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From: "sgk at troutmask dot apl.washington.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libquadmath/65757] gfortran gives incorrect result for anint with real*16 argument
Date: Tue, 14 Apr 2015 17:26:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65757

--- Comment #2 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Tue, Apr 14, 2015 at 01:38:00AM +0000, kargl at gcc dot gnu.org wrote:
> Using -fdump-tree-original (and removing the IO code), 
> one gets
> 
> anintbug ()
> {
>   real(kind=16) q1;
>   real(kind=16) q2;
>   real(kind=16) q3;
> 
>   q1 = 2.33181505644407999969482421875e+14;
>   q2 = roundq (q1);
>   q3 = 2.33181505644408e+14;
> 
> }
> 
> So, gfortran is calling the libquadmath routine roundq.
> I've changed the component from fortran to libquadmath.
> This also appears to fail with all versions of gfortran.
> 

The following patch appears to fix this issue, but I
do not know the internals of __float128_t so the bit
twiddling may not be right.

Index: libquadmath/math/roundq.c
===================================================================
--- libquadmath/math/roundq.c    (revision 222054)
+++ libquadmath/math/roundq.c    (working copy)
@@ -69,7 +69,7 @@ roundq (__float128 x)
     }
   else
     {
-      uint64_t i = -1ULL >> (j0 - 48);
+      uint64_t i = -1ULL >> (j0 - 47);
       if ((i1 & i) == 0)
     /* X is integral.  */
     return x;
@@ -77,7 +77,7 @@ roundq (__float128 x)
       if (huge + x > 0.0)
     {
       /* Raise inexact if x != 0.  */
-      uint64_t j = i1 + (1LL << (111 - j0));
+      uint64_t j = i1 + (1LL << (110 - j0));
       if (j < i1)
         i0 += 1;
       i1 = j;