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* [Bug c++/47600] New: gcc optimizer seems to avoid necessary floating-point addition
@ 2011-02-03 16:37 acfbuerger at googlemail dot com
  2011-02-03 16:49 ` [Bug c++/47600] " rguenth at gcc dot gnu.org
                   ` (5 more replies)
  0 siblings, 6 replies; 7+ messages in thread
From: acfbuerger at googlemail dot com @ 2011-02-03 16:37 UTC (permalink / raw)
  To: gcc-bugs

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47600

           Summary: gcc optimizer seems to avoid necessary floating-point
                    addition
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: acfbuerger@googlemail.com


Created attachment 23237
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23237
assembly and g++ -v output

g++ versions tested: g++-4.5 (Ubuntu/Linaro 4.5.1-7ubuntu2)  4.5.1
                     g++-4.4 (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5

Problem: I have code like this:

double fakeManipulation(double d) { return d; }
...
dist = <about 100>
const double d = <very small number, 1e-16>;
const double ndist = dist+d; // does not work with optimizer
//const double ndist = fakeManipulation(dist+d); // works with optimizer
if( ndist <= dist )
    <avoid endless loop>

The comparison in the if statement is intended exactly like this: check if d is
so small that adding it to dist does not make any difference because of the
limited floating-point precision. I do not know a simpler way to do that which
is equally fast and independent of the actual values of d and dist.

When compiling this code without optimization, the comparison is handled
correctly and the endless loop is avoided.

When compiling with -O or -O2, the comparison is NOT handled correctly and the
program runs into an endless loop. With -O and -O2 and the fakeManipulation
call, it is also handled correctly.

My suspicion is that the optimizer replaces the comparison with (d<=0) which is
not numerically correct. 

The full code is rather lengthy and I did not succeed to write a small
demonstration program, so I attached parts of the generated assembler code.


^ permalink raw reply	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2011-03-08 19:10 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-02-03 16:37 [Bug c++/47600] New: gcc optimizer seems to avoid necessary floating-point addition acfbuerger at googlemail dot com
2011-02-03 16:49 ` [Bug c++/47600] " rguenth at gcc dot gnu.org
2011-02-03 16:56 ` dominiq at lps dot ens.fr
2011-02-03 16:59 ` rguenth at gcc dot gnu.org
2011-02-03 17:56 ` redi at gcc dot gnu.org
2011-02-03 19:03 ` dominiq at lps dot ens.fr
2011-03-08 19:10 ` acfbuerger at googlemail dot com

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