Re: Fix scalb spurious "invalid" exceptions (bug 16770)

[Rich Felker <dalias at aerifal dot cx>]

On Sat, Mar 29, 2014 at 04:21:23PM +0000, Joseph S. Myers wrote:
> This patch fixes bug 16770, spurious "invalid" exceptions from scalb
> when testing whether the second argument is an integer, by inserting
> appropriate range checks to determine whether a cast to int is safe.
> (Note that invalid_fn is a function that handles both nonintegers and
> large integers, distinguishing them reliably using functions such as
> __rint; note also that there are no issues with scalb needing to avoid
> spurious "inexact" exceptions - it's an old-POSIX XSI function, not a
> standard C function bound to an IEEE 754 operation - although the
> return value is still fully determined.)
> 
> Tested x86_64 and x86.
> 
> 2014-03-29  Joseph Myers  <joseph@codesourcery.com>
> 
> 	[BZ #16770]
> 	* math/e_scalb.c (__ieee754_scalb): Check second argument is not
> 	too large before casting to int.
> 	* math/e_scalbf.c (__ieee754_scalbf): Likewise.
> 	* math/e_scalbl.c (__ieee754_scalbl): Likewise.
> 	* math/libm-test.inc (scalb_test_data): Add more tests.
> 
> diff --git a/math/e_scalb.c b/math/e_scalb.c
> index bddedfa..146d49e 100644
> --- a/math/e_scalb.c
> +++ b/math/e_scalb.c
> @@ -50,7 +50,7 @@ __ieee754_scalb (double x, double fn)
>  	return x;
>        return x / -fn;
>      }
> -  if (__glibc_unlikely ((double) (int) fn != fn))
> +  if (__glibc_unlikely (fabs (fn) >= 0x1p31 || (double) (int) fn != fn))

Off-by-one for the negative case: -0x1p31 is representable as int.
Maybe it doesn't matter anyway, but if it does, you could test
fn>=0x1p31 and fn<-0x1p31 separately.

Rich