gh-154146: Fix accuracy of the math.acospi() fallback near 1#154148
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Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
skirpichev
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Jul 20, 2026
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The fallback computed
acos(x)/pi, and some libms (old fdlibm derivatives on NetBSD, OpenBSD and DragonFly) lose precision inacos(x)near x = 1. Compute it as2*asin(sqrt((1 - x)/2))/pifor x >= 0.5, where1 - xis exact andasin()is accurate.Tested on NetBSD 10.1, OpenBSD 7.9 and DragonFly 6.4 (the failing mtest cases pass now) and FreeBSD 15.1 (no regression).
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