[newlib-cygwin] Fix error in powf for x close to 1 and large y

[newlib-cygwin] Fix error in powf for x close to 1 and large y

[Jeff Johnston]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=cf1ef2dc5b95342fa337141b3b1ec85aac73229f

commit cf1ef2dc5b95342fa337141b3b1ec85aac73229f
Author: Fabian Schriever <fabian.schriever@gtd-gmbh.de>
Date:   Fri Dec 11 16:57:35 2020 +0100

    Fix error in powf for x close to 1 and large y
    
    This patch fixes the error found by Paul Zimmermann (see
    https://homepages.loria.fr/PZimmermann/papers/#accuracy) regarding x
    close to 1 and rather large y (specifically he found the case
    powf(0x1.ffffeep-1,-0x1.000002p+27) which returns +Inf instead of the
    correct value). We found 2 more values for x which show the same faulty
    behaviour, and all 3 are fixed with this patch. We have tested all
    combinations for x in [+1.fffdfp-1, +1.00020p+0] and y in
    [-1.000007p+27, -1.000002p+27] and [1.000002p+27,1.000007p+27].

Diff:
---
 newlib/libm/math/ef_pow.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/newlib/libm/math/ef_pow.c b/newlib/libm/math/ef_pow.c
index e3579f071..07b225f8c 100644
--- a/newlib/libm/math/ef_pow.c
+++ b/newlib/libm/math/ef_pow.c
@@ -138,7 +138,7 @@ ivln2_l  =  7.0526075433e-06; /* 0x36eca570 =1/ln2 tail*/
     /* |y| is huge */
 	if(iy>0x4d000000) { /* if |y| > 2**27 */
 	/* over/underflow if x is not close to one */
-	    if(ix<0x3f7ffff8) return (hy<0)? __math_oflowf(0):__math_uflowf(0);
+	    if(ix<0x3f7ffff4) return (hy<0)? __math_oflowf(0):__math_uflowf(0);
 	    if(ix>0x3f800007) return (hy>0)? __math_oflowf(0):__math_uflowf(0);
 	/* now |1-x| is tiny <= 2**-20, suffice to compute 
 	   log(x) by x-x^2/2+x^3/3-x^4/4 */