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| author | Jakub Jelinek <jakub@redhat.com> | 2025-04-09 18:24:11 +0200 |
|---|---|---|
| committer | Jakub Jelinek <jakub@redhat.com> | 2025-04-09 18:24:11 +0200 |
| commit | 63c99cd50bc9c10f0692f7cb31f4c5f02ff526df (patch) | |
| tree | 814264c88a645a8287934a717022cb729a181194 | |
| parent | 12a497c716f0a06be5946cabb8c3ec22a079771e (diff) | |
| download | glibc-63c99cd50bc9c10f0692f7cb31f4c5f02ff526df.zip glibc-63c99cd50bc9c10f0692f7cb31f4c5f02ff526df.tar.gz glibc-63c99cd50bc9c10f0692f7cb31f4c5f02ff526df.tar.bz2 | |
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 <josmyers@redhat.com>
| -rw-r--r-- | sysdeps/ieee754/ldbl-128/e_expl.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/sysdeps/ieee754/ldbl-128/e_expl.c b/sysdeps/ieee754/ldbl-128/e_expl.c index 25b45c7..4855c6b 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] |