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Re: [PATCH] Use fabs(f/l) rather than __fabs
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>
- Cc: nd at arm dot com
- Date: Tue, 03 Oct 2017 15:32:50 +0100
- Subject: Re: [PATCH] Use fabs(f/l) rather than __fabs
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On 29/09/17 14:02, Wilco Dijkstra wrote:
> diff --git a/sysdeps/ieee754/dbl-64/e_log10.c b/sysdeps/ieee754/dbl-64/e_log10.c
> index dfb7d056caee5f592048559b448c899aa11e9de2..677cbc4df8739521c50a72d5ab657f438c74c129 100644
> --- a/sysdeps/ieee754/dbl-64/e_log10.c
> +++ b/sysdeps/ieee754/dbl-64/e_log10.c
> @@ -65,7 +65,7 @@ __ieee754_log10 (double x)
> if (hx < 0x00100000)
> { /* x < 2**-1022 */
> if (__glibc_unlikely (((hx & 0x7fffffff) | lx) == 0))
> - return -two54 / __fabs (x); /* log(+-0)=-inf */
> + return -two54 / fabs (x); /* log(+-0)=-inf */
> if (__glibc_unlikely (hx < 0))
> return (x - x) / (x - x); /* log(-#) = NaN */
> k -= 54;
> diff --git a/sysdeps/ieee754/dbl-64/e_log2.c b/sysdeps/ieee754/dbl-64/e_log2.c
> index 2f3da129f83dd0dc2fc7da78cfce3804febdbac5..e4a6aff9a3b5aa68c03603bf4cb8bb78507ea9c6 100644
> --- a/sysdeps/ieee754/dbl-64/e_log2.c
> +++ b/sysdeps/ieee754/dbl-64/e_log2.c
> @@ -83,7 +83,7 @@ __ieee754_log2 (double x)
> if (hx < 0x00100000)
> { /* x < 2**-1022 */
> if (__glibc_unlikely (((hx & 0x7fffffff) | lx) == 0))
> - return -two54 / __fabs (x); /* log(+-0)=-inf */
> + return -two54 / fabs (x); /* log(+-0)=-inf */
> if (__glibc_unlikely (hx < 0))
> return (x - x) / (x - x); /* log(-#) = NaN */
> k -= 54;
note that on aarch64 wrong code is generated for these fabs
changes, most likely a gcc bug, still investigating.
it is not visible on aarch64 because the compat wrapper
handles the log(0) case, but without the wrapper (e.g. ilp32)
log(0) in downward rounding becomes +inf instead of -inf,
because fabs(x) is compiled to x-x for x==0 for some reason.