On Tue, 5 Aug 2025, Thirumalai Nagalingam wrote:
> Hi all,
>
> This patch adds support for the `fsqrt` instruction on AArch64 platforms
> in the `__FLT_ABI(sqrt)` implementation.
This looks OK as far as it goes, but I have a few thoughts.
>From the comments, it appears this code originally came from mingw-w64.
Their current version of this code has aarch64 implementations. The
difference with this one is they have a version for float as well as
double. The versions here seem to only be used for long double (which on
aarch64 is the same as double).
Given that long double is the same as double on aarch64, might it make
sense to redirect/alias the long double names to the double
implementations in the def file (cygwin.din) on aarch64, rather than
providing two different implementations (one in newlib for double and one
in this cygwin/math directory for long double)? It seems like that's
asking for subtle discrepancies between the implementations. I'm not
seeing any obvious preprocesor-like operations in gendef (mingw-w64 uses
cpp to preprocess .def.in => .def files for arch-specific #ifdefs) so
maybe this would be more complicated.
>
> In-lined Patch:
>
> From aee895b4b7c4045dea64d1206731dc01d29c155c Mon Sep 17 00:00:00 2001
> From: Thirumalai Nagalingam <thirumalai.nagalin...@multicorewareinc.com>
> Date: Wed, 23 Jul 2025 00:37:09 -0700
> Subject: [PATCH] Cygwin: math: Add AArch64 support for sqrt()
>
> Extend `__FLT_ABI(sqrt)` implementation to support AArch64 using the
> `fsqrt` instruction for double-precision floating point values.
> This addition enables square root computation on 64-bit ARM platforms
> improving compatibility and performance.
>
> Signed-off-by: Thirumalai Nagalingam
> <thirumalai.nagalin...@multicorewareinc.com>
> ---
> winsup/cygwin/math/sqrt.def.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/winsup/cygwin/math/sqrt.def.h b/winsup/cygwin/math/sqrt.def.h
> index 3d1a00908..598614d3a 100644
> --- a/winsup/cygwin/math/sqrt.def.h
> +++ b/winsup/cygwin/math/sqrt.def.h
> @@ -89,6 +89,8 @@ __FLT_ABI (sqrt) (__FLT_TYPE x)
> __fsqrt_internal(x);
> #elif defined(_X86_) || defined(__i386__) || defined(_AMD64_) ||
> defined(__x86_64__)
> asm volatile ("fsqrt" : "=t" (res) : "0" (x));
> +#elif defined(__aarch64__)
> + asm volatile ("fsqrt %d0, %d1" : "=w"(res) : "w"(x));
> #else
> #error Not supported on your platform yet
> #endif
> --
> 2.49.0.windows.1
>
> Thanks,
> Thirumalai Nagalingam
>
>
>
