Re: strtod ("nan") returns negative NaN

[Craig Howland <howland at LGSInnovations dot com>]

On 08/14/2018 08:02 PM, Masamichi Hosoda wrote:
> > On Wed, 15 Aug 2018, Masamichi Hosoda wrote:
> >
> > > On Linux with glibc, both strtod ("nan")
> > > and strtod ("-nan") return positive NaN.
> > >
> > > So I've created the patch that behaves like glibc.
> > > Both strtod ("nan") and strtod ("-nan") return positive NaN.
> > I would suggest that you should not consider fixed bugs in glibc (bug
> > 23007 in this case) to be appropriate to emulate in other libraries.
> I've create the patch that behaves to preserve the sign bit.
>
> The result on Cygwin 64 bit (newlib, x86_64) with the patch:
> ```
> strtof ("nan", NULL) = nan
> strtof ("-nan", NULL) = -nan
> strtod ("nan", NULL) = nan
> strtod ("-nan", NULL) = -nan
> strtold ("nan", NULL) = nan
> strtold ("-nan", NULL) = -nan
> ```
>
> Thank you for your suggestion.
     The f_QNAN value should be 0x7fc00000 regardless of byte ordering.  In 
addition, the d_QNAN* values should be 0x0 and 0x7FF80000, with only the index 
changing based on byte ordering.  So instead of them being inside of a 
x86/x86_64 define, they should just have their values corrected in the 
LITTLE_ENDIAN clause.
     x86 does use a different coding for their 80-bit long double, so the 
ldus_QNAN* defines could belong within an x86 define.  On the other hand, the 
ldus_QNAN* defines only apply for Intel 80-bit, so in that respect don't need to 
be within a guard.
     The ld_QNAN defines are not actually used anywhere.  If they were, 
however, Intel 80-bit would require different values than 128-bit.  However, the 
long double support in Newlib really only works when long double is the same 
size as double.  Some functions can work with Intel 80-bit, but almost none of 
them work with 128-bit.
     Given the prior considerations, I suggest that only the values get fixed 
so that Cygwin works, but the #if x86 is not added.  For this to work on other 
platforms (128-bit long double), more work will be needed.  Again, sorry that I 
can't provide a git diff patch, but here's a suggested one with diff -pu.  It 
contains the value corrections from Masamichi, but skips the #if x86 and adds a 
comments about some of the deficiencies.
     The changes to strtod.c look OK.
                Craig

--- gd_qnan.h.20150311	2015-03-11 10:27:17.160725969 -0400
+++ gd_qnan.h	2018-08-14 21:45:01.316310256 -0400
@@ -26,18 +26,22 @@
 #elif defined(__IEEE_LITTLE_ENDIAN)
 
 #if !defined(__mips)
-#define f_QNAN 0xffc00000
+#define f_QNAN 0x7fc00000
 #define d_QNAN0 0x0
-#define d_QNAN1 0xfff80000
+#define d_QNAN1 0x7ff80000
+/* NOTE (FIXME):  Newlib only can do long double in limited situations.  The
+ * ld_QNAN* and ldus_QNAN* defines are for Intel 80-bit.  Additional C function
+ * work (both here and in multiple *.c files) is needed to support standard
+ * 128-bit long double.  */
 #define ld_QNAN0 0x0
 #define ld_QNAN1 0xc0000000
-#define ld_QNAN2 0xffff
+#define ld_QNAN2 0x7fff
 #define ld_QNAN3 0x0
 #define ldus_QNAN0 0x0
 #define ldus_QNAN1 0x0
 #define ldus_QNAN2 0x0
 #define ldus_QNAN3 0xc000
-#define ldus_QNAN4 0xffff
+#define ldus_QNAN4 0x7fff
 #elif defined(__mips_nan2008)
 #define f_QNAN 0x7fc00000
 #define d_QNAN0 0x0