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: unassig...@gcc.gnu.org
ReportedBy: vbyakov...@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.
