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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.
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Paul E. Murphy
Paul E. Murphy committed Feb 29, 2016
1 parent 8305663 commit 37a4c70bd4c5c74ac562072e450dc02e8cb4c150
Showing with 42 additions and 11 deletions.
  1. +14 −0 ChangeLog
  2. +10 −4 stdio-common/tst-sprintf3.c
  3. +18 −7 sysdeps/ieee754/ldbl-128ibm/ldbl2mpn.c
@@ -1,3 +1,17 @@
2016-03-31 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.

2016-03-31 H.J. Lu <hongjiu.lu@intel.com>

[BZ #19881]
@@ -38,31 +38,37 @@ 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); \
result = 1; \
} \
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);
@@ -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;

@@ -72,27 +83,27 @@ __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
hi--;
}
#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

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