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Implement space-efficient ValueTuple.ToString and GetHashCode for NativeAOT#124450

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Implement space-efficient ValueTuple.ToString and GetHashCode for NativeAOT#124450
Copilot wants to merge 5 commits intomainfrom
copilot/investigate-value-tuple-tostring

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Copilot AI commented Feb 16, 2026 •
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Description

Adds shared ToStringShared and GetHashCodeShared helper methods on the non-generic ValueTuple struct that use __GetFieldHelper to enumerate fields at runtime under #if NATIVEAOT. These replace the per-arity ToString(), ToStringEnd(), and GetHashCode() implementations (arities 1–8) with single shared implementations, leading to smaller code size in NativeAOT. The existing per-arity implementations are preserved under #else for non-NativeAOT runtimes.

How it works

ToString

  • The shared helper calls ValueType.__GetFieldHelper(-1, out _) to get the total field count, then iterates fields using __GetFieldHelper(i, out MethodTable*) to obtain each field's byte offset and type.
  • Each field is boxed via RuntimeImports.RhBoxAny and ToString() is called on the boxed value.
  • For the 8-tuple (ValueTuple<T1..T7, TRest>), when the last field (TRest) implements IValueTupleInternal, the helper calls ToStringEnd() instead of ToString() to flatten nested tuple representations (e.g., (1, 2, 3, 4, 5, 6, 7, 8) instead of (1, 2, 3, 4, 5, 6, 7, (8))). This check is guarded by numFields == 8 && i == 7 to ensure it only applies to the 8-tuple, not to other arities that might happen to have a ValueTuple as their last field.
  • The includeOpenParen parameter supports both ToString() (includes opening () and ToStringEnd() (omits it).

GetHashCode

  • Uses the same __GetFieldHelper field enumeration pattern to box each field and call GetHashCode() on it.
  • For arities 1–7, combines all field hash codes using the HashCode struct (Add/ToHashCode).
  • For the 8-tuple, replicates the existing complex flattening logic: when Rest implements IValueTupleInternal, it uses Rest.Length and Rest.GetHashCode() to determine which subset of Item1..Item7 to include in the hash, matching the original behavior. When Rest is not IValueTupleInternal, it hashes only the first 7 fields.

Changes

  • src/libraries/System.Private.CoreLib/src/System/ValueTuple.cs:

    • Added NativeAOT-specific using directives (System.Runtime, System.Text, Internal.Runtime)
    • Added ValueTuple.ToStringShared(ValueType, bool) static helper under #if NATIVEAOT
    • Added ValueTuple.GetHashCodeShared(ValueType) static helper under #if NATIVEAOT
    • Wrapped all ToString() and ToStringEnd() methods (arities 1–8) with #if NATIVEAOT / #else / #endif, delegating to the shared helper on NativeAOT
    • Wrapped all GetHashCode() overrides (arities 1–8) with #if NATIVEAOT / #else / #endif, delegating to the shared helper on NativeAOT

  • src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/CompilerTypeSystemContext.GetFieldMethodOverrides.cs:

    • Changed RequiresValueTypeGetFieldHelperMethod to return true for ValueTuple types (was previously returning false since ValueTuples override Equals/GetHashCode directly). ValueTuple types now need __GetFieldHelper for field enumeration in ToString and GetHashCode.

  • src/coreclr/tools/Common/TypeSystem/IL/Stubs/GetFieldHelperMethodOverride.cs:

    • Updated EmitILCommon to skip the memcompare shortcut for ValueTuple types. Previously, ValueTuple instantiations with simple fields (e.g., ValueTuple<byte, byte, nint>) would get a __GetFieldHelper that only returned a negative memcompare size instead of per-field enumeration, causing the shared helpers to fail with an assertion.

Testing

  • ✅ CoreCLR CoreLib builds successfully
  • ✅ NativeAOT CoreLib builds successfully
  • ✅ ILCompiler.Compiler builds successfully
  • ✅ All 97 existing ValueTuple tests pass
Original prompt

This section details on the original issue you should resolve

<issue_title>Investigate space efficient ValueTuple.ToString</issue_title>
<issue_description>‎src/libraries/System.Private.CoreLib/src/System/ValueTuple.cs contains definition for all ValueTuples. Each of these defines their own ToString override in the shape return $"({Item1}, {Item2})";. We could however have a more space-efficient version of this for native AOT (under a #if NATIVEAOT ifdef).

In native AOT, the ValueType base type (src/coreclr/nativeaot/System.Private.CoreLib/src/System/ValueType.cs) defines a int __GetFieldHelper(int index, out MethodTable* mt) that allows enumerating fields on a value type (see sample usage of the __GetFieldHelper method in the same file).

ValueTuple could use this method to enumerate fields. The implementation could be shared for all value tuples (i.e. the body of ValueTuple.ToString would just call the shared helper).

This would lead to smaller code size.</issue_description>

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@github-actions github-actions bot added the needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners label Feb 16, 2026
@MichalStrehovsky
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MichalStrehovsky commented Feb 16, 2026

@copilot

Copilot AI and others added 2 commits February 16, 2026 06:08
@MichalStrehovsky
Implement space-efficient ValueTuple.ToString for NativeAOT
15e4544 
Add a shared ToStringShared helper on ValueTuple that uses
__GetFieldHelper to enumerate fields, replacing per-arity
ToString/ToStringEnd implementations under #if NATIVEAOT.
This leads to smaller code size in NativeAOT by sharing a
single implementation across all ValueTuple arities.

