FSharp.Core `#if NET` fast-path opportunities — SIMD / span / TensorPrimitives / formatting (unblocked by #20229)

[T-Gro]

#20229 ships FSharp.Core with a net10.0 TFM. A #if NET8_0_OR_GREATER / #if NET block therefore compiles only into the net asset — netstandard2.0/netstandard2.1 stay bit-identical. This tracks the additive, guarded fast paths that unlocks.

Important gate distinction some span APIs are already in netstandard2.1, so FSharp.Core's ns2.1 asset could use them too — those are perf wins but not strictly gated on #20229.

Format per row: F# affected → FSharp.Core file:line → .NET API → guard → win.

1 · Numeric reductions → SIMD TensorPrimitives (net-only)

F# affected	FSharp.Core file	.NET API	guard	win
Array.sum / Array.average	array.fs:1628/1722	TensorPrimitives.Sum (net8)	#if NET8_0_OR_GREATER	2–5× · gated: float reassociation; int drops Checked.(+) overflow
Array.max / Array.min	array.fs:1685/1648	TensorPrimitives.Max/Min (net8)	#if NET8_0_OR_GREATER	3–4× · gated: IEEE vs F# NaN — safe for ints
Seq.sum/average/min/max (array fast-path)	seq.fs:1465/1485/1517/1557	TensorPrimitives behind :? ('T[]) (net8)	#if NET8_0_OR_GREATER	2–4× · inherits above; array-backed only

2 · Primitive-array equality & comparison (ns2.1+)

F# affected	FSharp.Core file	.NET API	guard	win
GenericEquality{Byte,Int32,Int64,Char}Array	prim-types.fs:1481/1496/1558/1543	MemoryExtensions.SequenceEqual<'T>	`#if NETSTANDARD2_1_OR_GREATER
GenericComparisonByteArray	prim-types.fs:1230	MemoryExtensions.SequenceCompareTo<byte>	`#if NETSTANDARD2_1_OR_GREATER

3 · Array search & reorder (ns2.1+)

F# affected	FSharp.Core file	.NET API	guard	win
Array.contains (single primitive)	array.fs:~488	MemoryExtensions.Contains / ReadOnlySpan.IndexOf	`#if NETSTANDARD2_1_OR_GREATER
Array.rev	array.fs:1386	System.Array.Reverse	any TFM	small

4 · Primitive-array hashing (net-only)

F# affected	FSharp.Core file	.NET API	guard	win
GenericHash{Byte,Int32,Int64}Array	prim-types.fs:964/975/986	HashCode.AddBytes (net6)	#if NET6_0_OR_GREATER	15–30% · keep in-process hash stability; 10-node cap

5 · String building (ns2.1+)

F# affected	FSharp.Core file	.NET API	guard	win
String.map / String.mapi / String.filter	string.fs:57/71/89	String.Create(len, state, writer)	`#if NETSTANDARD2_1_OR_GREATER

6 · Formatting / printf (mixed)

F# affected	FSharp.Core file	.NET API	guard	win
%O/%P/interpoland/user types/decimal/DateTime	printf.fs:911–929 ObjectToString/InterpolandToString (+ span sink 1073–1083)	ISpanFormattable.TryFormat (net6) + String.Create	#if NET6_0_OR_GREATER	skip 1 temp string per arg; polymorphic (no per-type match). No-width case first — padding is string-based today
numeric specifiers %d %i %u %x %X %f %e %g %M (no width/prec)	printf.fs:746–770/852–859	concrete Int32/Int64/Double/Single/Decimal.TryFormat into Span<char>	`#if NETSTANDARD2_1_OR_GREATER
StringBuilder / TextWriter sinks	printf.fs:1073–1083	StringBuilder.Append(ReadOnlySpan<char>) / TextWriter.Write(ReadOnlySpan<char>)	`#if NETSTANDARD2_1_OR_GREATER
sprintf string sink	printf.fs:1029–1071	pooled char[] (ArrayPool) + String.Create	`#if NETSTANDARD2_1_OR_GREATER

7 · Generic math (net-only)

F# affected	FSharp.Core file	.NET API	guard	win
GenericZero/OneDynamicImplTable (Half)	prim-types.fs:2667/2700	typeof<System.Half> fast case (net5)	#if NET5_0_OR_GREATER	avoid reflection

No boost (documented negatives)

• GenericEquality{Single,Double}Array (prim-types.fs:1511/1527) — NaN-reflexivity (er flag: NaN = NaN) diverges from IEEE SequenceEqual; keep scalar.
• DefaultInterpolatedStringHandler as the printf sink — it is a ref struct, cannot be a field of the heap PrintfEnv class; and sprintf's ToStringAndClear() is itself just String.Create over a pool. DISH belongs to a compiler lowering.