[thweetkomputer]

Fixed-buffer reads and writes pass the 24-byte FixedBuffer descriptor by value through Worker, StorageBackend and the selected backend. GCC retains repeated caller-side stack copies, including when storage uses SPDK. Pass the descriptor by const& throughout synchronous submission to remove those copies.

The descriptor is borrowed only until submission returns. io_uring retains its value-capturing preparation lambda and queues prepared SQEs; SPDK snapshots request fields into its pending requests. Neither backend queues the descriptor reference. Awaitables continue to own their descriptors by value, and data buffers and registrations must still survive through completion.

Add io_uring and SPDK stub regressions that reuse and clear the descriptor after submitting fixed reads/writes under backpressure. They verify the saved buffer address, size, offset, completion result and, for io_uring, a nonzero registered-buffer index.

Validation:

cmake --build build-testcases --parallel 4
ctest --test-dir build-testcases --output-on-failure
cmake --build build-testcases-bypass --parallel 4
ctest --test-dir build-testcases-bypass --output-on-failure
pre-commit run

Test configurations use CMAKE_BUILD_TYPE=Debug and BYCORF_BUILD_EXAMPLES=OFF; the bypass build additionally uses BYCORF_KERNEL_BYPASS=ON. All 96 normal and 123 bypass-capable tests passed; formatting and whitespace checks passed.

GCC 13.3/AArch64 static inspection at -O3 confirms the caller-side descriptor copies disappear in both configurations. Without LTO, the complete SizeIoAwaitable::await_suspend instruction count falls from 138 to 125 in the normal build and from 159 to 150 in the bypass-capable build. SPDK fixed-read/write callee counts remain unchanged; io_uring callees grow by two static instructions, so caller-only reductions are not total-path counts. In an isolated LTO link of actual storage/network/io_uring translation units, the storage await-suspend harness frame shrinks from 176 to 96 bytes. This is code-generation evidence, not a hardware throughput or latency measurement; SPDK LTO was not measured.

Ordinary calls, including temporary descriptors, remain source-compatible. The submission signatures and corresponding member-function-pointer types change, so dependent binaries must be rebuilt. Ownership, queue ordering and backend dispatch remain unchanged.

[coderabbitai]

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