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Message-ID: <20240612141256.GR10433@brightrain.aerifal.cx>
Date: Wed, 12 Jun 2024 10:12:58 -0400
From: Rich Felker <dalias@...c.org>
To: Meng Zhuo <mzh@....io>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH 2/2] math: add riscv64 round/roundf
On Wed, Jun 12, 2024 at 05:00:18PM +0800, Meng Zhuo wrote:
> ---
> src/math/riscv64/round.c | 21 +++++++++++++++++++++
> src/math/riscv64/roundf.c | 21 +++++++++++++++++++++
> 2 files changed, 42 insertions(+)
> create mode 100644 src/math/riscv64/round.c
> create mode 100644 src/math/riscv64/roundf.c
>
> diff --git a/src/math/riscv64/round.c b/src/math/riscv64/round.c
> new file mode 100644
> index 00000000..12e670a5
> --- /dev/null
> +++ b/src/math/riscv64/round.c
> @@ -0,0 +1,21 @@
> +#include <math.h>
> +
> +#if __riscv_flen >= 64
> +
> +double round(double x)
> +{
> + if (isnan(x)) return x;
> + if (x == INFINITY || x == -INFINITY) return x;
> + double tmp;
> + long long n;
> + __asm__ ("fcvt.l.d %0, %1, rmm" : "=r"(n) : "f"(x));
> + __asm__ ("fcvt.d.l %0, %1" : "=f"(tmp) : "r"(n));
> + x = copysign(tmp, x);
> + return x;
> +}
> +
> +#else
> +
> +#include "../round.c"
> +
> +#endif
> diff --git a/src/math/riscv64/roundf.c b/src/math/riscv64/roundf.c
> new file mode 100644
> index 00000000..8da3c65a
> --- /dev/null
> +++ b/src/math/riscv64/roundf.c
> @@ -0,0 +1,21 @@
> +#include <math.h>
> +
> +#if __riscv_flen >= 32
> +
> +float roundf(float x)
> +{
> + if (isnan(x)) return x;
> + if (x == INFINITY || x == -INFINITY) return x;
> + float tmp;
> + long n;
> + __asm__ ("fcvt.w.s %0, %1, rmm" : "=r"(n) : "f"(x));
> + __asm__ ("fcvt.s.w %0, %1" : "=f"(tmp) : "r"(n));
> + x = copysign(tmp, x);
> + return x;
> +}
> +
> +#else
> +
> +#include "../roundf.c"
> +
> +#endif
> --
> 2.39.2
These functions clearly have the wrong behavior for inputs like
0x1p1000. As a general principle, you can't implement round in terms
of lround/llround, only the opposite direction. You may be able to do
it by first performing a range check, though. For example,
if (!(fabs(x) < 0x1p54)) return x;
Note that this also covers the non-finite cases, but I'm not sure if
it avoids raising exceptions it shouldn't. If not, or if you weren't
doing this, you can collapse the multiple inf/nan checks you have to:
if (!isfinite(x)) return x;
You should also probably use copysignf rather than copysign for
roundf, or do an inline version of it.
Rich
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