public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug c/50195] New: Linking time eroor with -fast-math -O0
@ 2011-08-26 13:22 vbyakovl23 at gmail dot com
  2011-08-26 13:26 ` [Bug middle-end/50195] [4.7 Regression] Linking time error " rguenth at gcc dot gnu.org
                   ` (5 more replies)
  0 siblings, 6 replies; 7+ messages in thread
From: vbyakovl23 at gmail dot com @ 2011-08-26 13:22 UTC (permalink / raw)
  To: gcc-bugs

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50195

             Bug #: 50195
           Summary: Linking time eroor with -fast-math -O0
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vbyakovl23@gmail.com


Following test fails in linking if compiled with ffast-math and O0,
but it compiled successfully with ffast-math and O2. Also no problem
if -lm is added.

$ cat t.c
#include <stdio.h>

float foo(float x)
{
  float y = 0;
  while (x > 0.00000001) {
    y += x*x*x*x*x*x*x*x*x*x*x*x*x;
    x = x/2;
  }
  return y;
}

int main (int argc, char    *argv[])
{
 float y = atoi(argv[1]);
 printf("%f\n", foo(y));
 return 0;
}


$ gcc  -ffast-math -O0   t.c
/tmp/cccA1sUB.o: In function `foo':
t.c:(.text+0x2c): undefined reference to `powf'
collect2: error: ld returned 1 exit status
$ gcc  -ffast-math -O2   t.c
$ ./a.out 5
1220852096.000000


FE with -ffast-math replaced x*x*...*x with __builtin_powf. Later with
-O2 this call is replaced back into multiplications in sincos phase.
The stability with -O0 is because sincos phase doesn't work on -O0.

I think we must avoid doing this optimization in FE and turn off
-ffast-math if -O0 is used. 

>From Richard Guenther:
No, I think we should avoid most of the builtin related folding at -O0.


^ permalink raw reply	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2011-10-10 16:26 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-08-26 13:22 [Bug c/50195] New: Linking time eroor with -fast-math -O0 vbyakovl23 at gmail dot com
2011-08-26 13:26 ` [Bug middle-end/50195] [4.7 Regression] Linking time error " rguenth at gcc dot gnu.org
2011-10-10 12:24 ` rguenth at gcc dot gnu.org
2011-10-10 13:04 ` rguenth at gcc dot gnu.org
2011-10-10 14:27 ` rguenth at gcc dot gnu.org
2011-10-10 15:48 ` rguenth at gcc dot gnu.org
2011-10-10 16:26 ` hubicka at ucw dot cz

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).