Gcc46 and 128 Bit Floating Point

[Bruce Evans]

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