Re: Ping Re: Fix IBM long double spurious overflows

[David Edelsohn <dje dot gcc at gmail dot com>]

On Tue, Jan 28, 2014 at 12:51 PM, Joseph S. Myers
<joseph@codesourcery.com> wrote:
> On Mon, 6 Jan 2014, David Edelsohn wrote:
>
>> On Sat, Jan 4, 2014 at 8:16 AM, Joseph S. Myers <joseph@codesourcery.com> wrote:
>> > This patch fixes various cases of spurious overflow exceptions in the
>> > IBM long double support code.  The generic issue is that an initial
>> > approximation is computed by using the relevant arithmetic operation
>> > on the high parts of the operands - but this may overflow double in
>> > some cases where the final result is large but still a long way (up to
>> > around 2^53 ulp) from overflowing long double.  For division overflow
>> > could occur not just from the initial a / c division but also from the
>> > subsequent multiplication of the result by c (in some cases where a is
>> > DBL_MAX, say), when the final result of the division need not be large
>> > at all.
>> >
>> > __gcc_qadd already tried to handle such overflow cases, but detected
>> > them by examining the result of the addition of high parts - which
>> > leaves a spurious overflow exception raised even if it returns the
>> > correct non-overflowing value.  This patch instead checks the operands
>> > and does appropriate scaling, in all of __gcc_qadd, __gcc_qmul and
>> > __gcc_qdiv, to avoid spurious overflow exceptions arising as well as
>> > avoiding the bad results arising from such overflows.
>> >
>> > Tested with no regressions with cross to powerpc-linux-gnu (and also
>> > ran the glibc libm tests, which provide a rather more thorough test of
>> > floating-point arithmetic than the GCC testsuite; this patch fixes, at
>> > least, bad results from cbrtl (LDBL_MAX) that arose from the second
>> > division issue mentioned, as well as the specific cases shown in the
>> > tests added to the GCC testsuite).  OK to commit?
>>
>> Before we go farther down this path, IBM needs to internally decide on
>> the end goal and the amount of language / library conformance that
>> makes sense for the IBM long double format.
>
> Ping.  Original patch:
> <http://gcc.gnu.org/ml/gcc-patches/2014-01/msg00157.html>.

Joseph,

Adding the various tests for overflow slows down some other code where
performance is important.  Explicitly changing rounding mode would be
even more invasive and have significant performance impact.

This long double format never was designed for the rounding features
of the current ISO C standard. The format has been used effectively
without the additional features and there have not been requests for
conformance from normal users of the long double format.

Without a clearer need, there is no urgency to make the format fully
conforming and implement all of the performance mitigation and
alternatives. We prefer to not pursue the fixes until circumstances
change.

Thanks, David