[PATCH v6 0/2] riscv: lib: add optimized memcmp() and extend KUnit tests

[Milan Tripkovic <milant2002@gmail.com>]

This v6 series provides an optimized RISC-V memcmp() implementation 
along with extended KUnit tests for functional verification and 
benchmarking

The memcmp() implementation now uses word-aligned loads for the generic 
path and leverages Zbb for fast sign resolution on supported hardware. 
A robust fallback to byte-by-byte comparison is implemented for non-Zbb 
systems to ensure correctness and stability across different toolchains 
and hardware revisions.

Signed-off-by: Milan Tripkovic <Milan.Tripkovic@rt-rk.com>

---
v6 changes:
    - Fixed build errors kernel test robot noticed. 
    - Link to v5: https://lore.kernel.org/all/20260519140029.1190381-1-milant2002@gmail.com/
    
v5 changes:
    - Removed redundant warm-up from test case
    - Optimized generic memcmp path with word-aligned loads.
    - Implemented non-ZBB fallback to byte comparison per review.
    - Link to v4: https://lore.kernel.org/all/20260518131407.1026049-2-milant2002@gmail.com/

v4 changes:
    - Fixed build error: call to undeclared function 'STRING_BENCH'
    - Link to v3: https://lore.kernel.org/all/20260515141019.1234037-1-milant2002@gmail.com/

v3 changes:
    - Split memcmp benchmark into wrapper (string_bench_memcmp) and worker
      function (do_string_bench_memcmp).
    - Removed all C99 mixed declarations; moved all variable declarations
      to the top of each function.
    - Converted len, iterations and loop counters in the benchmark to u64
      to avoid implicit casts.
    - Cleaned up spacing, indentation and minor style issues.
    - Added #if defined(CONFIG_RISCV_ISA_ZBB)... in  memcmp.S
    - Link to v2: https://lore.kernel.org/all/20260514121359.931999-1-milant2002@gmail.com/

v2 changes:
    - Added alignment checks for buffers to avoid expensive misaligned loads.
    - Optimized the loop using end-pointers to reduce per-iteration overhead.
    - Implemented word-aligned tail handling using ZBB shifts.
    - Removed redundant pointer equality (a0 == a1) check.
    - Retained BE support via #ifndef; ZBB rev8 is used for the LE fast-path.
    - Fixed KUnit build failures for Clang and non-benchmark configs.
    - Link to v1: https://lore.kernel.org/all/20260512141007.1193033-1-milant2002@gmail.com/

Milan Tripkovic (2):
  riscv: lib: add memcmp() implementation
  lib/string_kunit: extend benchmarks and unit test to memcmp()

 arch/riscv/include/asm/string.h |   2 +
 arch/riscv/lib/Makefile         |   1 +
 arch/riscv/lib/memcmp.S         | 107 ++++++++++++++++++++++++++++++++
 arch/riscv/purgatory/Makefile   |   5 +-
 lib/tests/string_kunit.c        | 106 +++++++++++++++++++++++++++++++
 5 files changed, 220 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/lib/memcmp.S

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2.43.0