Allocate pinned buffer for vectorized code

[bitfaster]

trafficstars

Current AVX2 vectorized code doesn't have much of an advantage on .NET8 and .NET9. We can gain some speed by using the pinned object heap introduced in .NET5 and eliminating the fixed statement. With a fixed address, we can also do a trick to align to 32 bytes, which is best for AVX instructions.

Use of fixed results in a fixed local variable in IL, the runtime overhead comes from the JITted code. Explanation here.

This has a greater impact on the increment code path, and when the table is smaller (because L1 cache misses are fewer so fixed is relatively more overhead).

Inc baseline

Tabular .NET8

Method	Size	Mean	Error	StdDev	Ratio	Allocated
IncFlat	32768	12.04 ns	0.127 ns	0.113 ns	1.00	-
IncBlockAvx	32768	12.15 ns	0.110 ns	0.092 ns	1.01	-
IncFlat	524288	22.14 ns	0.414 ns	0.323 ns	1.00	-
IncBlockAvx	524288	17.32 ns	0.250 ns	0.222 ns	0.78	-
IncFlat	8388608	67.38 ns	1.336 ns	1.184 ns	1.00	-
IncBlockAvx	8388608	62.36 ns	0.408 ns	0.362 ns	0.93	-
IncFlat	134217728	88.89 ns	1.592 ns	1.834 ns	1.00	-
IncBlockAvx	134217728	75.63 ns	0.415 ns	0.388 ns	0.85	-

Inc pinned + 32 byte align

(chart title is wrong, but this is the inc test)

Tabular .NET8

Method	Size	Mean	Error	StdDev	Ratio	Allocated
IncFlat	32768	12.07 ns	0.109 ns	0.097 ns	1.00	-
IncBlockAvx	32768	11.49 ns	0.043 ns	0.041 ns	0.95	-
IncFlat	524288	21.93 ns	0.391 ns	0.573 ns	1.00	-
IncBlockAvx	524288	20.58 ns	0.331 ns	0.310 ns	0.93	-
IncFlat	8388608	66.81 ns	0.571 ns	0.477 ns	1.00	-
IncBlockAvx	8388608	48.79 ns	0.421 ns	0.394 ns	0.73	-
IncFlat	134217728	88.23 ns	0.950 ns	0.793 ns	1.00	-
IncBlockAvx	134217728	60.81 ns	0.559 ns	0.523 ns	0.69	-

Freq Baseline

Tabular .NET8

Method	Size	Mean	Error	StdDev	Ratio	Allocated
FrequencyFlat	32768	19.50 ns	0.075 ns	0.058 ns	1.00	-
FrequencyBlockAvx	32768	15.13 ns	0.150 ns	0.133 ns	0.78	-
FrequencyFlat	524288	32.67 ns	0.650 ns	0.773 ns	1.00	-
FrequencyBlockAvx	524288	23.40 ns	0.398 ns	0.458 ns	0.72	-
FrequencyFlat	8388608	118.10 ns	0.665 ns	0.590 ns	1.00	-
FrequencyBlockAvx	8388608	64.05 ns	0.471 ns	0.393 ns	0.54	-
FrequencyFlat	134217728	148.67 ns	1.288 ns	1.075 ns	1.00	-
FrequencyBlockAvx	134217728	77.84 ns	1.510 ns	1.484 ns	0.53	-

Freq pinned + 32 byte align

Tabular .NET8

Method	Size	Mean	Error	StdDev	Ratio	Allocated
FrequencyFlat	32768	20.50 ns	0.151 ns	0.134 ns	1.00	-
FrequencyBlockAvx	32768	13.72 ns	0.093 ns	0.087 ns	0.67	-
FrequencyFlat	524288	31.98 ns	0.609 ns	1.460 ns	1.00	-
FrequencyBlockAvx	524288	22.36 ns	0.439 ns	0.431 ns	0.66	-
FrequencyFlat	8388608	119.80 ns	2.053 ns	1.921 ns	1.00	-
FrequencyBlockAvx	8388608	61.45 ns	0.596 ns	0.558 ns	0.51	-
FrequencyFlat	134217728	148.59 ns	1.448 ns	1.284 ns	1.00	-
FrequencyBlockAvx	134217728	72.96 ns	0.530 ns	0.496 ns	0.49	-