[cfe-dev] default setting of -fmath-errno

[Jay Foad]

Hi,

I was surprised by a difference between clang and gcc. On gcc, the
docs for -fno-math-errno say:

"This option is not turned on by any -O option since it can result in
incorrect output for programs which depend on an exact implementation
of IEEE or ISO rules/specifications for math functions."

But with clang, it seems that when I turn on any optimization (-O1 or
higher), I *do* get the effect of -fno-math-errno. Is this
intentional?

There's a test case below. If I compile with "clang -O" and run it, I
get "result -nan, errno 0". I'm using the 3.0rc3 release on
Linux/x86_64.

Thanks,
Jay.


#include <errno.h>
#include <math.h>
#include <stdio.h>

int main(int argc, const char *argv[])
{
	double d;
	errno = 0;
	d = sqrt(argc - 5);
	printf("result %g, errno %d\n", d, errno);
	return 0;
}