This is a reland of e16eca95f5
Original change's description:
> Create a basic IRNode pooling system.
>
> Allocations are redirected by overriding `operator new` and `operator
> delete` on the IRNode class. This allows us to use our existing
> `unique_ptr` and `make_unique` calls as-is. The Pool class is simple;
> it holds a fixed number of nodes and recycles them as they are returned.
>
> A fixed pool size of 2000 nodes was chosen. That is large enough to hold
> the contents of `sksl_large` during compilation, but it can be
> overflowed by very large shaders, or if multiple programs are converted
> at the same time. Exhausting the pool is not a problem; if this happens,
> additional nodes will be allocated via the system allocator as usual.
> More elaborate schemes are possible but might not add a lot of value.
>
> Thread safety is accomplished by placing the pool in a `thread_local`
> static during a Program's creation and destruction; the pool is freed
> when the program is destroyed. One important consequence of this
> strategy is that a program must free every node that it allocated during
> its creation, or else the node will be leaked. In debug, leaking a node
> will be detected and causes a DEBUGFAIL. In release, the pool will be
> freed despite having a live node in it, and if that node is later freed,
> that pointer will be passed to the system `free` (which is likely to
> cause a crash).
>
> In this CL, iOS does not support pooling, since support for
> `thread_local` was only added on iOS 9. This is fixed in the followup
> CL, http://review.skia.org/328837, which uses pthread keys on iOS.
>
> Nanobench shows ~15% improvement:
> (last week) http://screen/5CNBhTaZApcDA8h
> (today) http://screen/8ti5Rymvf6LUs8i
>
> Change-Id: I559de73606ee1be54e5eae7f82129dc928a63e3c
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326876
> Commit-Queue: John Stiles <johnstiles@google.com>
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>
Change-Id: I114971e8e7ac0fabaf26216ae8813eeeaad0d4a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329086
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5b09e6a007