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.

diff --git a/ChangeLog b/ChangeLog
index 1a87d43e1139da917296a74dc947148543a8d274..29dd1648022ca6d878ef37fe387ded33a5b38d01 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -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]

diff --git a/stdio-common/tst-sprintf3.c b/stdio-common/tst-sprintf3.c
index cffd1b69d0dc6c44c090ed50f9a3c1d5a1b86115..dad216b941995aa3124c04084d0687b729e4ca03 100644 (file)
--- 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 4f550ef47c54a20f005cca5eeb6856d9e172fc00..30d9bc34433af0c85a5db6f105776b60fa67e4fd 100644 (file)
--- 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