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From: "jsm28 at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug math/18956] New: powf inaccuracy
Date: Fri, 11 Sep 2015 19:51:00 -0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: math
X-Bugzilla-Version: 2.22
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https://sourceware.org/bugzilla/show_bug.cgi?id=18956

            Bug ID: 18956
           Summary: powf inaccuracy
           Product: glibc
           Version: 2.22
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

Even in round-to-nearest mode, powf can have errors beyond the 9ulp maximum
accepted in glibc (seen on x86_64 and mips64):

Failure: Test: pow (0xd.73035p-4, -0x1.47bb8p+8)
Result:
 is:          5.22400435890634124322e+24   0x1.148e7600000000000000p+82
 should be:   5.22399859429881820899e+24   0x1.148e6200000000000000p+82
 difference:  5.76460752303423488000e+18   0x1.40000000000000000000p+62
 ulp       :  10.0000
 max.ulp   :  3.0000
Failure: Test: pow_downward (0xd.73035p-4, -0x1.47bb8p+8)
Result:
 is:          5.22398418278001062340e+24   0x1.148e3000000000000000p+82
 should be:   5.22399801783806590556e+24   0x1.148e6000000000000000p+82
 difference:  1.38350580552821637120e+19   0x1.80000000000000000000p+63
 ulp       :  24.0000
 max.ulp   :  4.0000
Failure: Test: pow_towardzero (0xd.73035p-4, -0x1.47bb8p+8)
Result:
 is:          5.22398360631925831997e+24   0x1.148e2e00000000000000p+82
 should be:   5.22399801783806590556e+24   0x1.148e6000000000000000p+82
 difference:  1.44115188075855872000e+19   0x1.90000000000000000000p+63
 ulp       :  25.0000
 max.ulp   :  8.0000
Failure: Test: pow_upward (0xd.73035p-4, -0x1.47bb8p+8)
Result:
 is:          5.22400435890634124323e+24   0x1.148e7600000000000000p+82
 should be:   5.22399859429881820899e+24   0x1.148e6200000000000000p+82
 difference:  5.76460752303423488000e+18   0x1.40000000000000000000p+62
 ulp       :  10.0000
 max.ulp   :  8.0000

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