IL: intern the attributes and type references read from metadata

[auduchinok]

A metadata reader dedups a row within its own assembly, but the same value is read again in every
assembly that mentions it, and readers are shared process-wide. Reading the 489 references of a
ReSharper F# plugin project:

read in ilread	values	distinct
ILAttribute (constructor + blob)	22983	2896
its constructor ILMethodSpec	22983	338
ILTypeRef (TypeRef table)	9350	1186

Each occurrence rebuilt the chain behind one attribute: constructor, argument types, type reference,
scope reference, assembly reference, public key. Three capped tables in ilread now keep one instance
of each across readers.

• Cleared from ClearAllILModuleReaderCache, so FSharpChecker.ClearCaches() reaches them: an entry
  outlives the reader that produced it.
• Interning past the cap is skipped rather than evicting — an unshared value is correct, just larger.
• Cache<_, _> from illib does not fit: no Clear, and adding one means touching the eviction
  machinery shared with the type-subsumption and overload-resolution caches.
• Constructors are interned inside the cached seekReadCustomAttrType, not at the use site, so a
  constructor chain is hashed once per row per assembly instead of once per attribute; attributes then
  compare their constructor by reference. Without both, small projects paid ~4.5% of their check time.

Retained memory

Whole solution checked and held at once, checker options as an editor passes them
(keepAllBackgroundResolutions and keepAllBackgroundSymbolUses off).

solution	base MB	change MB	delta
consoleapp	29.17	27.96	-1.21 (-4.1%)
oxpecker	130.95	126.64	-4.31 (-3.3%)
prime	105.49	103.44	-2.05 (-1.9%)
fstoolkit	161.31	158.37	-2.94 (-1.8%)
resharper	385.08	378.40	-6.68 (-1.7%)
icedtasks	96.63	94.96	-1.67 (-1.7%)
fantomas	349.57	346.82	-2.75 (-0.8%)
fcsrepo	877.28	875.18	-2.10 (-0.2%)

The saving is a roughly fixed 1-7 MB per configuration rather than a share of the heap, so it reads
largest where the heap is smallest.

[github-actions]

⚠️ Release notes required, but author opted out

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cc @dotnet/fsharp-team-msft

[T-Gro]

🤖🕵️ [P2] Editing one assembly's imported attribute also changes another assembly's exported attribute. Keep mutable blobs reader-local.

open FSharp.Compiler.AbstractIL.IL
open FSharp.Compiler.AbstractIL.ILBinaryReader

let opts =
    { pdbDirPath = None; reduceMemoryUsage = ReduceMemoryFlag.Yes
      metadataOnly = MetadataOnlyFlag.Yes; tryGetMetadataSnapshot = fun _ -> None }

let clsBlob name =
    let sdk = @".dotnet\sdk\11.0.100-preview.6.26359.118\"
    let reader = AssemblyReader.GetILModuleReader(sdk + name + ".dll", opts)
    reader.ILModuleDef.Manifest.Value.CustomAttrs.AsArray()
    |> Array.pick (function
        | ILAttribute.Encoded(m, data, _)
            when m.MethodRef.DeclaringTypeRef.FullName = "System.CLSCompliantAttribute" -> Some data
        | _ -> None)

let a = clsBlob "Microsoft.Build"
let b = clsBlob "Microsoft.Build.Framework"
a[2] <- 0uy
printfn "B CLSCompliant = %b" (b[2] <> 0uy) // base: true; head: false

Reproduced with exact base/head builds in all four reduceMemoryUsage × metadataOnly combinations. Exporting B with AssemblyBuilder.SetCustomAttribute changes its runtime CLSCompliantAttribute.IsCompliant from true to false too.

[github-actions]

🔍 Tooling Safety Check — Affects-Bootstrap, Affects-Compiler-Output
Affects-Bootstrap: Changes compiler metadata-reading code used during bootstrap.
Affects-Compiler-Output: Changes compiler metadata interning and memory behavior.

Generated by PR Tooling Safety Check · gpt56 595.2K · ◷