benchmark: add benchmark for coverage

[bcoe]

Working on #36956, I became a bit worried that as we extend code coverage to support more types of branching, performance might start to become unacceptable.

I created this benchmark to experiment:

Node 14.15.4, NODE_V8_COVERAGE, vs., no coverage:

process/coverage.js n=100000 *** -18.94 % ±0.77% ±1.02% ±1.30%

Node 15.6.0, NODE_V8_COVERAGE, vs., no coverage:

process/coverage.js n=100000 *** -17.95 % ±1.08% ±1.43% ±1.83%

Coverage with #36956, vs., no coverage:

process/coverage.js n=100000 *** -19.37 % ±1.36% ±1.79% ±2.30%

To Summarize

In the current release of 14/15, we see that running Node with coverage decreases performance by about 18%.

With the introduction of optional chaining in this benchmark, we see a 1 - 2% additional hit to performance.

Other interesting facts

• I benchmarked coverage.js using Istanbul, vs., NODE_V8_COVERAGE, it was 37.46% slower than no coverage.
• I benchmarked deps: V8: cherry-pick fe191e8d05cc #36956 with no coverage, vs., 15.6.0 with no coverage; there did not appear to be a significant difference in performance (0.37 % ±1.14% ±1.50% ±1.92%).

Conclusions

• My gut is that it's worth taking the 1 - 2% performance hit to support a new type of branching in our coverage reporting.
• As new constructs like this are added to JavaScript, we need to be mindful that the cost of branch coverage will continue to increase.
• I was excited to see that v8's coverage offers a significant performance benefit over pre-instrumenting the JavaScript code.

CC: @schuay

[bcoe]

@joyeecheung I was thinking this benchmark would be a really good use of v8.startCoverage, because it would let us turn on coverage collection programmatically, at which point it would be easier to use this benchmark to detect regressions in coverage performance in the future.

[Trott]

Subsystem in commit message should be benchmark rather than test.

[bcoe]

@Trott refactored so that the benchmark is easier to use, it now spawns a subprocess with coverage enabled (_so no need to create a separate bin) 👍

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/35626/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/35683/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/35719/ ✅

[bcoe]

Landed in 8c9dc4e