[PATCH 0/1] riscv: Use Zbkb pack* in repeat_bytes for

[Pincheng Wang]

This patch introduces a RISC-V specific helper in string-misc.h that
leverages the Zbkb extension to optimize the construction of broadcast
byte masks used by memchr and memrchr. The optimization replaces the
generic multiply-based idiom with a sequence of pack* instructions,
reducing instruction count and avoiding the high-latency mul
instruction.

Motivation

Functions such as memchr and memrchr call repeat_bytes to prepare a word
with every byte set to the target character. In the generic
implementation this is done by multiplying the input byte with the
constat 0x010101..., which requires a wide constant load and a 64-bit
multiply. On RISC-V targets that provide Zbkb, we can use packh, packw,
and pack (RV64) or packh+pack (RV32) to achieve the same broadcast with
fewer and cheaper instructions

Static analysis

- RV64: generic path requires ~4 instructions including a mul; optimized
  path requires 3 simple pack* instructions.
- RV32: generic path requires a 32-bit multiply; optimized path requires
  2 pack instructions.
- The critical path is shorter and avoids mul, which often has 3-5 cycle
  latency.
- The design reduces front-end pressure (no large immediate load) and
  makes the sequence more predictable for the pipeline.

Benchmarking (emulation-only; directional evidence)

Due to hardware limitations, we conducted experiments under QEMU.
- Framework: glibc benchtests, slightly modified to also report retired
  instructions.
- Metric: geometric mean reduction in retired instructions.
- Result: 13.275% reduction with 95% CI [12.212%,14.432%].
- Methodology: QEMU was run with deterministic settings (e.g, -icount
  arg) to stablilize runs.

Interpretation:

- Retired instructions provide a structural, frequency-agnostic metric:
  fewer instructions for the same semantics means less work.
- When CPI does not worsen (which is the case here, since pack* is
  cheaper than mul), this maps to real-time improvements.
- Gains are expected mainly for small-size buckets; for large scans,
  memory bandwidth dominates and both paths converge.

We therefore treat these results as directional evidence. Absolute
speedup on real Zbkb hardware will be confirmed with PMU counters and
per-size breakdowns.

Compatibility

- ABI and API remain unchanged
- The optimization is guarded by __riscv_zbkb macros; systems without
  Zbkb continue to use the generic path.
- Builds with -march lacking Zbkb are unaffected.

Patch layout

- 1/1: riscv: add Zbkb optimized repeat_bytes helper in string-misc.h

Pincheng Wang (1):
  riscv: Add Zbkb optimized repeat_bytes helper

 sysdeps/riscv/string-misc.h | 64 +++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)
 create mode 100644 sysdeps/riscv/string-misc.h

-- 
2.39.5