e081ced345
Here is a cherry-pick from glibc [BZ #32411] fix. As mentioned by the reporter in a pull request against gcc-mirror, the THREEp96 constant in e_expl.c is incorrect, it is actually 0x3.p+94f128 rather than 0x3.p+96f128. The algorithm uses that to compute the t2 integer (tval2), by whose delta it adjusts the x+xl pair and then in the result uses the precomputed exp value for that entry. Using 0x3.p+94f128 rather than 0x3.p+96f128 results in tval2 sometimes being one smaller, sometimes one larger than the desired value, thus can mean the x+xl pair after adjustment will be larger in absolute value than it should be. DesWursters created a test program for this https://github.com/DesWurstes/comparefloats and his results were total: 1135000000 not_equal: 4322 earlier_score: 674 later_score: 3648 I've modified this so with https://sourceware.org/bugzilla/show_bug.cgi?id=32411#c3 so that it actually tests pseudo-random _Float128 values with range (-16384.,16384) with strong bias on values larger than 0.0002 in absolute value (so that tval1/tval2 aren't zero most of the time) and that gave total: 10000000000 not_equal: 29861 earlier_score: 4606 later_score: 25255 So, in both cases, in most cases the change doesn't result in any differences, and in those rare cases where does, about 85% have smaller ulp than without the patch. Additionally I've tried https://sourceware.org/bugzilla/show_bug.cgi?id=32411#c4 and in 2 billion iterations it didn't find any case where x+xl after the adjustments without this change would be smaller in absolute value compared to x+xl after the adjustments with this change. 2025-04-09 Jakub Jelinek <jakub@redhat.com> * math/expq.c (C): Fix up THREEp96 constant. |
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