Use indirect field access for types spanning multiple inexact compilation units#129830
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…tion units Fixes the Verify_FieldOffset failfast in crossgen2-composite Large Version Bubble R2R legs (dotnet#128968). When a reference type's layout chain spans 2+ inexact compilation units, crossgen2 conservatively aligns the derived type's fields after a cross-compilation-unit base. At runtime those framework modules can instead bind into a single composite image, where the runtime back-fills the derived fields into the base type's trailing alignment padding. Neither a baked relative (ENCODE_FIELD_BASE_OFFSET) nor absolute offset is valid for both layouts, so route these fields to an indirect, runtime-resolved field offset (ENCODE_FIELD_OFFSET), which is correct regardless of how the modules are grouped and emits no layout-verification fixup. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
This PR updates crossgen2 ReadyToRun field-access encoding to avoid baking field offsets for reference types whose layout can differ at runtime when their layout “spans” multiple inexact compilation units, which can otherwise trigger Verify_FieldOffset failfasts (and potentially incorrect field accesses when verification is off).
Changes:
- In
EncodeFieldBaseOffset, detectHasMultipleInexactCompilationUnitsfor the owning type and switch to an indirect, runtime-resolved field offset encoding (ENCODE_FIELD_OFFSETviaSymbolNodeFactory.FieldOffset). - Ensure this indirection path runs before the existing
IsInheritanceChainLayoutFixedInCurrentVersionBubble(...)decision so a potentially unstable baked offset isn’t used.
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/azp run runtime-coreclr crossgen2-composite |
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This was referenced Jun 25, 2026
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Fixes #128968.
Problem
In crossgen2-composite Large Version Bubble (LVB) R2R test legs (compiled with
--verify-type-and-field-layout), the runtime hits aVerify_FieldOffsetfailfast, e.g.:Root cause
This is a cross-module layout divergence between crossgen2 and the runtime:
The two layouts disagree, so a baked relative offset (
ENCODE_FIELD_BASE_OFFSET) or absolute offset with a relative field-offset verification is invalid for one of them — producing the failfast (and, with verification off, an incorrect memory access).This reproduces for types like
UnixConsoleStream._useReadLine(System.Console+ corelib) andXmlJsonReader._complexTextMode(System.Private.DataContractSerialization+System.Private.Xml+ corelib).Fix
In
EncodeFieldBaseOffset, whenTypeLayoutCompilationUnits(pMT).HasMultipleInexactCompilationUnitsis true, route field access to an indirect, runtime-resolved field offset (ENCODE_FIELD_OFFSETviaSymbolNodeFactory.FieldOffset). This is correct regardless of how the modules are ultimately grouped and emits no layout-verification fixup. The new branch is placed before theIsInheritanceChainLayoutFixedInCurrentVersionBubblebranch, which would otherwise bake a wrong offset for these types.Validation
Validated end-to-end against a composite framework image (
framework-r2r.dll) plus a separately-compiled test composite, the exact configuration that triggered the failure:binarytrees-6(writes to Console ->UnixConsoleStream): passed, no assert.JsonBenchmarksSerialize/Deserialize (DataContractJsonSerializer->XmlJsonReader): passed, no assert.straceconfirmed bothframework-r2r.dlland the patched testcomposite-r2r.dllare loaded in-process during the run (not a JIT fallback).Note
This pull request was authored with the assistance of GitHub Copilot.