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Re: Anyone working on 64-bit octave

From: Thomas D. Dean
Subject: Re: Anyone working on 64-bit octave
Date: Fri, 17 Feb 2012 14:03:04 -0800
User-agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:9.0) Gecko/20120126 Thunderbird/9.0

On 02/17/12 12:25, Jordi GutiƩrrez Hermoso wrote:
On 17 February 2012 15:12, Matyas Sustik<address@hidden>  wrote:

I am looking at building BLAS and LAPACK with

    gfortran -fdefault-integer-8 and -default-real-8

This makes double precision 128 bits.

My search turned up the following that may help:

If you got it working, let me (us) know...

Note that even if you do get this working, essentially all of the
Octave source assumes double precision. There are no quadruple
precision types in Octave.

More investigation shows this to be a really hard problem. The state of computing with household PC's is not equal to the task. Maybe in a few years when compilers catch up with hardware.

sizeof(long double) returns 16(bytes) == 128-bits. But, the gcc libs, etc., change this into 80-bits precision.

The __float128 type will cause gcc46 to issue 128-bit instructions. But, to do i/o, special function calls must be made. And, libquadmath linked.

I fear this would lead to non-standard variable definitions in the C code and lead to a non-supportable situation when compilers catch up.

On FreeBSD and most likely any system that uses gcc46 or earlier:

As it turns out, gcc46 does not support the full precision of long double. Or, is it just because the port lang/gcc46 uses the system libc, which is gcc 4.2.1?

#include <quadmath.h>
#include <stdio.h>
int main() {
  __float128 f128;
  long double ld;
  char s[512];

  f128 = (__float128)1.0;
  f128 += ((__float128)2.0) * FLT128_EPSILON;
  ld = (long double)f128;

  quadmath_snprintf(s, sizeof s, "%.70Qe", f128);
  if (ld != f128) printf("not equal\n");

  return 0;

> env | grep LD
> which gcc46
> locate

> gcc46 -march=corei7 -msse2 -Wall quadtest.c -o quadtest -lquadmath
> ./quadtest
not equal


The conversion from __float128 to long double uses 80-bit floating point registers.

In reality, long double does not provide 16 bytes precision as reported by sizeof(), but, is 80-bits, like the original 386 FPU's.

gcc docs, in discussing 128-bit floats:

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a `long double'.

Tom Dean

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