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

[Osama Abdelkader]

On Thu, Oct 23, 2025 at 06:06:26PM +0300, Osama Abdelkader wrote:
> 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
> 

Hi All,

I wanted to follow up on this patch series sent on Oct 23. I've addressed
the feedback from Wilco, Adhemerval, and Paul:

- ~ 40% performance improvement for double precision
- ~ 32% performance improvement for float
- Arithmetic approach (better on ARM/RISC-V per godbolt)
- All tests passing

Is there anything else needed for this to move forward?

Thanks,
Osama