Re: Fix powl inaccuracy for ldbl-128ibm (bug 14914)

[David Miller <davem at davemloft dot net>]

From: "Joseph S. Myers" <joseph@codesourcery.com>
Date: Tue, 4 Dec 2012 00:30:32 +0000

> On Tue, 4 Dec 2012, David Miller wrote:
> 
>> From: "Joseph S. Myers" <joseph@codesourcery.com>
>> Date: Mon, 3 Dec 2012 22:42:26 +0000
>> 
>> > Bug 14914 is inaccuracy of powl for ldbl-128ibm, which showed up in
>> > validation for 2.17.
>> > 
>> > In various places the code, based on that for ldbl-128, tries to split
>> > numbers into high and low parts in order to carry out operations on
>> > those parts separately, with at least some of the high-part operations
>> > needing to be exact.  However, the way the code extracts high parts is
>> > to clear the low 47 bits of the mantissa.  I've no idea how that
>> > figure was chosen - the ldbl-128 code clears 59 bits, i.e. more than
>> > half the mantissa - but as shown by the large errors for powl in the
>> > testsuite, it doesn't work well.  This patch changes the code to clear
>> > the whole low double of the long double value.  This eliminates all
>> > the ulps seen for the tests recently added or enabled for long double;
>> > no ulps at all are needed for pow for powerpc test-ldouble after this
>> > patch.  (Tested powerpc32, hard float; if this goes in then I'll
>> > retest powerpc-nofpu to see if any pow ulps updates are needed there.)
>> 
>> There aren't any major powl ULPs for sparc, so presumably ldbl-128 is
>> OK?
> 
> Yes.  The problem is with how the ldbl-128 code was adapated for 
> ldbl-128ibm.

Thanks for the clarification.  I'm personally OK with this change.