From 596259874b1ff910d137e7c7c0d0b25357836744 Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Wed, 12 Aug 2020 11:43:55 +0100
Subject: [PATCH] Fix frexp for mpfr: Make sure we call the correct API, add
 motivating test case.

---
 include/boost/multiprecision/mpfr.hpp | 28 ++++++++++++++++++++++-----
 test/Jamfile.v2                       |  1 +
 test/git_issue_265.cpp                | 27 ++++++++++++++++++++++++++
 3 files changed, 51 insertions(+), 5 deletions(-)
 create mode 100644 test/git_issue_265.cpp

diff --git a/include/boost/multiprecision/mpfr.hpp b/include/boost/multiprecision/mpfr.hpp
index d987845f..b774e60e 100644
--- a/include/boost/multiprecision/mpfr.hpp
+++ b/include/boost/multiprecision/mpfr.hpp
@@ -1366,16 +1366,34 @@ inline void eval_ldexp(mpfr_float_backend<Digits10, AllocateType>& result, const
 template <unsigned Digits10, mpfr_allocation_type AllocateType>
 inline void eval_frexp(mpfr_float_backend<Digits10, AllocateType>& result, const mpfr_float_backend<Digits10, AllocateType>& val, int* e)
 {
-   long v;
-   mpfr_get_d_2exp(&v, val.data(), GMP_RNDN);
+   if (mpfr_zero_p(val.data()))
+   {
+      *e = 0;
+      result = val;
+      return;
+   }
+   mp_exp_t v = mpfr_get_exp(val.data());
    *e = v;
-   eval_ldexp(result, val, -v);
+   if (v)
+      eval_ldexp(result, val, -v);
+   else
+      result = val;
 }
 template <unsigned Digits10, mpfr_allocation_type AllocateType>
 inline void eval_frexp(mpfr_float_backend<Digits10, AllocateType>& result, const mpfr_float_backend<Digits10, AllocateType>& val, long* e)
 {
-   mpfr_get_d_2exp(e, val.data(), GMP_RNDN);
-   return eval_ldexp(result, val, -*e);
+   if (mpfr_zero_p(val.data()))
+   {
+      *e = 0;
+      result = val;
+      return;
+   }
+   mp_exp_t v = mpfr_get_exp(val.data());
+   *e = v;
+   if(v)
+      eval_ldexp(result, val, -v);
+   else
+      result = val;
 }
 
 template <unsigned Digits10, mpfr_allocation_type AllocateType>
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 16959a5f..593857cd 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -1014,6 +1014,7 @@ test-suite misc :
       [ run git_issue_167.cpp ]
       [ run git_issue_175.cpp ]
       [ run git_issue_248.cpp ]
+      [ run git_issue_265.cpp : : : [ check-target-builds ../config//has_mpfr : <source>gmp <source>mpfr : <build>no ] ]
       [ compile git_issue_98.cpp : 
          [ check-target-builds ../config//has_float128 : <define>TEST_FLOAT128 <source>quadmath : ]
          [ check-target-builds ../config//has_gmp : <define>TEST_GMP <source>gmp : ]
diff --git a/test/git_issue_265.cpp b/test/git_issue_265.cpp
new file mode 100644
index 00000000..5f6b97cb
--- /dev/null
+++ b/test/git_issue_265.cpp
@@ -0,0 +1,27 @@
+///////////////////////////////////////////////////////////////////////////////
+//  Copyright 2020 John Maddock. Distributed under the Boost
+//  Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/multiprecision/mpfr.hpp>
+#include <boost/math/special_functions/next.hpp>
+#include "test.hpp"
+
+using namespace boost::multiprecision;
+
+int main()
+{
+   mpfr_float_50 half = 0.5;
+   mpfr_float_50 under_half = boost::math::float_prior(half);
+   BOOST_CHECK_NE(half, under_half);
+   int e1, e2;
+   mpfr_float_50 norm1, norm2;
+   norm1 = frexp(half, &e1);
+   norm2 = frexp(under_half, &e2);
+   BOOST_CHECK_EQUAL(norm1, half);
+   BOOST_CHECK_EQUAL(e1, 0);
+   BOOST_CHECK_GT(1, norm2);
+   BOOST_CHECK_LE(0.5, norm2);
+   BOOST_CHECK_EQUAL(e2, -1);
+   return boost::report_errors();
+}