[PATCH] fix strtod and strtof rounding with many trailing zeros

[Szabolcs Nagy <nsz@port70.net>]

in certain cases excessive trailing zeros could cause incorrect
rounding from long double to double or float in decfloat.

e.g. in strtof("9444733528689243848704.000000", 0) the argument
is 0x1.000001p+73, exactly halfway between two representible floats,
this incorrectly got rounded to 0x1.000002p+73 instead of 0x1p+73,
but with less trailing 0 the rounding was fine.

the fix makes sure that the z index always points one past the last
non-zero digit in the base 10^9 representation, this way trailing
zeros don't affect the rounding logic.
---
i think x[z-1] is fine in the loop here as z>0 and x[0]!=0
and later code does not turn x[z-1] into 0.

the upscaling code path already removed trailing zeros but
that was not always taken.

only tested on x86, the bad rounding cases depend on long double
representation.

 src/internal/floatscan.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/internal/floatscan.c b/src/internal/floatscan.c
index 17d8e70..66d01ef 100644
--- a/src/internal/floatscan.c
+++ b/src/internal/floatscan.c
@@ -172,6 +172,9 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 			return sign * (long double)x[0] * p10s[rp-10];
 	}
 
+	/* Drop trailing zeros */
+	for (; !x[z-1]; z--);
+
 	/* Align radix point to B1B digit boundary */
 	if (rp % 9) {
 		int rpm9 = rp>=0 ? rp%9 : rp%9+9;
-- 
2.9.3