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Fix XCHG atomic operation to fetch old value per eBPF spec#41

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Officeyutong merged 2 commits intomainfrom
copilot/fix-bpf-conformance-tests
Feb 12, 2026
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Fix XCHG atomic operation to fetch old value per eBPF spec#41
Officeyutong merged 2 commits intomainfrom
copilot/fix-bpf-conformance-tests

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Copilot AI commented Feb 11, 2026 •
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The lock_xchg and lock_xchg32 conformance tests were failing because the atomic exchange operation wasn't storing the old memory value back to the source register.

Changes

  • src/compiler.cpp: Changed is_fetch parameter from false to true in EBPF_ATOMIC_OP_XCHG handler

The eBPF spec defines EBPF_ATOMIC_OP_XCHG as (0xe0 | EBPF_ATOMIC_OP_FETCH), meaning the fetch flag is always set. The operation must atomically exchange values and return the old memory value in the source register.

// Before: old value discarded
case EBPF_ATOMIC_OP_XCHG: {
    emitAtomicBinOp(builder, &regs[0], 
                    llvm::AtomicRMWInst::BinOp::Xchg, inst,
                    inst.opcode == EBPF_ATOMIC_OPCODE_64,
                    false);  // ❌ Missing fetch
    break;
}

// After: old value stored in src register
case EBPF_ATOMIC_OP_XCHG: {
    emitAtomicBinOp(builder, &regs[0],
                    llvm::AtomicRMWInst::BinOp::Xchg, inst,
                    inst.opcode == EBPF_ATOMIC_OPCODE_64,
                    true);   // ✅ Correct per spec
    break;
}

All 34 lock-related conformance tests now pass.

Original prompt

This section details on the original issue you should resolve

<issue_title>[BUG] lock_xchg insn fail in the bpf_conformance test</issue_title>
<issue_description>Describe the bug

Test "./bpf_conformance/tests/lock_xchg.data" failed: Plugin returned incorrect return value 1229782938247303441 expected 0
Test "./bpf_conformance/tests/lock_xchg32.data" failed: Plugin returned incorrect return value 286331153 expected 0
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@Officeyutong</author><body>
![image](https://github.com/user-attachments/assets/78b5ee86-a85a-4e4d-b35f-bf8bbc61e935)

llvm-jit is able to pass these tests in previous version</body></comment_new>
<comment_new><author>@yunwei37</author><body>
That's interesting...maybe they change the testcases</body></comment_new>
</comments>

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@yunwei37
Fix lock_xchg atomic operation to properly fetch old value
8f0079c 
Co-authored-by: yunwei37 <34985212+yunwei37@users.noreply.github.com>
Copilot AI changed the title [WIP] Fix lock_xchg instruction failure in BPF conformance tests Fix XCHG atomic operation to fetch old value per eBPF spec Feb 11, 2026
Copilot AI requested a review from yunwei37 February 11, 2026 04:46
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codecov bot commented Feb 11, 2026 •
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.00%. Comparing base (256540e) to head (8f0079c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files 
@@           Coverage Diff           @@
##             main      #41   +/-   ##
=======================================
  Coverage   79.00%   79.00%           
=======================================
  Files           5        5           
  Lines        1229     1229           
  Branches      134      134           
=======================================
  Hits          971      971           
  Misses        258      258
Flag Coverage Δ
bpf_conformance 63.12% <ø> (ø)
spirv_opencl 41.25% <ø> (ø)
unittests 50.83% <ø> (ø)

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@yunwei37
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yunwei37 commented Feb 12, 2026

can you update the bpf conformance_test so the Ci is actually exam the tests cases? Currently ci will pass without valid the testcases of bpf conformance

@yunwei37 yunwei37 marked this pull request as ready for review February 12, 2026 01:35
Copilot AI review requested due to automatic review settings February 12, 2026 01:35
Copilot AI reviewed Feb 12, 2026
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Pull request overview

This PR fixes a bug in the LLVM JIT compiler where the EBPF_ATOMIC_OP_XCHG operation was not returning the old memory value to the source register as required by the eBPF specification. The fix resolves failures in the lock_xchg and lock_xchg32 conformance tests.

Changes:

  • Changed the is_fetch parameter from false to true in the EBPF_ATOMIC_OP_XCHG handler to comply with the eBPF specification

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@Officeyutong Officeyutong merged commit 18ae036 into main Feb 12, 2026
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This was referenced Feb 23, 2026
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[BUG] lock_xchg insn fail in the bpf_conformance test

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