[lewing]

Problem

crossgen2 neither precompiles nor inlines a generic method or type with a where T : struct or where T : unmanaged constraint when it is instantiated over a CoreLib primitive (for example S<char> or GS<int>.Get) from code outside CoreLib's version bubble. The same generic without the constraint compiles and inlines fine. When inlining fails, the JIT dump shows:

INLINER: ... result 'failed this callee' reason 'cannot get method info' for 'V:CallS(char):int' calling 'V:S[char](char):int'

Root cause

ReadyToRunCompilationModuleGroupBase.ComputeInstantiationVersionsWithCode counts a primitive type argument as part of the generic's version bubble only when the generic parameter has no type constraints at all. C# writes where T : struct and where T : unmanaged into metadata as a System.ValueType type constraint (the unmanaged modreq is not visible in the type system). That makes the constraint list non-empty, so VersionsWithMethodBody returns false, and two things follow:

• the instantiation is not compiled into the defining module, and
• CorInfoImpl.getMethodInfo → CanInline returns false, which the JIT reports as "cannot get method info".

The rule dates from #1889. Its design doc already says primitives are in the bubble "as long as the generic is not constrained on an interface or class". System.ValueType is just how C# encodes struct: every primitive satisfies it, and it exposes nothing an unconstrained T can't already use. The runtime needs no change. ReadyToRunInfo::ComputeAlternateGenericLocationForR2RCode skips primitive arguments, so these instantiations are looked up in the defining module, the same path the unconstrained case already uses.

Fix

When the type argument is a primitive, ignore System.ValueType constraints. Any other constraint, interface or class, still disqualifies the instantiation, as before. The object/string branch is unchanged.

Other changes in this PR:

• R2R reader: ILCompiler.Reflection.ReadyToRun's CrossModuleInliningInfoSection now names local inlinees that have no compiled body of their own, such as AggressiveInlining helpers, using the image metadata. Previously they showed as <MethodDef 0x…>. The new test's inlining assertion needs this, and it also improves R2RDump output.
• Docs: docs/design/features/crossgen2-compilation-structure-enhancements.md now states the rule.
• Test: new R2RTestSuites.StructConstrainedPrimitiveInstantiations ([Fact], fixture TestCases/PrimitiveInstantiations/StructConstrainedGenerics.cs). It asserts that StructConstrained<int>, UnmanagedConstrained<int> and StructConstrainedBox<int>.GetValueHashCode are compiled, and that the struct- and unmanaged-constrained AggressiveInlining helpers are inlined.

Impact

• Affected:

  • native per-assembly R2R without --opt-cross-module;
  • app-only composite without --opt-cross-module or --inputbubble.

  These instantiations are jitted at startup instead of using R2R code, and they miss R2R inlining.

• Not affected, so this change is a no-op there:

  • browser-wasm defaults: per-assembly plus --opt-cross-module:*, used for both app publish and the shared framework;
  • composites that include the framework: browser composite, WASI and Apple mobile.

Interface constraints still disqualify primitive instantiations by design. The separate generic-math issue (calls to static virtual methods such as INumber<T> operators that can't be resolved across the version bubble) is tracked in #134559 and is not addressed here.

Related: #135899 (the same crossgen2 area, found while working on it).

Validation

All runs on osx-arm64, checked:

• ./build.sh clr+libs -lc Release -c checked: succeeded with 0 warnings and 0 errors.
• ./build.sh -s clr.r2rtests -lc Release -c checked -test: 51 total, 36 passed, 0 failed, 15 skipped (target-specific tests).
• I ran the same command with only the crossgen2 change reverted. StructConstrainedPrimitiveInstantiations failed with Expected compiled method 'StructConstrainedGenerics.StructConstrained<int>' not found, and passed again once the fix was restored.
• I also ran a standalone repro through the built crossgen2 for both --targetos:osx --targetarch:arm64 -O and --targetos:browser --targetarch:wasm --obj-format:wasm -O, with default args.
  • SN<int>, MN<int> and GS<int>.Get are now compiled.
  • CallS(char) and CallM(char) now have the same code size as the unconstrained CallU(char), which shows the call is inlined.
  • IN<int>, which has an IEquatable<T> constraint, is still not compiled, as intended.

Not run: the CI browser-wasm R2R test leg (CLR_R2R_Tests_browser_wasm), which needs a browser CoreCLR build. The fix doesn't depend on the target, and the new test is a plain [Fact], so that leg will run it.

Note

This PR description and the change were generated with GitHub Copilot.

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