <dbl> <bch:byt> <dbl> <int> <dbl> <bch:tm> <list> <list> <list> <list> 1 test(x) 232.65ms 236.53ms 4.09 7.63MB 7.77 20 38 4.89s <chr [1,000,000]> <Rprofmem [149 × 3]> <bench_tm [20]> <tibble [20 × 3]>
2 test2(x) 4.39ms 4.53ms 191. 7.63MB 38.2 20 4 104.82ms <chr [1,000,000]> <Rprofmem [1 × 3]> <bench_tm [20]> <tibble [20 × 3]>

#299 :

> bench::mark(test(x), test2(x), iterations = 20)
# A tibble: 2 × 13
expression min median `itr/sec` mem_alloc `gc/sec` n_itr n_gc total_time result memory time gc <bch:expr> <bch:tm> <bch:tm> <dbl> <bch:byt> <dbl> <int> <dbl> <bch:tm> <list> <list> <list> <list> 1 test(x) 32.17ms 34.15ms 25.9 7.63MB 38.9 20 30 772ms <chr [1,000,000]> <Rprofmem [225 × 3]> <bench_tm [20]> <tibble [20 × 3]>
2 test2(x) 4.43ms 4.54ms 144. 7.63MB 36.0 20 5 139ms <chr [1,000,000]> <Rprofmem [1 × 3]> <bench_tm [20]> <tibble [20 × 3]>

[DavisVaughan]

@romainfrancois we should also try looking at this comment from Jim, possibly as a (nicer?) alternative to this PR 9a62c3a#r90124412