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Fix assertion failure with sub-8B kernel arguments#34

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Fix assertion failure with sub-8B kernel arguments#34
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copilot/fix-sub-8b-kernel-arguments

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Copilot AI commented Mar 16, 2026 •
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When a kernel has pointer arguments followed by sub-8B arguments (e.g. int), the injected dh_comms* in the instrumented clone lands at a 4-byte-aligned offset, so the explicit args end at a non-8-byte-aligned boundary (e.g. 44 bytes). The old roundArgsLength() call rounded that up to 48, overshooting the actual hidden-args start and causing:

assert(desc.clone_hidden_args_length <= desc.kernarg_length - desc.explicit_args_length)
// e.g. assert(72 <= 116 - 48)  →  assert(72 <= 68)  →  ABORT

Reproducer:

__global__ void hgemm_kernel(const __half* A, const __half* B, __half* C,
                              int M, int N, int K)  // ← int args trigger the bug

Changes

  • src/utils.cc — KernelArgHelper::computeKernargData: use the offset of the first hidden argument from the kernel metadata as explicit_args_length. This is the exact explicit/hidden boundary as encoded by the compiler — no rounding needed. roundArgsLength is retained only for kernels with no hidden arguments (e.g. some Triton kernels), where it is both harmless and preserves existing behaviour.

  • tests/test_kernels/sub8b_args_test.cpp: regression kernel with (const int*, const int*, int*, int M, int N, int K) arguments — same layout as the failing kernel above.

  • tests/test_kernels/CMakeLists.txt, tests/run_basic_tests.sh: wire the new kernel into the build and basic test suite.

Original prompt

This section details on the original issue you should resolve

<issue_title>Error with sub-8B kernel arguments</issue_title>
<issue_description>Possibly related to this comment in intercept.cc,

it seems like Omniprobe returns the following when each kernel argument is smaller that 8 Bytes.

>>>>>>>> HSA intercept registered.
ADDRESS_MESSAGE,timestamp,kernel,src_line,dispatch,exec_mask,xcc_id,se_id,cu_id,kind,address
host_device_combined_omniprobe: /app/omniprobe/src/interceptor.cc:649: void hsaInterceptor::fixupKernArgs(void *, void *, void *, arg_descriptor_t): Assertion `desc.clone_hidden_args_length <= desc.kernarg_length - desc.explicit_args_length' failed.

This was encountered with the following kernel argument - when 32b integer arguments are used,

__global__ void hgemm_kernel(const __half* __restrict__ A,
                             const __half* __restrict__ B,
                             __half* __restrict__ C,
                             int M, int N, int K)

but not encountered (proceeds to instrumentation) with the following kernel argument - when replaced with 64b integers,

__global__ void hgemm_kernel(const __half* __restrict__ A,
                             const __half* __restrict__ B,
                             __half* __restrict__ C,
                             int64_t M, int64_t N, int64_t K)
```</issue_description>

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

<comments>
<comment_new><author>@rwvo</author><body>
This may have been fixed already; I looked into kernel argument alignment before. Should be testable by writing a kernel that has similar arguments as the one above with the three int arguments, launch it with a single block of 64 threads, let thread 0 printf the values of the integer arguments. Then instrument it using instrumentation scoping such that nothing is actually instrumented, but a cloned kernel with an extra void* argument is created. Then run with and without instrumentation under omniprobe. Both runs should print the same values.</body></comment_new>
</comments>

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@rwvo
Fix assertion failure with sub-8B kernel arguments
2dcf6c1 
In KernelArgHelper::computeKernargData(), the explicit_args_length was
computed as roundArgsLength(last_explicit_arg_offset + size), rounding up
to the next 8-byte boundary.  When a kernel has pointer args followed by
32-bit int args (e.g. hgemm_kernel(half*, half*, half*, int, int, int)),
the last explicit arg in the instrumented kernel (dh_comms*) is placed at a
4-byte-aligned offset.  The rounded explicit_args_length overshot the actual
hidden-args boundary, so kernarg_length - explicit_args_length became smaller
than clone_hidden_args_length, triggering:

  assert(desc.clone_hidden_args_length <=
         desc.kernarg_length - desc.explicit_args_length)

Fix: use the offset of the FIRST hidden argument from the kernel metadata
as explicit_args_length.  This is the exact explicit/hidden boundary with no
rounding required.  Fall back to roundArgsLength only for kernels that have
no hidden arguments (e.g. some Triton kernels), where rounding is harmless.

Also:
- Add sub8b_args_test.cpp regression test kernel (ptr + int args)
- Wire it into CMakeLists.txt and run_basic_tests.sh

Co-authored-by: rwvo <21990117+rwvo@users.noreply.github.com>
Copilot AI changed the title [WIP] [ISSUE-123] Fix error with sub-8B kernel arguments Fix assertion failure with sub-8B kernel arguments Mar 16, 2026
Copilot AI requested a review from rwvo March 16, 2026 20:18
Copilot finished work on behalf of rwvo March 16, 2026 20:18
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Error with sub-8B kernel arguments

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