On 11/03/2010 05:10 PM, Eric Blake wrote: > I'm running out of time today to get to the bottom of this, so if anyone > else wants to help out, great. If not, I'll see if I can resume testing > tomorrow and get a minimal test case that exposes the compiler bug, as > well as try and isolate ways to work around it. > > I'm on a Linux x86_64 machine with icc (ICC) 10.0 20070426. > > I hate it when a heisenbug disappears because of the mere act of adding > debugging hooks. :(
I've reduced it to a minimal testcase:
$ cat foo.c
#define _GNU_SOURCE 1
#include <assert.h>
#include <math.h>
double zero;
double bar (void) { return -zero; }
int main (void)
{
assert (!signbit (zero));
assert (signbit (bar ()));
return 0;
}
$ icc -g -o foo foo.c
$ ./foo
$ icc -o foo foo.c
$ ./foo
foo: foo.c:9: main: Assertion `(( sizeof( bar () ) > sizeof( double )) ?
__signbitl( (long double)(bar ()) ) : (( sizeof( bar () ) == sizeof(
float )) ? __signbitf( (float)(bar ()) ) : __signbit( (double)(bar ())
)))' failed.
Aborted (core dumped)
$
Looks like icc with debugging computes -0.0 correctly, but without -g it
makes a mistaken optimization. Comparing the assembly, I see:
with -g:
Dump of assembler code for function bar:
0x00000000004005f8 <+0>: push %rbp
0x00000000004005f9 <+1>: mov %rsp,%rbp
0x00000000004005fc <+4>: sub $0x10,%rsp
0x0000000000400600 <+8>: fldl 0x20087a(%rip) # 0x600e80 <zero>
0x0000000000400606 <+14>: fchs
0x0000000000400608 <+16>: fstpl -0x10(%rbp)
0x000000000040060b <+19>: movsd -0x10(%rbp),%xmm0
0x0000000000400610 <+24>: leaveq
0x0000000000400611 <+25>: retq
without -g:
Dump of assembler code for function bar:
0x0000000000400b3a <+0>: pxor %xmm0,%xmm0
0x0000000000400b3e <+4>: subsd 0x203542(%rip),%xmm0 #
0x604088 <zero>
0x0000000000400b46 <+12>: retq
0x0000000000400b47 <+13>: nop
Gnulib's testsuite already has several workarounds for computing -0.0 in
spite of buggy compilers, so I think it's just a matter of making all
the tests uniformly use those same workarounds.
In particular, I tested that this works for icc regardless of whether
CFLAGS includes -g (borrowing from test-ceill.c's workarounds for -0.0L
as my starting point):
#if defined __hpux || defined __sgi || defined __ICC
static double
compute_minus_zero (void)
{
return -DBL_MIN * DBL_MIN;
}
# define minus_zero compute_minus_zero ()
#else
double minus_zero = -0.0;
#endif
--
Eric Blake ebl...@redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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