From f664b663118642490b8776dcf4f30524a646dcbc Mon Sep 17 00:00:00 2001
From: "Paul E. Murphy" <murphyp@linux.vnet.ibm.com>
Date: Tue, 12 Apr 2016 13:29:53 -0500
Subject: [PATCH 60/65] Increase internal precision of ldbl-128ibm decimal
printf [BZ #19853]
When the signs differ, the precision of the conversion sometimes
drops below 106 bits. This strategy is identical to the
hexadecimal variant.
I've refactored tst-sprintf3 to enable testing a value with more
than 30 significant digits in order to demonstrate this failure
and its solution.
Additionally, this implicitly fixes a typo in the shift
quantities when subtracting from the high mantissa to compute
the difference.
(cherry picked from commit 37a4c70bd4c5c74ac562072e450dc02e8cb4c150)
---
ChangeLog | 14 ++++++++++++++
NEWS | 2 +-
stdio-common/tst-sprintf3.c | 14 ++++++++++----
sysdeps/ieee754/ldbl-128ibm/ldbl2mpn.c | 25 ++++++++++++++++++-------
4 files changed, 43 insertions(+), 12 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index b686231..f7664af 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,17 @@
+2016-04-12 Paul E. Murphy <murphyp@linux.vnet.ibm.com>
+
+ [BZ #19853]
+ * stdio-common/tst-sprintf3.c [TEST_N]: Refactor
+ TEST to take significant digits as second parameter.
+ [TEST]: Redefine in terms of TEST_N taking 30
+ significant digits.
+ (do_test): Add test case to demonstrate precision
+ failure in the ldbl-128ibm printf.
+ * sysdeps/ieee754/ldbl-128ibm/ldbl2pm.c:
+ (__mpn_extract_long_double): Carry 7 extra intermediate
+ bits of precision to aide computing difference when
+ signs differ.
+
2016-04-09 Mike Frysinger <vapier@gentoo.org>
* sysdeps/i386/configure.ac: Change == to = when calling test.
diff --git a/NEWS b/NEWS
index 8aa1206..2a43aec 100644
--- a/NEWS
+++ b/NEWS
@@ -25,7 +25,7 @@ Version 2.22.1
17905, 18420, 18421, 18480, 18589, 18743, 18778, 18781, 18787, 18796,
18870, 18887, 18921, 18928, 18969, 18985, 19003, 19018, 19058, 19174,
- 19178, 19590, 19682, 19791, 19822, 19879.
+ 19178, 19590, 19682, 19791, 19822, 19853, 19879.
* The getnetbyname implementation in nss_dns had a potentially unbounded
alloca call (in the form of a call to strdupa), leading to a stack
diff --git a/stdio-common/tst-sprintf3.c b/stdio-common/tst-sprintf3.c
index e1e5317..cae8ed1 100644
--- a/stdio-common/tst-sprintf3.c
+++ b/stdio-common/tst-sprintf3.c
@@ -38,11 +38,11 @@ do_test (void)
# define COMPARE_LDBL(u, v) ((u).l == (v).l)
#endif
-#define TEST(val) \
+#define TEST_N(val, n) \
do \
{ \
u.l = (val); \
- snprintf (buf, sizeof buf, "%.30LgL", u.l); \
+ snprintf (buf, sizeof buf, "%." #n "LgL", u.l); \
if (strcmp (buf, #val) != 0) \
{ \
printf ("Error on line %d: %s != %s\n", __LINE__, buf, #val); \
@@ -50,19 +50,25 @@ do_test (void)
} \
if (sscanf (#val, "%Lg", &v.l) != 1 || !COMPARE_LDBL (u, v)) \
{ \
- printf ("Error sscanf on line %d: %.30Lg != %.30Lg\n", __LINE__, \
- u.l, v.l); \
+ printf ("Error sscanf on line %d: %." #n "Lg != %." #n "Lg\n", \
+ __LINE__, u.l, v.l); \
