Remove buffer copy when writing base64 JSON value

[habbes]

Fix #97628

Benchmark results

I used the dotnet-performance benchmarks with *WriteJson<BinaryData>* as the filter.

Results before

BenchmarkDotNet v0.13.11-nightly.20231126.107, Windows 11 (10.0.22621.3007/22H2/2022Update/SunValley2)
Intel Xeon W-2123 CPU 3.60GHz, 1 CPU, 8 logical and 4 physical cores
.NET SDK 9.0.100-preview.2.24078.1
  [Host]     : .NET 9.0.0 (9.0.24.7604), X64 RyuJIT AVX-512F+CD+BW+DQ+VL
  Job-SULXBD : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX-512F+CD+BW+DQ+VL

PowerPlanMode=00000000-0000-0000-0000-000000000000  Toolchain=CoreRun  IterationTime=250.0000 ms
MaxIterationCount=20  MinIterationCount=15  WarmupCount=1

Method	Mode	Mean	Error	StdDev	Median	Min	Max	Gen0	Allocated
SerializeToString	Reflection	555.3 ns	15.82 ns	18.22 ns	557.8 ns	520.9 ns	589.0 ns	0.6458	2792 B
SerializeToUtf8Bytes	Reflection	407.3 ns	12.77 ns	14.19 ns	405.9 ns	385.1 ns	431.4 ns	0.3258	1408 B
SerializeToWriter	Reflection	220.6 ns	8.85 ns	9.84 ns	218.4 ns	206.0 ns	240.0 ns	-	-
SerializeObjectProperty	Reflection	768.6 ns	90.76 ns	97.11 ns	733.1 ns	667.4 ns	966.3 ns	0.7232	3120 B
SerializeToString	SourceGen	508.6 ns	20.22 ns	22.48 ns	504.3 ns	474.5 ns	553.1 ns	0.6465	2792 B
SerializeToUtf8Bytes	SourceGen	350.0 ns	9.83 ns	10.92 ns	348.9 ns	335.8 ns	374.5 ns	0.3263	1408 B
SerializeToWriter	SourceGen	201.0 ns	9.64 ns	10.72 ns	197.1 ns	186.6 ns	218.5 ns	-	-
SerializeObjectProperty	SourceGen	711.5 ns	46.65 ns	51.86 ns	706.9 ns	639.6 ns	823.8 ns	0.7232	3120 B
SerializeToStream	Reflection	397.4 ns	15.65 ns	17.40 ns	394.9 ns	378.7 ns	438.7 ns	0.0349	152 B
SerializeToStream	SourceGen	329.3 ns	15.47 ns	17.19 ns	325.2 ns	307.9 ns	366.7 ns	0.0065	32 B

Results After

BenchmarkDotNet v0.13.11-nightly.20231126.107, Windows 11 (10.0.22621.3007/22H2/2022Update/SunValley2)
Intel Xeon W-2123 CPU 3.60GHz, 1 CPU, 8 logical and 4 physical cores
.NET SDK 9.0.100-preview.2.24078.1
  [Host]     : .NET 9.0.0 (9.0.24.7604), X64 RyuJIT AVX-512F+CD+BW+DQ+VL
  Job-ZXNFMG : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX-512F+CD+BW+DQ+VL

PowerPlanMode=00000000-0000-0000-0000-000000000000  Toolchain=CoreRun  IterationTime=250.0000 ms
MaxIterationCount=20  MinIterationCount=15  WarmupCount=1

Method	Mode	Mean	Error	StdDev	Median	Min	Max	Gen0	Allocated
SerializeToString	Reflection	485.4 ns	3.89 ns	3.04 ns	485.3 ns	480.0 ns	489.8 ns	0.6469	2792 B
SerializeToUtf8Bytes	Reflection	346.7 ns	4.64 ns	4.11 ns	346.3 ns	341.8 ns	355.1 ns	0.3253	1408 B
SerializeToWriter	Reflection	173.0 ns	5.06 ns	5.41 ns	173.5 ns	163.4 ns	182.3 ns	-	-
SerializeObjectProperty	Reflection	643.4 ns	13.66 ns	15.73 ns	639.5 ns	626.3 ns	674.2 ns	0.7212	3120 B
SerializeToString	SourceGen	456.6 ns	8.78 ns	8.62 ns	454.0 ns	447.3 ns	473.0 ns	0.6468	2792 B
SerializeToUtf8Bytes	SourceGen	297.6 ns	5.76 ns	5.11 ns	295.4 ns	293.2 ns	310.0 ns	0.3259	1408 B
SerializeToWriter	SourceGen	134.1 ns	0.69 ns	0.54 ns	134.1 ns	133.2 ns	134.8 ns	-	-
SerializeObjectProperty	SourceGen	634.5 ns	25.21 ns	29.03 ns	644.0 ns	591.5 ns	684.3 ns	0.7225	3120 B
SerializeToStream	Reflection	344.3 ns	9.61 ns	10.68 ns	338.7 ns	335.3 ns	369.1 ns	0.0343	152 B
SerializeToStream	SourceGen	277.8 ns	1.28 ns	1.07 ns	277.6 ns	276.4 ns	279.5 ns	0.0067	32 B

Profiler results

While the benchmark so some speedup, they don't show any improvements in allocations. Either because the buffers are too small that they're allocated on the stack, or, since the buffer are rented, subsequent writes will likely reuse the same buffer and it probably won't show up in the out-of-process benchmark results. To demonstrate the allocations improvements, I created a sample repo that performs concurrent: https://github.com/habbes/experiments/tree/master/WriteBase64JsonMemoryRepro

CPU usage before

Span<byte>.CopyTo() and Buffer._Memmove() contribute to a relative high % of CPU use

CPU usage after

No traces of the buffer copies

Allocations before

Several large byte[] allocations from SharedArrayPool<byte>.Rent under Base64EncodeAndWrite.

Allocations after

No traces of Base64EncodeAndWrite in allocations

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
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Issue Details

---

Fix #97628

Author:	habbes
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[stephentoub]

LGTM. Thanks.

[eiriktsarpalis]

Thanks!