[Bug tree-optimization/52907] New: Underflowing floating point expressions wrongly folded to zero

[Bug tree-optimization/52907] New: Underflowing floating point expressions wrongly folded to zero

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52907

             Bug #: 52907
           Summary: Underflowing floating point expressions wrongly folded
                    to zero
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jsm28@gcc.gnu.org


fold-const.c:const_binop avoids folding overflowing floating-point operations
on constant operands to constants if flag_trapping_math:

      /* Don't constant fold this floating point operation if
         the result has overflowed and flag_trapping_math.  */
      if (flag_trapping_math
          && MODE_HAS_INFINITIES (mode)
          && REAL_VALUE_ISINF (result)
          && !REAL_VALUE_ISINF (d1)
          && !REAL_VALUE_ISINF (d2))
        return NULL_TREE;

However, there is no such check for underflow.  This has the effect of
miscompiling various code in glibc's libm that expects operations on constant
values to be usable to produce an overflow or underflow exception when
required.  In particular, this appears to cause
<http://sourceware.org/bugzilla/show_bug.cgi?id=10846>.  Testcase, tested x86
and x86_64 (-O2 -ftrapping-math, wrongly compiled to return 0 rather than doing
the given computation):

static const double a = 1e-300;
double f (void) { return a * a; }

[Bug tree-optimization/52907] Underflowing floating point expressions wrongly folded to zero

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52907

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-04-10
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-04-10 12:18:12 UTC ---
Does real.c even communicate this fact?

[Bug tree-optimization/52907] Underflowing floating point expressions wrongly folded to zero

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52907

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-04-23 20:02:50 UTC ---
On Tue, 10 Apr 2012, rguenth at gcc dot gnu.org wrote:

> Does real.c even communicate this fact?

You can identify underflow from the result of multiplication or division 
being zero or subnormal (unless an operand is zero or infinite), just as 
overflow is identified from an infinite result.  Information about 
subnormal results may not be conveniently communicated by real.c, but 
simply handling zero results here would be enough for the use cases in 
glibc.

[Bug middle-end/52907] Underflowing floating point expressions wrongly folded to zero

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52907

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2012-04-10 00:00:00         |2024-3-10

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Still happens.