Date: Wed, 29 Feb 2012 19:23:36 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: "Thomas D. Dean" <tomdean@speakeasy.org> Cc: freebsd-amd64@FreeBSD.org Subject: Re: Gcc46 and 128 Bit Floating Point Message-ID: <20120229191750.Y3167@besplex.bde.org> In-Reply-To: <4F4DD942.8070106@speakeasy.org> References: <4F3EA37F.9010207@speakeasy.org> <CAGE5yCpvF0-b1iKAVGbya=fUNaYbGyrpj1PHSQxw4BvycNMLDg@mail.gmail.com> <4F3EC0B4.6050107@speakeasy.org> <4F4DA398.6070703@speakeasy.org> <20120229161408.G2514@besplex.bde.org> <4F4DD942.8070106@speakeasy.org>
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On Tue, 28 Feb 2012, Thomas D. Dean wrote:
> On 02/28/12 22:03, Bruce Evans wrote:
>>
>>> #include <quadmath.h>
>>> #include <stdio.h>
>>> int main() {
>>> char buf[128];
>>> __float128 x = sqrtq(2.0Q);
>>> quadmath_snprintf(buf, sizeof buf, "%.45Qf",x);
>>> printf("sin(%s) = ",buf);
>>> quadmath_snprintf(buf, sizeof buf, "%.45Qf",sinq(x));
>>> printf("%s\n",buf);
>>> return 0;
>>> }
>>>
>>> gcc46 math.c -o math /usr/local/lib/gcc46/libquadmath.a /usr/lib/libm.a
>
>> objdump -d math | grep fsqrt
> 4014fd: d9 fa fsqrt
> 407bb4: d9 fa fsqrt
>
> Comes from the libs.
It's not unreasonable in the libraries. A lower-precision sqrt gives
a good place to start for a Newton approximation method. I wouldn't
have expected fsqrt to be a better place to start that a 64-bit sqrt
using SSE though. SSE also provides 32-bit sqrt and an even
lower-precision but much faster reciprocal square root to start from.
Bruce
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