Re: calling sincosf on subnormal argument is wrong

["Joseph S. Myers" <joseph at codesourcery dot com>]

On Thu, 16 Aug 2012, Dmitrieva Liubov wrote:

> The reason of fails is that the ccos, ccosh, cexp, csin, csinh
> functions must not raise Underflow, because result is not subnormal.
> But all they call to sincosf(x) on x=0x1p-149, that now raises
> Underflow (new sincosf is correct here).
> To fix the problems, the mentioned functions (ccos,etc) must not call
> sincosf on subnormal argument.

See what I said in 
<http://sourceware.org/ml/libc-alpha/2012-07/msg00080.html> about issues 
with rounding of results of sincos in directed rounding modes - the two 
issues (rounding yielding inaccurate results, and spurious underflows) are 
related, but the issues appeared to be latent.  (I mentioned ctan/ctanh 
there among the functions calling sincos internally, but actually it looks 
like they really will have an underflowing result when the input to sincos 
is subnormal, so the only issue for them may be missing underflow 
exceptions from sincos resulting in missing underflow exceptions from 
ctan/ctanh.)

Note that *some* cases of the affected functions calling sincos *should* 
underflow - the final results may or may not be underflowing, but when 
they are, the exception should be raised.  But such cases (where all the 
intermediate calculations being exact means underflow exceptions are 
missing) would be separate bugs, already present in the current code, so 
could have bugs filed to be fixed separately.

As usual, fixes should fix functions for all floating-point types at the 
same time - so change all of cexpf, cexp, cexpl in one patch to use (x, 1) 
instead of sincos (x), for example.  (This should be done for the least 
normal x as well as for subnormal x, e.g. fabs (x) <= DBL_MIN; whether sin 
(DBL_MIN) should raise underflow is architecture-dependent.)  Since the 
bugs arising from calling sincos are currently latent, no need to file 
them in Bugzilla before sending a patch - but the bugs of underflow 
exceptions being missing (when the final result still has at least one 
underflowing part) *should* be filed (as should the problem of most sin 
and sincos implementations not raising underflow when appropriate).

-- 
Joseph S. Myers
joseph@codesourcery.com