Mark all objects reachable from roots as live before doing main cyclic GC pass

Objects can only be cyclic garbage if they are not reachable.
So, if we can cheaply identify the majority of reachable objects before performing the (relatively slow) cycle detecting pass, we can save a lot of time.

Performing a transitive closure of all objects reachable from global roots (the sys and builtins modules as well as builtin class's dicts and sublasses) plus a transitive closure of all objects reachable from the stacks can eliminate >90% of all objects relatively cheaply.

Initial experiments show a ~3% speedup, with an almost 50% speedup of the most gc-heavy benchmark.

This idea has been proposed a few times.
@nascheme has definitely suggested it before. Perhaps he can add links to any prior discussion and/or experiments?

What makes this more feasible now is that the GC can see the evaluation stack of frames, thanks to #124392, so we would now expect that the vast majority of reachable objects can be cheaply marked, thus improving the efficiency of cycle detection considerably.

Linked PRs

• gh-126502
• gh-126983
• gh-126984
• gh-127110
• gh-127519
• gh-127770

从本 issue 中的可达性提案、#124392 中对求值栈的更改，以及链接的 PR #126502、#126983、#126984、#127110、#127519 和 #127770 开始。先审阅这些讨论和实验；完成的标准是：在主循环 GC 遍历之前标记可达对象，并对声称的性能改进进行测量和验证。