Implement a workaround for ld-prime hitting an assert on large addends by filipnavara · Pull Request #124721 · dotnet/runtime

filipnavara · 2026-02-22T08:57:29Z

We translate the IMAGE_REL_BASED_RELPTR32 relocation into ARM64_RELOC_SUBTRACTOR and ARM64_RELOC_UNSIGNED pair on ARM64. To emulate the behavior of PC relative relocation we bake the section-relative PC offset into the addend with negative sign. The ARM64_RELOC_SUBTRACTOR relocation then subtract the base address of the section and finally the ARM64_RELOC_UNSIGNED relocation adds the target symbol address.

This works fine for addends that fit into signed 20-bit integer, but ld-prime hits an assert when the addend is larger. The workaround is to generate a temporary label at the offset of the relocated address and adjust the addend to be relative to that label. We reuse the last temporary label for following relocations in the same section until we hit another overflow. This way we minimize the number of temporary labels we need to generate while still satisfying the ld-prime requirement.

Same logic applies to X86_64_RELOC_SUBTRACTOR + X86_64_RELOC_UNSIGNED pair for x64 targets.

Fixes #119380

filipnavara · 2026-02-22T09:48:18Z

Needs more work. I'll investigate the unit test failures first.

filipnavara · 2026-02-22T16:16:30Z

I don't get the System.Linq.Expressions test failure on Xcode 26.2 anymore. According the log the compiler/linker version on CI is ~~some version of Xcode 26.x~~ Xcode 16.4.

filipnavara · 2026-02-22T16:41:45Z

Going back to the drawing board now.

filipnavara · 2026-02-23T12:09:14Z

@akoeplinger Is there some easy way to switch some CI lane to use newer Xcode? They should be available in the runner images, just not as default.

akoeplinger · 2026-02-23T12:28:27Z

I assume adding xcode-select to the path mentioned in https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md#xcode somewhere early in the build should suffice?
You could add it to build.sh (if it's just for testing).

filipnavara · 2026-02-23T15:55:02Z

I assume adding xcode-select to the path mentioned in https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md#xcode somewhere early in the build should suffice?

Thanks, seems to pass the smoke test with Xcode 26.2. I'll probably open a separate PR to check what is the behavior just with the Xcode bump alone.

dotnet-policy-service · 2026-03-25T20:14:14Z

Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.

Copilot

Pull request overview

This PR adds a workaround in the Mach-O object writer to avoid Apple ld-prime assertions when emitting IMAGE_REL_BASED_RELPTR32 as *_RELOC_SUBTRACTOR + *_RELOC_UNSIGNED with large addends, by introducing reusable per-section temporary labels to keep addends within the linker’s expected signed 20-bit range.

Changes:

Track and emit per-section temporary labels to bound relocation addends for IMAGE_REL_BASED_RELPTR32 on ARM64 and x64 .eh_frame.
Adjust Mach relocation emission to optionally reference a temporary label symbol (instead of the section base symbol) for SUBTRACTOR relocs.
Modify build.sh to attempt switching Xcode versions on macOS.

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Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/coreclr/tools/Common/Compiler/ObjectWriter/MachObjectWriter.cs	Adds temporary-label generation/reuse and uses label symbol indices to avoid `ld-prime` large-addend assertions.
build.sh	Adds macOS-only logic to switch Xcode via `sudo xcode-select`.

filipnavara · 2026-05-06T20:53:17Z

Apple support engineers confirmed that 1 MB boundary for the temporary labels (as currently implemented in this PR) should be sufficient not to hit the linker limits. The PR needs to be rebased and the temporary Xcode version bump removed once the tests pass.

We translate the IMAGE_REL_BASED_RELPTR32 relocation into ARM64_RELOC_SUBTRACTOR and ARM64_RELOC_UNSIGNED pair on ARM64. To emulate the behavior of PC relative relocation we bake the section-relative PC offset into the addend with negative sign. The ARM64_RELOC_SUBTRACTOR relocation then subtract the base address of the section and finally the ARM64_RELOC_UNSIGNED relocation adds the target symbol address. This works fine for addends that fit into signed 24-bit integer, but ld-prime hits an assert when the addend is larger. The workaround is to generate a temporary label at the offset of the relocated address and adjust the addend to be relative to that label. We reuse the last temporary label for following relocations in the same section until we hit another overflow. This way we minimize the number of temporary labels we need to generate while still satisfying the ld-prime requirement. Same logic applies to X86_64_RELOC_SUBTRACTOR + X86_64_RELOC_UNSIGNED pair for x64 targets.

This reverts commit 610e0f1.

