*: use Equal for more efficient equality comparisons

[jbowens]

Use base.Equal for equality comparisons in more places. Equal(a,b) is faster
than Compare(a,b)==0. Additionally, embed the Comparer directly into the
Iterator struct for better cache locality and to avoid the additional pointer
indirection.

Informs #1819.
Informs cockroachdb/cockroach#82559.

name                                                                     old time/op    new time/op    delta
MVCCScan_Pebble/rows=1/versions=1/valueSize=64/numRangeKeys=0-24           6.80µs ± 1%    6.72µs ± 2%    ~     (p=0.165 n=5+10)
MVCCScan_Pebble/rows=1/versions=1/valueSize=64/numRangeKeys=1-24           14.1µs ± 1%    14.0µs ± 2%    ~     (p=0.107 n=5+10)
MVCCScan_Pebble/rows=1/versions=1/valueSize=64/numRangeKeys=100-24          154µs ± 2%     152µs ± 2%    ~     (p=0.075 n=5+10)
MVCCScan_Pebble/rows=1/versions=2/valueSize=64/numRangeKeys=0-24           7.16µs ± 2%    7.10µs ± 1%    ~     (p=0.147 n=5+9)
MVCCScan_Pebble/rows=1/versions=2/valueSize=64/numRangeKeys=1-24           15.0µs ± 1%    14.9µs ± 1%  -0.94%  (p=0.007 n=5+9)
MVCCScan_Pebble/rows=1/versions=2/valueSize=64/numRangeKeys=100-24          155µs ± 1%     155µs ± 1%    ~     (p=0.513 n=5+10)
MVCCScan_Pebble/rows=1/versions=10/valueSize=64/numRangeKeys=0-24          9.15µs ± 2%    8.91µs ± 0%  -2.61%  (p=0.003 n=5+7)
MVCCScan_Pebble/rows=1/versions=10/valueSize=64/numRangeKeys=1-24          18.2µs ± 2%    18.1µs ± 1%    ~     (p=0.518 n=5+9)
MVCCScan_Pebble/rows=1/versions=10/valueSize=64/numRangeKeys=100-24         147µs ± 4%     145µs ± 2%    ~     (p=0.129 n=5+10)
MVCCScan_Pebble/rows=1/versions=100/valueSize=64/numRangeKeys=0-24         18.1µs ± 2%    17.7µs ± 1%  -1.98%  (p=0.013 n=5+10)
MVCCScan_Pebble/rows=1/versions=100/valueSize=64/numRangeKeys=1-24         27.5µs ± 1%    27.2µs ± 2%    ~     (p=0.206 n=5+10)
MVCCScan_Pebble/rows=1/versions=100/valueSize=64/numRangeKeys=100-24        112µs ± 4%     109µs ± 1%    ~     (p=0.075 n=5+10)
MVCCScan_Pebble/rows=10/versions=1/valueSize=64/numRangeKeys=0-24          12.1µs ± 2%    11.8µs ± 2%  -3.00%  (p=0.003 n=5+10)
MVCCScan_Pebble/rows=10/versions=1/valueSize=64/numRangeKeys=1-24          21.5µs ± 1%    20.8µs ± 1%  -3.02%  (p=0.001 n=5+9)
MVCCScan_Pebble/rows=10/versions=1/valueSize=64/numRangeKeys=100-24         105µs ± 3%     102µs ± 1%  -2.73%  (p=0.013 n=5+10)
MVCCScan_Pebble/rows=10/versions=2/valueSize=64/numRangeKeys=0-24          13.8µs ± 2%    13.4µs ± 4%  -2.89%  (p=0.013 n=5+10)
MVCCScan_Pebble/rows=10/versions=2/valueSize=64/numRangeKeys=1-24          24.3µs ± 2%    23.8µs ± 2%    ~     (p=0.075 n=5+10)
MVCCScan_Pebble/rows=10/versions=2/valueSize=64/numRangeKeys=100-24         109µs ± 2%     105µs ± 2%  -3.43%  (p=0.005 n=5+10)
MVCCScan_Pebble/rows=10/versions=10/valueSize=64/numRangeKeys=0-24         25.0µs ± 1%    24.2µs ± 3%  -3.27%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=10/versions=10/valueSize=64/numRangeKeys=1-24         42.6µs ± 2%    42.0µs ± 2%    ~     (p=0.055 n=5+10)
MVCCScan_Pebble/rows=10/versions=10/valueSize=64/numRangeKeys=100-24        129µs ± 3%     127µs ± 2%  -1.61%  (p=0.040 n=5+10)
MVCCScan_Pebble/rows=10/versions=100/valueSize=64/numRangeKeys=0-24        63.2µs ± 1%    61.7µs ± 1%  -2.30%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=10/versions=100/valueSize=64/numRangeKeys=1-24        83.4µs ± 1%    81.6µs ± 1%  -2.16%  (p=0.003 n=5+10)
MVCCScan_Pebble/rows=10/versions=100/valueSize=64/numRangeKeys=100-24       178µs ± 2%     173µs ± 1%  -2.46%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=100/versions=1/valueSize=64/numRangeKeys=0-24         47.4µs ± 2%    46.0µs ± 1%  -2.89%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=100/versions=1/valueSize=64/numRangeKeys=1-24         74.2µs ± 1%    71.2µs ± 2%  -4.06%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=100/versions=1/valueSize=64/numRangeKeys=100-24        173µs ± 1%     170µs ± 3%    ~     (p=0.055 n=5+10)
