[PATCH v2 0/5] math: Optimize frexp functions with fast path

[Osama Abdelkader]

This patch series optimizes the frexp family of functions by adding a fast
path for normal floating-point numbers, which represent the vast majority of
real-world usage.

Changes in v2:
- Use arithmetic approach instead of bit masking (better on ARM/RISC-V)
- Removed ancient 1995 copyright and rcsid tags (significant rewrite)
- Use stdc_leading_zeros for subnormal normalization (float/double)
- Simplified zero/inf/NaN check to (int32_t)(hx << 1) <= 0
- Added separate benchmark workloads for zero and denormal cases
- All tests pass including subnormal edge cases

The optimization uses a single unsigned comparison (ex - 1) < MAX_NORMAL_EXP
to identify normal numbers and returns immediately via arithmetic on the bit
representation, avoiding branches and complex operations.

Benchmark results on Intel Core i9-13900H (13th Gen):

  Function     Baseline    Optimized   Speedup    Workload Breakdown
  --------     --------    ---------   -------    ------------------
  frexp        6.778 ns    4.007 ns    1.69x      Zero: 3.58ns, Denormal: 6.10ns
  frexpf       5.858 ns    4.003 ns    1.46x      Zero: 3.58ns, Denormal: 5.60ns
  frexpl      25.543 ns   25.531 ns    1.00x      Zero: 17.8ns, Denormal: 23.9ns

The optimization provides good improvements for float (31.7%) and double
(40.9%) precision. All existing tests pass, maintaining exact correctness for
edge cases including zero, subnormal, infinity, and NaN values.

The arithmetic approach generates better code than bit masking on ARM (fused
shift-subtract) and RISC-V (avoids loading large constants), as verified via
godbolt compiler explorer.

Suggested-by: Wilco Dijkstra <wilco.dijkstra@arm.com>

Osama Abdelkader (5):
  benchtests: Add benchmarks for frexp functions
  math: Optimize frexpf (binary32) with fast path for normal numbers
  math: Optimize frexp (binary64) with fast path for normal numbers
  math: Optimize frexpl (intel96) with fast path for normal numbers
  math: Optimize frexpl (binary128) with fast path for normal numbers

 benchtests/Makefile                 |    6 +
 benchtests/frexp-inputs             | 1013 ++++++++++++++++++++++++++
 benchtests/frexpf-inputs            | 1013 ++++++++++++++++++++++++++
 benchtests/frexpl-inputs            | 1014 +++++++++++++++++++++++++++
 sysdeps/ieee754/dbl-64/s_frexp.c    |   55 +-
 sysdeps/ieee754/flt-32/s_frexpf.c   |   82 ++-
 sysdeps/ieee754/ldbl-128/s_frexpl.c |  100 +--
 sysdeps/ieee754/ldbl-96/s_frexpl.c  |  101 +--
 8 files changed, 3221 insertions(+), 163 deletions(-)
 create mode 100644 benchtests/frexp-inputs
 create mode 100644 benchtests/frexpf-inputs
 create mode 100644 benchtests/frexpl-inputs

-- 
2.43.0