From: keithp@keithp.com ("Keith Packard")
To: newlib@sourceware.org <newlib@sourceware.org>
Subject: Unreachable code in ef_j0.c differs from glibc
Date: Tue, 17 Mar 2020 13:01:30 -0700 [thread overview]
Message-ID: <874kumn42t.fsf@keithp.com> (raw)
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Newlib and glibc both inherited a bunch of libm code from the venerable
SunPro C library, and it's still possible to diff the sources and see
how little they have diverged...
In trying to discover why newlib's bessel functions for 32-bit floats
are less accurate than glibc's, I compared the source code and found
some strange differences. While none of these explain the precision
issue, it seems like there are bugs here to fix.
(- is newlib, + is glibc)
(line 81, in j0f)
- if(ix>0x80000000) z = (invsqrtpi*cc)/__ieee754_sqrtf(x);
+ if(ix>0x48000000) z = (invsqrtpi*cc)/sqrtf(x);
(line 167 in y0f
- if(ix>0x80000000) z = (invsqrtpi*ss)/__ieee754_sqrtf(x);
+ if(ix>0x48000000) z = (invsqrtpi*ss)/sqrtf(x);
Line 81 and line 167 look like bugs to me -- 'ix' is the float with the
sign bit masked off, so those conditions can never be true.
openlibm has a different value that has a comment:
https://github.com/JuliaMath/openlibm/blob/master/src/e_j0f.c:
if(ix>0x58000000) z = (invsqrtpi*cc)/sqrtf(x); /* |x|>2**49 */
Apple has the same value as newlib:
https://opensource.apple.com/source/Libm/Libm-47.1/i386.subproj/e_j0f.c.auto.html
if((unsigned long)ix>0x80000000ul) z = (invsqrtpi*cc)/sqrtf(x);
An older version from freebsd has the newlib value as well:
web.mit.edu/freebsd/head/lib/msun/src/e_j0f.c
if(ix>0x80000000) z = (invsqrtpi*cc)/sqrtf(x);
(line 174 in y0f)
- if(ix<=0x32000000) { /* x < 2**-27 */
+ if(ix<=0x39800000) { /* x < 2**-13 */
return(u00 + tpi*__ieee754_logf(x));
}
z = x*x;
Line 174 just seems like a missed optimization case; y0f for values <
2**-13 can be approximated with the shorter computation.
As to the accuracy issues, I haven't found the root cause yet.
--
-keith
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next reply other threads:[~2020-03-17 20:01 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-17 20:01 "Keith Packard" [this message]
2020-03-17 22:31 ` Joseph Myers
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