MVCCScan_Pebble/rows=100/versions=2/valueSize=64/numRangeKeys=0-24         60.8µs ± 2%    59.1µs ± 2%  -2.82%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=100/versions=2/valueSize=64/numRangeKeys=1-24         95.8µs ± 1%    94.2µs ± 2%  -1.75%  (p=0.019 n=5+10)
MVCCScan_Pebble/rows=100/versions=2/valueSize=64/numRangeKeys=100-24        197µs ± 2%     193µs ± 2%  -1.85%  (p=0.019 n=5+10)
MVCCScan_Pebble/rows=100/versions=10/valueSize=64/numRangeKeys=0-24         148µs ± 1%     145µs ± 1%  -1.66%  (p=0.003 n=5+10)
MVCCScan_Pebble/rows=100/versions=10/valueSize=64/numRangeKeys=1-24         247µs ± 1%     241µs ± 2%  -2.32%  (p=0.003 n=5+10)
MVCCScan_Pebble/rows=100/versions=10/valueSize=64/numRangeKeys=100-24       390µs ± 2%     382µs ± 2%  -1.95%  (p=0.028 n=5+10)
MVCCScan_Pebble/rows=100/versions=100/valueSize=64/numRangeKeys=0-24        441µs ± 2%     437µs ± 1%    ~     (p=0.222 n=5+8)
MVCCScan_Pebble/rows=100/versions=100/valueSize=64/numRangeKeys=1-24        558µs ± 1%     547µs ± 1%  -2.01%  (p=0.004 n=4+10)
MVCCScan_Pebble/rows=100/versions=100/valueSize=64/numRangeKeys=100-24      790µs ± 2%     774µs ± 2%  -1.93%  (p=0.008 n=5+10)
MVCCScan_Pebble/rows=1000/versions=1/valueSize=64/numRangeKeys=0-24         350µs ± 1%     346µs ± 1%    ~     (p=0.075 n=5+10)
MVCCScan_Pebble/rows=1000/versions=1/valueSize=64/numRangeKeys=1-24         539µs ± 1%     534µs ± 2%    ~     (p=0.206 n=5+10)
MVCCScan_Pebble/rows=1000/versions=1/valueSize=64/numRangeKeys=100-24       759µs ± 1%     747µs ± 1%  -1.61%  (p=0.008 n=5+10)
MVCCScan_Pebble/rows=1000/versions=2/valueSize=64/numRangeKeys=0-24         472µs ± 0%     466µs ± 0%  -1.12%  (p=0.003 n=4+9)
MVCCScan_Pebble/rows=1000/versions=2/valueSize=64/numRangeKeys=1-24         755µs ± 1%     740µs ± 1%  -2.03%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=1000/versions=2/valueSize=64/numRangeKeys=100-24      1.02ms ± 3%    0.96ms ± 2%  -5.05%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=1000/versions=10/valueSize=64/numRangeKeys=0-24       1.29ms ± 1%    1.28ms ± 1%  -1.42%  (p=0.005 n=5+10)
MVCCScan_Pebble/rows=1000/versions=10/valueSize=64/numRangeKeys=1-24       2.19ms ± 1%    2.13ms ± 1%  -2.64%  (p=0.001 n=5+10)
MVCCScan_Pebble/rows=1000/versions=10/valueSize=64/numRangeKeys=100-24     2.82ms ± 2%    2.77ms ± 2%    ~     (p=0.129 n=5+10)
MVCCScan_Pebble/rows=1000/versions=100/valueSize=64/numRangeKeys=0-24      4.11ms ± 1%    4.08ms ± 2%    ~     (p=0.859 n=5+10)
MVCCScan_Pebble/rows=1000/versions=100/valueSize=64/numRangeKeys=1-24      5.17ms ± 2%    5.08ms ± 2%    ~     (p=0.055 n=5+10)
MVCCScan_Pebble/rows=1000/versions=100/valueSize=64/numRangeKeys=100-24    6.69ms ± 2%    6.61ms ± 4%    ~     (p=0.206 n=5+10)

[cockroach-teamcity]

This change is 

[erikgrinaker]

LGTM at high level, thanks!

[sumeerbhola]

Reviewed 14 of 14 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jbowens)

---

internal/keyspan/defragment.go line 24 at r1 (raw file):

	// ShouldDefragment takes two abutting spans and returns whether the two
	// spans should be combined into a single, defragmented Span.
	ShouldDefragment(comparer *base.Comparer, left, right *Span) bool

is the change to pointer indirection in this file not a performance regression?

[jbowens]

tftrs!

Reviewable status: 10 of 14 files reviewed, 1 unresolved discussion (waiting on @sumeerbhola)

---

internal/keyspan/defragment.go line 24 at r1 (raw file):

Previously, sumeerbhola wrote…

is the change to pointer indirection in this file not a performance regression?

it doesn't seem to be a measurable one, but I removed the indirection just in case.