[Bug target/12639] long double comparison is broken

[christopher dot eltschka at physik dot uni-regensburg dot de]

--- Comment #3 from christopher dot eltschka at physik dot uni-regensburg 
dot de  2005-10-27 09:11 ---
I just stumbled on the same problem, but with much more conventional numbers.
The following C++ test program demonstrates it:

#include 

// this just serves to calculate a positive number which the optimizer
// will not figure out at compile time
long double f()
{
  long double value = 1.0;
  for (int i = 0; i < 10; ++i)
value += (long double)i;
  return value;
}

int main()
{
  long double a = f();
  std::cout << a;
  if (a > 0)
std::cout << " > 0\n";
  else if (a < 0)
std::cout << " < 0\n";
  else if (a == 0)
std::cout << " == 0\n";
  else
std::cout << "oops!\n";
}

Compiling and running gives:
> g++ longdouble.cc -O3 && a.out
46 == 0

which is clearly wrong. The same result happens with -O2 and -O1. With -O0 I
correctly get the output 46 > 0.

The compiler is:

> g++ -v
Reading specs from /usr/local/lib/gcc-lib/alphaev67-dec-osf5.1/3.3/specs
Configured with: ../gcc-3.3/configure --with-libiconv-prefix=/usr/local
--with-included-gettext
Thread model: single
gcc version 3.3

As workaround for == 0, !! seems to work. Unfortunately I don't know a
workaround for > 0 and < 0.


-- 

christopher dot eltschka at physik dot uni-regensburg dot de changed:

   What|Removed |Added

 CC||christopher dot eltschka at
   ||physik dot uni-regensburg
   ||dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12639

[Bug target/12639] long double comparison is broken

[christopher dot eltschka at physik dot uni-regensburg dot de]

--- Comment #4 from christopher dot eltschka at physik dot uni-regensburg 
dot de  2005-10-27 10:38 ---
(In reply to comment #3)
> As workaround for == 0, !! seems to work. Unfortunately I don't know a
> workaround for > 0 and < 0.
> 

Sorry for self-reply, but now I've found a simple workaround for all cases:
Instead of comparing with 0, just add 1 and compare with 1.
For some reason this works even if the number to compare with happens to be -1
(i.e. if after adding 1 you get 0).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12639

[Bug target/12639] long double comparison is broken

[belyshev at depni dot sinp.msu.ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12639

Serge Belyshev  changed:

   What|Removed |Added

   Keywords||wrong-code
 Status|REOPENED|RESOLVED
 CC||belyshev at depni dot
   ||sinp.msu.ru
  Known to work||4.2.4, 4.3.5, 4.4.5, 4.5.2
 Resolution||FIXED
   Severity|critical|normal

--- Comment #5 from Serge Belyshev  
2011-03-05 18:37:52 UTC ---
Works on ev56 and ev67 linux-gnu with gcc 4.2 .. 4.5, assuming fixed.