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From: "tydeman at tybor dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/53216] New: fmaf() alters rounding mode of sse2 FPU
Date: Thu, 03 May 2012 18:02:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53216

             Bug #: 53216
           Summary: fmaf() alters rounding mode of sse2 FPU
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tydeman@tybor.com


The following code shows that the rounding mode of the sse2 FPU is altered by a
call to fmaf().

#include <stdio.h>
#include <float.h>
#include <math.h>

static  float res0, res1, res2, res3;
static  float half = 0.5f;

void chk( void ){
  res2 = res0 + half;        /* should round down to even */
  if( res2 == res0 ){ puts("OK1"); }else{ puts("Bad1"); }
  res3 = res1 + half;        /* should round up to even */
  if( res3 != res1 ){ puts("OK2"); }else{ puts("Bad2"); }
}

int main(void){

  res0 = FLT_EPSILON;
  res0++;
  res0--;
  res0 = 1.f / res0;        /* even; 1/EPSILON is a magic number */
  res1 = res0 + 1.f;        /* odd */

  chk();

  res2 = sinf( 1.f );        /* sse2 rounding is not altered */

  chk();

  res2 = fmaf( 1.f, 1.f, 1.f );    /* this messes up sse2 rounding */

  chk();

  return 0;
}


Hardware:  Intel Core 2 Duo in 32-bit mode
O.S.    :  Fedora Core 17 in 32-bit mode
Compiler:  gcc 4.7.0-2
Library :  glibc 2.15-32

Compiler options: -std=gnu11 -O0 -mfpmath=sse -msse2 -fno-builtin
-frounding-math -mieee-fp -ffloat-store -fexcess-precision=standard