[Bug math/34376] New: inaccurate sin/cos/tan functions

zimmerma+gcc at loria dot fr via Glibc-bugs <[email protected]> Wed, 08 Jul 2026 12:09:38 +0000

Newsgroups	gmane.comp.lib.glibc.bugs
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https://sourceware.org/bugzilla/show_bug.cgi?id=34376

            Bug ID: 34376
           Summary: inaccurate sin/cos/tan functions
           Product: glibc
           Version: 2.43
            Status: NEW
          Severity: critical
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: zimmerma+gcc at loria dot fr
  Target Milestone: ---

as reported by Stefan Kanthak in
https://sourceware.org/pipermail/libc-alpha/2026-July/178756.html, the binary64
trigonometric functions can yield huge errors of more than 100,000 ulps:

Checking sin with glibc-2.43/build and rndn
sin 0 -1 0x1.4c96c11134d36p+578 [102825] [1.03e+05] 102826 102825.2960633666

Checking cos with glibc-2.43/build and rndn
cos 0 -1 0x1.69eab0985179bp+246 [136921] [1.37e+05] 136921 136920.7555915861

Checking tan with glibc-2.43/build and rndn
tan 0 -1 0x1.69eab0985179bp+246 [143270] [1.44e+05] 143270 143269.8972757733

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