Co-authored-by: MichalStrehovsky <[email protected]>
@MichalStrehovsky
Fix IValueTupleInternal check to only apply for 8-tuples
0f675d0 
The ToStringEnd() call on the last field should only happen for
the 8-tuple (which has exactly 8 fields), not for any ValueTuple
where the last field happens to implement IValueTupleInternal.

Co-authored-by: MichalStrehovsky <[email protected]>
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Copilot AI commented Feb 16, 2026

@copilot

Implemented the space-efficient ValueTuple.ToString() for NativeAOT in commits 15e4544 and 0f675d0.

The shared ToStringShared helper on ValueTuple uses __GetFieldHelper to enumerate fields and RuntimeImports.RhBoxAny to box each field value, then calls ToString() on each. For the 8-tuple recursive case, when the last field (TRest) is an IValueTupleInternal, it calls ToStringEnd() to flatten the representation. All ValueTuple arities (1-8) delegate to this single shared helper under #if NATIVEAOT, keeping the existing per-arity implementations under #else.

Both CoreCLR and NativeAOT CoreLib build successfully, and all 97 ValueTuple tests pass.

Copilot AI changed the title [WIP] Investigate space efficient ValueTuple.ToString implementation Implement space-efficient ValueTuple.ToString for NativeAOT Feb 16, 2026
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MichalStrehovsky commented Feb 16, 2026

@copilot test failure:

07:12:22.208 Running test: Loader\classloader\explicitlayout\Regressions\100220\Runtime_100220\Runtime_100220.dll
Process terminated. Assertion failed.
numFields > 0
   at System.Diagnostics.DebugProvider.Fail(String, String)
   at System.Diagnostics.Debug.Fail(String, String)
   at System.ValueTuple.ToStringShared(ValueType, Boolean)
   at Runtime_100220.TestEntryPoint()
   at Program.<<Main>$>g__TestExecutor95|0_96(StreamWriter, StreamWriter, Program.<>c__DisplayClass0_0&)
   at Program.<Main>$(String[] args)
Expected: 100
Actual: -1073740791
END EXECUTION - FAILED

@MichalStrehovsky
Ensure NativeAOT compiler generates per-field __GetFieldHelper for Va…
236dd9f 
…lueTuple types

The compiler was previously skipping __GetFieldHelper generation for
ValueTuple types, and also short-circuiting to a memcompare-only
path for ValueTuples with simple fields. Both of these prevented
the ToStringShared helper from working. Now ValueTuple types always
get per-field __GetFieldHelper overrides, enabling field enumeration
for ToString.

Co-authored-by: MichalStrehovsky <[email protected]>
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Copilot AI commented Feb 16, 2026

@copilot test failure:

...

Fixed in 236dd9f. The NativeAOT compiler was not generating per-field __GetFieldHelper overrides for ValueTuple types — it was explicitly skipping them (line 106 in CompilerTypeSystemContext.GetFieldMethodOverrides.cs) and also short-circuiting to a memcompare-only path in GetFieldHelperMethodOverride.EmitILCommon for ValueTuples with simple fields like ValueTuple<byte, byte, nint, byte, nint, nint>. Both paths now correctly generate per-field enumeration for ValueTuple types.

@github-actions github-actions bot added area-NativeAOT-coreclr and removed needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners labels Feb 16, 2026
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dotnet-policy-service bot commented Feb 16, 2026

Tagging subscribers to this area: @agocke, @dotnet/ilc-contrib
See info in area-owners.md if you want to be subscribed.

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MichalStrehovsky commented Feb 16, 2026

@copilot now add similar optimization to the GetHashCode overrides. Introduce a shared helper based on __GetFieldHelper override and call it from GetHashCode under a #if NATIVEAOT ifdef.

@MichalStrehovsky
Add shared GetHashCodeShared helper for NativeAOT ValueTuple
cd06740 
Add a shared GetHashCodeShared helper on ValueTuple that uses
__GetFieldHelper to enumerate fields under #if NATIVEAOT. This
replaces per-arity GetHashCode() implementations with a single
shared implementation, leading to smaller code size in NativeAOT.

For arities 1-7, the helper uses HashCode struct to combine
field hash codes. For the 8-tuple, it replicates the existing
flattening behavior based on Rest.Length.

The existing per-arity implementations are preserved under #else
for non-NativeAOT runtimes.

Co-authored-by: MichalStrehovsky <[email protected]>
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Copilot AI commented Feb 16, 2026

@copilot now add similar optimization to the GetHashCode overrides. Introduce a shared helper based on __GetFieldHelper override and call it from GetHashCode under a #if NATIVEAOT ifdef.

Added in cd06740. The shared GetHashCodeShared helper on ValueTuple uses __GetFieldHelper to enumerate fields and HashCode struct to combine hash codes. For arities 1-7, it simply combines all field hash codes. For the 8-tuple, it replicates the existing flattening behavior based on Rest.Length and IValueTupleInternal. All 97 existing ValueTuple tests pass.

Copilot AI changed the title Implement space-efficient ValueTuple.ToString for NativeAOT Implement space-efficient ValueTuple.ToString and GetHashCode for NativeAOT Feb 16, 2026
@build-analysis build-analysis bot mentioned this pull request Feb 16, 2026
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@MichalStrehovsky MichalStrehovsky deleted the copilot/investigate-value-tuple-tostring branch February 16, 2026 12:11
@github-actions github-actions bot locked and limited conversation to collaborators Mar 19, 2026
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Investigate space efficient ValueTuple.ToString

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