result = 1; \
} \
/* printf ("%s %Lg %016Lx %016Lx\n", #val, u.l, u.x[0], u.x[1]); */ \
} \
while (0)
+#define TEST(val) TEST_N (val,30)
+
#if LDBL_MANT_DIG >= 106
# if LDBL_MANT_DIG == 106
TEST (2.22507385850719347803989925739e-308L);
TEST (2.22507385850719397210554509863e-308L);
TEST (2.22507385850720088902458687609e-308L);
+
+ /* Verify precision is not lost for long doubles
+ of the form +1.pN,-1.pM. */
+ TEST_N (3.32306998946228968225951765070082e+35L, 34);
# endif
TEST (2.22507385850720138309023271733e-308L);
TEST (2.22507385850720187715587855858e-308L);
diff --git a/sysdeps/ieee754/ldbl-128ibm/ldbl2mpn.c b/sysdeps/ieee754/ldbl-128ibm/ldbl2mpn.c
index e9b5803..030a2aa 100644
--- a/sysdeps/ieee754/ldbl-128ibm/ldbl2mpn.c
+++ b/sysdeps/ieee754/ldbl-128ibm/ldbl2mpn.c
@@ -28,6 +28,12 @@
bits (106 for long double) and an integral power of two (MPN
frexpl). */
+
+/* When signs differ, the actual value is the difference between the
+ significant double and the less significant double. Sometimes a
+ bit can be lost when we borrow from the significant mantissa. */
+#define EXTRA_INTERNAL_PRECISION (7)
+
mp_size_t
__mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
int *expt, int *is_neg,
@@ -45,10 +51,15 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
lo = ((long long) u.d[1].ieee.mantissa0 << 32) | u.d[1].ieee.mantissa1;
hi = ((long long) u.d[0].ieee.mantissa0 << 32) | u.d[0].ieee.mantissa1;
+ /* Hold 7 extra bits of precision in the mantissa. This allows
+ the normalizing shifts below to prevent losing precision when
+ the signs differ and the exponents are sufficiently far apart. */
+ lo <<= EXTRA_INTERNAL_PRECISION;
+
/* If the lower double is not a denormal or zero then set the hidden
53rd bit. */
if (u.d[1].ieee.exponent != 0)
- lo |= 1ULL << 52;
+ lo |= 1ULL << (52 + EXTRA_INTERNAL_PRECISION);
else
lo = lo << 1;
@@ -72,12 +83,12 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
if (u.d[0].ieee.negative != u.d[1].ieee.negative
&& lo != 0)
{
- lo = (1ULL << 53) - lo;
+ lo = (1ULL << (53 + EXTRA_INTERNAL_PRECISION)) - lo;
if (hi == 0)
{
/* we have a borrow from the hidden bit, so shift left 1. */
- hi = 0x0ffffffffffffeLL | (lo >> 51);
- lo = 0x1fffffffffffffLL & (lo << 1);
+ hi = 0x000ffffffffffffeLL | (lo >> (52 + EXTRA_INTERNAL_PRECISION));
+ lo = 0x0fffffffffffffffLL & (lo << 1);
(*expt)--;
}
else
@@ -85,14 +96,14 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
}
#if BITS_PER_MP_LIMB == 32
/* Combine the mantissas to be contiguous. */
- res_ptr[0] = lo;
- res_ptr[1] = (hi << (53 - 32)) | (lo >> 32);
+ res_ptr[0] = lo >> EXTRA_INTERNAL_PRECISION;
+ res_ptr[1] = (hi << (53 - 32)) | (lo >> (32 + EXTRA_INTERNAL_PRECISION));
res_ptr[2] = hi >> 11;
res_ptr[3] = hi >> (32 + 11);
#define N 4
#elif BITS_PER_MP_LIMB == 64
/* Combine the two mantissas to be contiguous. */
- res_ptr[0] = (hi << 53) | lo;
+ res_ptr[0] = (hi << 53) | (lo >> EXTRA_INTERNAL_PRECISION);
res_ptr[1] = hi >> 11;
#define N 2
#else
--
2.7.4
