fix(build): build xxHash objects with PIC for MllmRT

[kkkzbh]

Summary

This PR builds the xxHash object library with position-independent code so it can be safely linked into MllmRT.

MllmRT is a shared library and links xxHash::xxhash. On Linux x86_64, the current build may fail when linking libMllmRT.so because xxhash_static is created from $<TARGET_OBJECTS:xxhash_impl>, while the xxhash_impl objects are not compiled with PIC.

Background

I found this while investigating the official x86 Qwen3-0.6B C++ runner/build path. The x86 build can reach the MllmRT link step and fail with:

/usr/bin/ld.bfd: lib/libxxhash_static.a(xxhash.c.o): relocation R_X86_64_32S against `.rodata` can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld.bfd: failed to set dynamic section sizes: bad value
collect2: error: ld returned 1 exit status

Other x86 backend findings from that investigation are intentionally kept out of scope. This PR only fixes the xxHash PIC linkage issue.

Root Cause

third_party/xxHash/CMakeLists.txt builds the xxHash sources through the xxhash_impl object target, then creates xxhash_static from those object files:

add_library(xxhash_impl OBJECT ${_src})
add_library(xxhash_static STATIC $<TARGET_OBJECTS:xxhash_impl>)

Setting PIC on the static archive target is not enough here, because the object files are already produced by xxhash_impl.

Fix

Enable POSITION_INDEPENDENT_CODE on xxhash_impl, the target that actually compiles xxhash.c and xxh_x86dispatch.c.

Validation

• git diff --check
• cmake -S . -B build-xxhash-pic -G Ninja -DCMAKE_BUILD_TYPE=Release -DHWY_ENABLE_TESTS=OFF -DHWY_ENABLE_EXAMPLES=OFF -DHWY_ENABLE_CONTRIB=OFF -DMLLM_CPU_BACKEND_COMPILE_OPTIONS="-march=native" -DMLLM_KERNEL_USE_THREADS=ON -DMLLM_KERNEL_THREADS_VENDOR_OPENMP=ON -DMLLM_KERNEL_USE_THREADS_VENDOR_MLLM=OFF
• cmake --build build-xxhash-pic --target MllmRT -j2

Summary by CodeRabbit

• Chores
  • Updated internal build configuration for improved code compatibility.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 90b587e0-4d31-4c36-9d45-96cd46b04aba

📥 Commits

Reviewing files that changed from the base of the PR and between b9c3070 and d061406.

📒 Files selected for processing (1)

• third_party/xxHash/CMakeLists.txt

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📝 Walkthrough

Walkthrough

This PR enables position-independent code compilation for the xxHash library's object library target by setting the POSITION_INDEPENDENT_CODE property to ON in its CMake configuration.

Changes

Position-Independent Code Configuration

Layer / File(s)	Summary
Enable PIC for xxhash_impl third_party/xxHash/CMakeLists.txt	xxhash_impl object library is configured to build with position-independent code via set_target_properties.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 A single line, so neat and small,
Makes hashes dance without a wall,
Position-independent now they stand,
Built with care by rabbit's hand,
Flexibility in every byte! 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: enabling position-independent code compilation for xxHash objects to fix a linking issue with MllmRT.
Description check	✅ Passed	The description provides comprehensive context including summary, background, root cause analysis, fix explanation, and validation steps, exceeding the minimal template requirement.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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