Copilot

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

- Replace (int) cast-based 20-bit overflow check with direct long comparison against signed 20-bit bounds [-524288, 524287] to avoid silent truncation for large addend values. - Add Debug.Assert before the int cast in WriteInt32LittleEndian to guard against unexpected out-of-range values. - Add Debug.Assert in EmitRelocationsX64/Arm64 to verify _temporaryLabelsBaseIndex has been initialized before use. - Improve comment on Addend repurposing to document the design choice. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

+                        // The SymbolicRelocation.Addend field is repurposed to carry the
+                        // 1-based temporary label index (0 means no temporary label). This
+                        // avoids introducing a side table and is consumed by
+                        // EmitRelocationsX64/EmitRelocationsArm64 when building the
+                        // subtractor relocation pair.
+                        addend = temporaryLabelIndex;


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Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

filipnavara · 2026-05-16T14:30:52Z

+            foreach (SymbolDefinition definition in _temporaryLabels)
+            {
+                temporaryLabelNameBuilder.Clear();
+                temporaryLabelNameBuilder.Append("ltemp"u8).Append((int)(symbolIndex - _temporaryLabelsBaseIndex));


Utf8StringBuilder doesn't have an overload that takes unsigned integers. This should not overflow for any reasonably sized object file.

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Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

+                        symbolicRelocation.Addend != 0 ?
+                        (uint)(_temporaryLabelsBaseIndex + symbolicRelocation.Addend - 1) :
+                        (uint)sectionIndex;
+                    Debug.Assert(baseSymbolIndex < _symbolTable.Count);


                            data,
                            BinaryPrimitives.ReadInt32LittleEndian(data) +
-                            (int)(addend - offset));
+                            (int)addend);


+                        // The SymbolicRelocation.Addend field is repurposed to carry the
+                        // 1-based temporary label index (0 means no temporary label). This
+                        // avoids introducing a side table and is consumed by
+                        // EmitRelocationsX64/EmitRelocationsArm64 when building the
+                        // subtractor relocation pair.
+                        addend = temporaryLabelIndex;


MichalStrehovsky · 2026-05-19T13:22:59Z

@agocke do you know who would be the best to review macho object writer changes?

We have a test that was running into this:

runtime/src/libraries/tests.proj

Lines 408 to 413 in e9dec1b

    
           <!-- https://github.com/dotnet/runtime/issues/128286 --> 
        
           <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Text.Json\tests\System.Text.Json.SourceGeneration.Tests\System.Text.Json.SourceGeneration.Roslyn4.4.Tests.csproj" /> 
        
           <!-- https://github.com/dotnet/runtime/issues/119380 --> 
        
           <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Text.Json\tests\System.Text.Json.SourceGeneration.Tests\System.Text.Json.SourceGeneration.Roslyn4.4.Tests.csproj" 
        
                             Condition="'$(TargetOS)' == 'osx' and '$(TargetArchitecture)' == 'arm64'" />

But I just had to disable the test on all platforms because the test became completely broken over the weekend or so (see the two quoted bugs above, it's two disablements for the same test for different reasons).

agocke · 2026-05-22T03:11:59Z

You and @elinor-fung 😄

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Feb 22, 2026

dotnet-policy-service Bot added the community-contribution Indicates that the PR has been added by a community member label Feb 22, 2026

filipnavara added os-mac-os-x macOS aka OSX os-ios Apple iOS area-NativeAOT-coreclr and removed area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI labels Feb 22, 2026

filipnavara marked this pull request as ready for review February 22, 2026 09:04

filipnavara requested a review from MichalStrehovsky as a code owner February 22, 2026 09:04

filipnavara marked this pull request as draft February 22, 2026 09:47

build-analysis Bot mentioned this pull request Feb 22, 2026

[android][clr] No peer certificates when executing System.Net.Http.Functional.Tests on Android emulator #124526

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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[android-arm64] The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#6408

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filipnavara mentioned this pull request Apr 13, 2026

ld64 crashing while Native AOT compiling Microsoft MCP for .NET 10 #119380

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System.Net.NameResolution.Tests DNS failures: Name or service not known #126641

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filipnavara added 8 commits May 16, 2026 08:31

Fix missing temporaryLabelIndex variable assignment

b99ee70

Prevent dead-striping of dehydrated data by the linker

2fa03fa

ltmp is special

c157abb

Name the local label differently

4a18e9e

Add N_NO_DEAD_STRIP to make Xcode 16 -ld_classic happy

be48d72

Be more conservative in the largest allowed addend

3f8dd48

Test: Use Xcode 26.2 for build

610e0f1

filipnavara force-pushed the ld-prime-addend branch from 3861310 to 610e0f1 Compare May 16, 2026 06:32

build-analysis Bot mentioned this pull request May 16, 2026

Unable to pull image from mcr.microsoft.com #117164

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filipnavara added 2 commits May 16, 2026 13:56

Revert "Test: Use Xcode 26.2 for build"

63cc8fd

This reverts commit 610e0f1.

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