* [PATCH] fix strtod and strtof rounding with many trailing zeros
@ 2016-09-04 2:46 Szabolcs Nagy
0 siblings, 0 replies; only message in thread
From: Szabolcs Nagy @ 2016-09-04 2:46 UTC (permalink / raw)
To: musl
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
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2016-09-04 2:46 [PATCH] fix strtod and strtof rounding with many trailing zeros Szabolcs Nagy
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