This CL makes a bunch of changes (mostly to raw_hash_set which
underlies flat_hash_set and flat_hash_map). Techniques used:
* Extract code that does not depend on the specific hash table type
into common (non-inlined) functions.
* Place ABSL_ATTRIBUTE_NOINLINE directives judiciously.
* Out-of-line some slow paths.
Reduces sizes of some large binaries by ~0.5%.
Has no significant performance impact on a few performance critical
binaries.
## Speed of fleetbench micro-benchmarks
Following is a histogram of %-age changes in
[fleetbench](https://github.com/google/fleetbench)
hot_swissmap_benchmark results. Negative numbers indicate a speedup
caused by this change. Statistically insignificant changes are mapped
to zero.
XXX Also run and merge in cold_swissmap_benchmark
Across all 351 benchmarks, the average speedup is 0.38%.
The best speedup was -25%, worst slowdown was +6.81%.
```
Count: 351 Average: -0.382764 StdDev: 3.77807
Min: -25 Median: 0.435135 Max: 6.81
---------------------------------------------
[ -25, -10) 16 4.558% 4.558% #
[ -9, -8) 2 0.570% 5.128%
[ -8, -7) 1 0.285% 5.413%
[ -7, -6) 1 0.285% 5.698%
[ -6, -5) 2 0.570% 6.268%
[ -5, -4) 5 1.425% 7.692%
[ -4, -3) 13 3.704% 11.396% #
[ -3, -2) 15 4.274% 15.670% #
[ -2, -1) 26 7.407% 23.077% ##
[ -1, 0) 14 3.989% 27.066% #
[ 0, 1) 185 52.707% 79.772% ############
[ 1, 2) 14 3.989% 83.761% #
[ 2, 3) 8 2.279% 86.040% #
[ 3, 4) 7 1.994% 88.034%
[ 4, 5) 32 9.117% 97.151% ##
[ 5, 6) 6 1.709% 98.860%
[ 6, 7) 4 1.140% 100.000%
```
We looked at the slowdowns and they do not seem worth worrying
about. E.g., the worst one was:
```
BM_FindHit_Hot<::absl::node_hash_set,64>/set_size:4096/density:0
2.61ns ± 1% 2.79ns ± 1% +6.81% (p=0.008 n=5+5)
```
## Detailed changes
* Out-of-line slow paths in hash table sampler methods.
* Explicitly unregister from sampler instead of from destructor.
* Introduced a non-templated CommonFields struct that holds some of
the hash table fields (infoz, ctrl, slots, size, capacity). This
struct can be passed to new non-templated helpers. The struct is
a private base class of raw_hash_set.
* Made non-inlined InitializeSlots<> that is only templated on
allocator and size/alignment of the slot type so that we can share
instantiations across types that have the same size/alignment.
* Moved some infrequently called code paths into non-inlined type-erased.
functions. Pass a suite of type-specific function pointers to these
routines for when they need to operate on slots.
* Marked some methods as non-inlined.
* Avoid unnecessary reinitialization in destructor.
* Introduce UpdateSpine type-erased helper that is called from
clear() and rehash().
PiperOrigin-RevId: 491413386
Change-Id: Ia5495c5a6ec73622a785a0d260e406ddb9085a7c
