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

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.

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.

Reviewed changes

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`.

Copilot · 2026-04-13T14:27:22Z

src/coreclr/tools/Common/Compiler/ObjectWriter/MachObjectWriter.cs

+                        var temporaryLabelOffset = _sections[sectionIndex].TemporaryLabelOffset;
+                        var temporaryLabelIndex = _sections[sectionIndex].TemporaryLabelIndex;
+                        addend -= offset - temporaryLabelOffset;
+                        if ((int)addend != (((int)addend << 12) >> 12))
+                        {
+                            addend += offset - temporaryLabelOffset;
+                            _temporaryLabels.Add(new SymbolDefinition(sectionIndex, offset, 0, false));
+                            _sections[sectionIndex].TemporaryLabelOffset = temporaryLabelOffset = offset;
+                            _sections[sectionIndex].TemporaryLabelIndex = temporaryLabelIndex = (uint)_temporaryLabels.Count;
+                        }


The signed-20-bit overflow check is done after casting addend to int. Since addend is a long and is adjusted by offset (also long), this cast can truncate for large section offsets/addends and make the overflow detection unreliable (potentially skipping label generation when it’s actually needed). Consider performing the range check using long arithmetic (e.g., compare against the signed 20-bit bounds or use a sign-extension check on long) before any narrowing conversions.

Copilot · 2026-04-13T14:27:22Z

src/coreclr/tools/Common/Compiler/ObjectWriter/MachObjectWriter.cs

+                        // Abuse the addend to store the temporary label index.
+                        addend = temporaryLabelIndex;


SymbolicRelocation.Addend is being repurposed to carry a temporary-label index (“Abuse the addend to store the temporary label index”). This makes the meaning of Addend context-dependent and couples EmitRelocation tightly to the later Mach relocation emission logic, which is easy to break during future refactors. Consider storing this index in a dedicated field/structure (or a side table keyed by relocation) so Addend consistently remains an addend.

Copilot · 2026-04-13T14:27:22Z

build.sh

+# switch to different Xcode version
+if [[ "$OSTYPE" == "darwin"* ]]; then
+  if [[ -d "/Applications/Xcode_26.2.app" ]]; then
+    echo "Switching to Xcode 26.2 for build"
+    sudo xcode-select -s "/Applications/Xcode_26.2.app"


This change makes the repo-level build.sh attempt to switch the active Xcode via sudo xcode-select whenever a specific path exists. This has major operational and security implications: it mutates global machine state, requires elevated privileges/interactive sudo in many environments, and can break CI or local builds unexpectedly. Repo build scripts should not implicitly run privileged commands; prefer documenting the required Xcode version or allowing an opt-in environment variable/argument without sudo.

Suggested change

# switch to different Xcode version

if [[ "$OSTYPE" == "darwin"* ]]; then

if [[ -d "/Applications/Xcode_26.2.app" ]]; then

echo "Switching to Xcode 26.2 for build"

sudo xcode-select -s "/Applications/Xcode_26.2.app"

# do not implicitly change the active Xcode for the machine

if [[ "$OSTYPE" == "darwin"* ]]; then

if [[ -d "/Applications/Xcode_26.2.app" ]]; then

echo "Xcode 26.2 is installed at /Applications/Xcode_26.2.app."

echo "This build does not automatically switch the active Xcode."

echo "If needed, select it explicitly before building:"

echo " sudo xcode-select -s \"/Applications/Xcode_26.2.app/Contents/Developer\""

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

Fix missing temporaryLabelIndex variable assignment

98ffbba

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

Open

filipnavara added 2 commits February 22, 2026 15:48

Prevent dead-striping of dehydrated data by the linker

28ea1b5

ltmp is special

01eb833

filipnavara closed this Feb 22, 2026

Name the local label differently

6ea9009

filipnavara reopened this Feb 22, 2026

filipnavara closed this Feb 22, 2026

filipnavara added 2 commits February 22, 2026 23:20

Add N_NO_DEAD_STRIP to make Xcode 16 -ld_classic happy

3df7606

Be more conservative in the largest allowed addend

1f951c0

filipnavara reopened this Feb 22, 2026

Test: Use Xcode 26.2 for build

154335c

dotnet-policy-service bot closed this Mar 25, 2026

filipnavara reopened this Mar 25, 2026

This was referenced Mar 26, 2026

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

Open

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

Open

Merge branch 'main' into ld-prime-addend

3861310

Copilot AI review requested due to automatic review settings April 13, 2026 14:18

Copilot started reviewing on behalf of filipnavara April 13, 2026 14:20 View session

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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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build-analysis bot mentioned this pull request Apr 13, 2026

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

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Implement a workaround for ld-prime hitting an assert on large addends#124721

Implement a workaround for ld-prime hitting an assert on large addends#124721
filipnavara wants to merge 9 commits intodotnet:mainfrom
filipnavara:ld-prime-addend

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		// Abuse the addend to store the temporary label index.
		addend = temporaryLabelIndex;

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