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diff -Naurd mpfr-3.1.6-a/PATCHES mpfr-3.1.6-b/PATCHES
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--- mpfr-3.1.6-a/PATCHES	2017-12-15 12:37:44.074256548 +0000
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+++ mpfr-3.1.6-b/PATCHES	2017-12-15 12:37:44.146256304 +0000
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@@ -0,0 +1 @@
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+root
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diff -Naurd mpfr-3.1.6-a/VERSION mpfr-3.1.6-b/VERSION
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--- mpfr-3.1.6-a/VERSION	2017-10-26 13:55:51.236013121 +0000
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+++ mpfr-3.1.6-b/VERSION	2017-12-15 12:37:44.142256319 +0000
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@@ -1 +1 @@
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-3.1.6-p1
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+3.1.6-p2
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diff -Naurd mpfr-3.1.6-a/src/mpfr.h mpfr-3.1.6-b/src/mpfr.h
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--- mpfr-3.1.6-a/src/mpfr.h	2017-10-26 13:55:51.232013138 +0000
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+++ mpfr-3.1.6-b/src/mpfr.h	2017-12-15 12:37:44.142256319 +0000
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@@ -27,7 +27,7 @@
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 #define MPFR_VERSION_MAJOR 3
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 #define MPFR_VERSION_MINOR 1
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 #define MPFR_VERSION_PATCHLEVEL 6
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-#define MPFR_VERSION_STRING "3.1.6-p1"
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+#define MPFR_VERSION_STRING "3.1.6-p2"
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 /* Macros dealing with MPFR VERSION */
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 #define MPFR_VERSION_NUM(a,b,c) (((a) << 16L) | ((b) << 8) | (c))
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diff -Naurd mpfr-3.1.6-a/src/root.c mpfr-3.1.6-b/src/root.c
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--- mpfr-3.1.6-a/src/root.c	2017-01-01 01:39:09.000000000 +0000
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+++ mpfr-3.1.6-b/src/root.c	2017-12-15 12:37:44.118256398 +0000
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@@ -107,6 +107,11 @@
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       MPFR_RET_NAN;
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     }
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+  /* Special case |x| = 1. Note that if x = -1, then k is odd
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+     (NaN results have already been filtered), so that y = -1. */
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+  if (mpfr_cmpabs (x, __gmpfr_one) == 0)
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+    return mpfr_set (y, x, rnd_mode);
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+
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   /* General case */
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   /* For large k, use exp(log(x)/k). The threshold of 100 seems to be quite
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@@ -188,6 +193,14 @@
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    Assume all special cases have been eliminated before.
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    In the extended exponent range, overflows/underflows are not possible.
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    Assume x > 0, or x < 0 and k odd.
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+   Also assume |x| <> 1 because log(1) = 0, which does not have an exponent
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+   and would yield a failure in the error bound computation. A priori, this
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+   constraint is quite artificial because if |x| is close enough to 1, then
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+   the exponent of log|x| does not need to be used (in the code, err would
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+   be 1 in such a domain). So this constraint |x| <> 1 could be avoided in
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+   the code. However, this is an exact case easy to detect, so that such a
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+   change would be useless. Values very close to 1 are not an issue, since
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+   an underflow is not possible before the MPFR_GET_EXP.
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 */
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 static int
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 mpfr_root_aux (mpfr_ptr y, mpfr_srcptr x, unsigned long k, mpfr_rnd_t rnd_mode)
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@@ -219,7 +232,8 @@
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       mpfr_log (t, absx, MPFR_RNDN);
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       /* t = log|x| * (1 + theta) with |theta| <= 2^(-w) */
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       mpfr_div_ui (t, t, k, MPFR_RNDN);
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-      expt = MPFR_GET_EXP (t);
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+      /* No possible underflow in mpfr_log and mpfr_div_ui. */
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+      expt = MPFR_GET_EXP (t);  /* assumes t <> 0 */
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       /* t = log|x|/k * (1 + theta) + eps with |theta| <= 2^(-w)
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          and |eps| <= 1/2 ulp(t), thus the total error is bounded
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          by 1.5 * 2^(expt - w) */
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diff -Naurd mpfr-3.1.6-a/src/version.c mpfr-3.1.6-b/src/version.c
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--- mpfr-3.1.6-a/src/version.c	2017-10-26 13:55:51.232013138 +0000
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+++ mpfr-3.1.6-b/src/version.c	2017-12-15 12:37:44.142256319 +0000
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@@ -25,5 +25,5 @@
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 const char *
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 mpfr_get_version (void)
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 {
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-  return "3.1.6-p1";
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+  return "3.1.6-p2";
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 }
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diff -Naurd mpfr-3.1.6-a/tests/troot.c mpfr-3.1.6-b/tests/troot.c
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--- mpfr-3.1.6-a/tests/troot.c	2017-01-01 01:39:09.000000000 +0000
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+++ mpfr-3.1.6-b/tests/troot.c	2017-12-15 12:37:44.118256398 +0000
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@@ -405,6 +405,26 @@
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   mpfr_clears (x, y1, y2, (mpfr_ptr) 0);
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 }
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+static void
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+bug20171214 (void)
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+{
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+  mpfr_t x, y;
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+  int inex;
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+
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+  mpfr_init2 (x, 805);
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+  mpfr_init2 (y, 837);
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+  mpfr_set_ui (x, 1, MPFR_RNDN);
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+  inex = mpfr_root (y, x, 120, MPFR_RNDN);
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+  MPFR_ASSERTN (inex == 0);
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+  MPFR_ASSERTN (mpfr_cmp_ui (y, 1) == 0);
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+  mpfr_set_si (x, -1, MPFR_RNDN);
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+  inex = mpfr_root (y, x, 121, MPFR_RNDN);
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+  MPFR_ASSERTN (inex == 0);
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+  MPFR_ASSERTN (mpfr_cmp_si (y, -1) == 0);
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+  mpfr_clear (x);
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+  mpfr_clear (y);
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+}
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+
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 int
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 main (void)
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 {
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@@ -415,6 +435,7 @@
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   tests_start_mpfr ();
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+  bug20171214 ();
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   exact_powers (3, 1000);
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   special ();
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   bigint ();