[Bug c/34261] New: Directed rounding doesn't work on MacOS X

["j dot d dot pryce at ntlworld dot com" <gcc-bugzilla at gcc dot gnu dot org>]

I research in interval arithmetic. I bought a MacBook Pro in Spring 07 (
MacBookPro2,2; Intel Core 2 Duo 2.33 GHz; Mac OS X 10.4.11 (8S2167), Darwin
8.11.1). 
Within MATLAB (Version 7.4.0.287 (R2007a) Jan 2007) I wanted to use Rump's
interval library INTLAB. On startup, INTLAB detected the needed changing of
rounding-mode wasn't working, and I couldn't fix it. 
So I tried a simple C program to check this with gcc as follows
/*--------roundtest.cpp--------*/
#include <iostream>
#include <fenv.h>
using namespace std;
int main(int argc, char* argv[]) {
  int flag;
  double x, y;
  double z0, z1, z2, z3;
  x = atof(argv[1]);
  y = atof(argv[2]);

  flag = fesetround(0); // round toward 0
  fprintf(stdout, "fesetround(0) returned %i\n", flag);
  fprintf(stdout, "Then fegetround() returned %i\n", fegetround());
  z0 = x/y;
  flag = fesetround(1); // round to nearest
  fprintf(stdout, "fesetround(1) returned %i\n", flag);
  fprintf(stdout, "Then fegetround() returned %i\n", fegetround());
  z1 = x/y;
  flag = fesetround(2); // round toward +oo
  fprintf(stdout, "fesetround(2) returned %i\n", flag);
  fprintf(stdout, "Then fegetround() returned %i\n", fegetround());
  z2 = x/y;
  flag = fesetround(3); // round toward -oo
  fprintf(stdout, "fesetround(3) returned %i\n", flag);
  fprintf(stdout, "Then fegetround() returned %i\n", fegetround());
  z3 = x/y;

  fesetround(1); // round to nearest
  fprintf(stdout, "z0=%20.18f, z1=%20.18f, z2-z1=%g, z1-z3=%g\n",
          z0, z1, z2-z1, z1-z3);

  return 0;
}
/*--------------*/
I compiled with this (I am a C++ beginner, for some reason "gcc" compiles but
can't link):
johnpryce 49$ g++ -frounding-math -Wall roundtest.cpp -o roundtest

Then I ran with this, for instance
johnpryce 54$roundtest 1 3
fesetround(0) returned 0
Then fegetround() returned 0
fesetround(1) returned 1
Then fegetround() returned 0
fesetround(2) returned 1
Then fegetround() returned 0
fesetround(3) returned 1
Then fegetround() returned 0
z0=0.333333333333333315, z1=0.333333333333333315, z2-z1=0, z1-z3=0

With correct rounding, z2 and z3 MUST be different, and z1 equals one of them.
But the fesetround/fegetround output seems to show
(a) Rounding mode can't be changed
(b) It is always toward zero (i.e. oldfashioned IBM360 style chopping) if this
output is to be believed. This violates the IEEE754 standard, which of course
mandates rounding to nearest as the default.

Actually I think rounding to nearest is what DOES happen - big number-crunching
calculations seem too accurate to be consistent with chopping - so the
fesetround/fegetround functions must be wrong.


-- 
           Summary: Directed rounding doesn't work on MacOS X
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: j dot d dot pryce at ntlworld dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34261