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ldtoa: fix dropping too many digits from output 

ldtoa cuts the number of digits it returns based on a computation of
number of supported bits (144) divide by log10(2).  Not only is the
integer approximation of log10(2) ~= 8/27 missing a digit here, it
also fails to take really small double and long double values into
account.

Allow for the full potential precision of long double values.  At the
same time, change the local string array allocation to request only as
much bytes as necessary to support the caller-requested number of
digits, to keep the stack size low on small targets.

In the long run a better fix would be to switch to gdtoa, as the BSD
variants, as well as Mingw64 do.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen 2021-11-04 11:30:44 +01:00
parent d4e42ceb96
commit 4d90e53359
1 changed files with 6 additions and 3 deletions
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9
newlib/libc/stdlib/ldtoa.c
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@ -34,9 +34,9 @@ void _IO_ldtostr (long double *, char *, int, int, char);
#define NBITS ((NI-4)*16)
/* Maximum number of decimal digits in ASCII conversion
* = NBITS*log10(2)
* Take full possible size of output into account
*/
#define NDEC (NBITS*8/27)
#define NDEC 1023
/* The exponent of 1.0 */
#define EXONE (0x3fff)
@ -2794,7 +2794,6 @@ _ldtoa_r (struct _reent *ptr, long double d, int mode, int ndigits,
LDPARMS rnd;
LDPARMS *ldp = &rnd;
char *outstr;
char outbuf[NDEC + MAX_EXP_DIGITS + 10];
union uconv du;
du.d = d;
@ -2841,6 +2840,8 @@ _ldtoa_r (struct _reent *ptr, long double d, int mode, int ndigits,
if (ndigits > NDEC)
ndigits = NDEC;
char outbuf[ndigits + MAX_EXP_DIGITS + 10];
etoasc (e, outbuf, ndigits, mode, ldp);
s = outbuf;
if (eisinf (e) || eisnan (e))
@ -3111,6 +3112,8 @@ tnzro:
else
{
emovi (y, w);
/* Note that this loop does not access the incoming string array,
* which may be shorter than NDEC + 1 bytes! */
for (i = 0; i < NDEC + 1; i++)
{
if ((w[NI - 1] & 0x7) != 0)