From: Jakub Jelinek Date: Wed, 9 Apr 2025 16:24:11 +0000 (+0200) Subject: math: Fix up THREEp96 constant in expf128 [BZ #32411] X-Git-Tag: glibc-2.42~320 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=63c99cd50bc9c10f0692f7cb31f4c5f02ff526df;p=thirdparty%2Fglibc.git math: Fix up THREEp96 constant in expf128 [BZ #32411] 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. Reviewed-by: Joseph Myers --- diff --git a/sysdeps/ieee754/ldbl-128/e_expl.c b/sysdeps/ieee754/ldbl-128/e_expl.c index 25b45c7123..4855c6b18b 100644 --- a/sysdeps/ieee754/ldbl-128/e_expl.c +++ b/sysdeps/ieee754/ldbl-128/e_expl.c @@ -79,7 +79,7 @@ L(-11433.4627433362978788372438434526231), /* 3x2^96 */ #define THREEp96 C[2] - L(59421121885698253195157962752.0), + L(237684487542793012780631851008.0), /* 3x2^103 */ #define THREEp103 C[3]