The implementation of iter.Pull and iter.Pull2 functions is based on
closures and sharing local state, which results in one heap allocation
for each captured variable.

The number of heap allocations can be reduced by grouping the state
shared between closures in a struct, allowing the compiler to allocate
all local variables in a single heap region instead of creating
individual heap objects for each variable.

This approach can sometimes have downsides when it couples unrelated
objects in a single memory region, preventing the garbage collector from
reclaiming unused memory. While technically only a subset of the local
state is shared between the next and stop functions, it seems unlikely
that retaining the rest of the state until stop is reclaimed would be
problematic in practice, since the two closures would often have very
similar lifetimes.

The change also reduces the total memory footprint due to alignment
rules, the two booleans can be packed in memory and sometimes can even
exist within the padding space of the v value. There is also less
metadata needed for the garbage collector to track each individual heap
allocation.

goos: darwin
goarch: arm64
pkg: iter
cpu: Apple M2 Pro
         │ /tmp/bench.old │           /tmp/bench.new            │
         │     sec/op     │   sec/op     vs base                │
Pull-12       218.6n ± 7%   146.1n ± 0%  -33.19% (p=0.000 n=10)
Pull2-12      239.8n ± 5%   155.0n ± 5%  -35.36% (p=0.000 n=10)
geomean       229.0n        150.5n       -34.28%

         │ /tmp/bench.old │           /tmp/bench.new           │
         │      B/op      │    B/op     vs base                │
Pull-12        288.0 ± 0%   176.0 ± 0%  -38.89% (p=0.000 n=10)
Pull2-12       312.0 ± 0%   176.0 ± 0%  -43.59% (p=0.000 n=10)
geomean        299.8        176.0       -41.29%

         │ /tmp/bench.old │           /tmp/bench.new           │
         │   allocs/op    │ allocs/op   vs base                │
Pull-12       11.000 ± 0%   5.000 ± 0%  -54.55% (p=0.000 n=10)
Pull2-12      12.000 ± 0%   5.000 ± 0%  -58.33% (p=0.000 n=10)
geomean        11.49        5.000       -56.48%

Change-Id: Iccbe233e8ae11066087ffa4781b66489d0d410a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/552375


Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>