[Bug-gsl] [bug #44865] bsimp/msbdf e5_bigt in ode-initval2/test.c is FMA-sensitive

[Patrick Alken]

URL:
  <http://savannah.gnu.org/bugs/?44865>

                 Summary: bsimp/msbdf e5_bigt in ode-initval2/test.c is
FMA-sensitive
                 Project: GNU Scientific Library
            Submitted by: psa
            Submitted on: Fri 17 Apr 2015 03:28:16 PM GMT
                Category: Accuracy problem
                Severity: 3 - Normal
        Operating System: 
                  Status: None
             Assigned to: None
             Open/Closed: Open
                 Release: 
         Discussion Lock: Any

    _______________________________________________________

Details:

Hello,

Please CC me directly on any replies because I'm not subscribed to the bugs
list.

When compiling gsl-1.16 using Clang/LLVM trunk for big-endian PPC64/Linux, and
using -ffp-contract=fast, make check reports one test failure, which is in
ode-initval2, with this error:

FAIL: bsimp/msbdf e5_bigt [0] (3.74930479547497807e-49 observed vs
3.75953510319630164e-49 expected) [1126]
FAIL: bsimp/msbdf e5_bigt [1] (6.24102529027298901e-21 observed vs
8.02997493696624951e-21 expected) [1127]
FAIL: bsimp/msbdf e5_bigt [2] (1.55381668382339737e-20 observed vs
1.27123535645485829e-20 expected) [1128]
FAIL: bsimp/msbdf e5_bigt [3] (5.6710668834932872e-65 observed vs
4.65237109886871277e-65 expected) [1129]

When compiling ode-initval2/test.c using -ffp-contract=on or -ffp-contract=off
instead, the test passes. Also, when compiling using -ffp-contract=fast but
placing '#pragma clang optimize off'/'#pragma clang optimize on' around only
the rhs_e5 function, the test also passes. I've examined the assembly produced
for the rhs_e5 (annotated below), and it looks okay. As a result, it seems
likely that this is a sensitivity of the test to the extra FMA precision.

Could the test be modified to make it less sensitive to FMA formation?

Thanks again,
Hal

int
rhs_e5 (double t, const double y[], double f[], void *params)
{
  const double a = 7.89e-10;
  const double b = 1.1e7;
  const double c = 1.13e3;
  const double m = 1.0e6;

  extern int nfe;
  nfe += 1;

  f[0] = -a * y[0] - b * y[0] * y[2];
  f[1] = a * y[0] - m * c * y[1] * y[2];
  f[3] = b * y[0] * y[2] - c * y[3];
  f[2] = f[1] - f[3];

  return GSL_SUCCESS;
}




    _______________________________________________________

Reply to this item at:

  <http://savannah.gnu.org/bugs/?44865>

_______________________________________________
  Message sent via/by Savannah
  http://savannah.gnu.org/