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msun: Fix math error in comment explaining y reduction

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x = k + y for some integer k and |y| < 1/2
exp2(x) = exp2(k + y) = exp2(k) * exp2(y)
which can be written as 2**k * exp2(y)

The original had x = 2**k + y, which is has an extra 2** in it which
isn't correct.

Confirmed by forumula 2 in Gal and Bachelis referenced in the comments
for the source of this method
	https://dl.acm.org/doi/pdf/10.1145/103147.103151

The actual code is correct.

Reviewed by: imp (who added s_exp2.c and wrote the commit message)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1127
This commit is contained in:
Henri Chataing 2024-04-12 16:08:01 -06:00 committed by Warner Losh
parent 2cf0c51793
commit 2b2cd97844
2 changed files with 2 additions and 2 deletions
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2
lib/msun/src/s_exp2.c
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@ -314,7 +314,7 @@ static const double tbl[TBLSIZE * 2] = {
* Method: (accurate tables)
*
* Reduce x:
* x = 2**k + y, for integer k and |y| <= 1/2.
* x = k + y, for integer k and |y| <= 1/2.
* Thus we have exp2(x) = 2**k * exp2(y).
*
* Reduce y:

2
lib/msun/src/s_exp2f.c
View file

@ -72,7 +72,7 @@ static const double exp2ft[TBLSIZE] = {
* Method: (equally-spaced tables)
*
* Reduce x:
* x = 2**k + y, for integer k and |y| <= 1/2.
* x = k + y, for integer k and |y| <= 1/2.
* Thus we have exp2f(x) = 2**k * exp2(y).
*
* Reduce y: