Target FP: Make use of MPFR if available

This second patch introduces mfpr_float_ops, an new implementation
of target_float_ops.  This implements precise emulation of target
floating-point formats using the MPFR library.  This is then used
to perform operations on types that do not match any host type.

Note that use of MPFR is still not required.  The patch adds
a configure option --with-mpfr similar to --with-expat.  If use of
MPFR is disabled via the option or MPFR is not available, code will
fall back to current behavior.  This means that operations on types
that do not match any host type will be implemented on the host
long double type instead.

A new test case verifies that we can correctly print the largest
__float128 value now.

gdb/ChangeLog:
2017-11-22  Ulrich Weigand  <uweigand@de.ibm.com>

	* NEWS: Document use of GNU MPFR.
	* README: Likewise.

	* Makefile.in (LIBMPFR): Add define.
	(CLIBS): Add $(LIBMPFR).
	* configure.ac: Add --with-mpfr configure option.
	* configure: Regenerate.
	* config.in: Regenerate.

	* target-float.c [HAVE_LIBMPFR]: Include <mpfr.h>.
	(class mpfr_float_ops): New type.
	(mpfr_float_ops::from_target): Two new overloaded functions.
	(mpfr_float_ops::to_target): Likewise.
	(mpfr_float_ops::to_string): New function.
	(mpfr_float_ops::from_string): Likewise.
	(mpfr_float_ops::to_longest): Likewise.
	(mpfr_float_ops::from_longest): Likewise.
	(mpfr_float_ops::from_ulongest): Likewise.
	(mpfr_float_ops::to_host_double): Likewise.
	(mpfr_float_ops::from_host_double): Likewise.
	(mpfr_float_ops::convert): Likewise.
	(mpfr_float_ops::binop): Likewise.
	(mpfr_float_ops::compare): Likewise.
	(get_target_float_ops): Use mpfr_float_ops if available.

gdb/doc/ChangeLog:
2017-11-22  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.texinfo (Requirements): Document use of GNU MPFR.

gdb/testsuite/ChangeLog:
2017-11-22  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.base/float128.c (large128): New variable.
	* gdb.base/float128.exp: Add test to print largest __float128 value.

1 files changed, 17 insertions, 0 deletions

diff --git a/gdb/README b/gdb/README
index d485536..e43887f 100644
--- a/gdb/README
+++ b/gdb/README
@@ -89,6 +89,14 @@ features.  Expat will be linked in if it is available at build time, or
 those features will be disabled.  The latest version of Expat should be
 available from `http://expat.sourceforge.net'.
 
+   GDB uses GNU MPFR, a library for multiple-precision floating-point
+computation with correct rounding, to emulate target floating-point
+arithmetic during expression evaluation when the target uses different
+floating-point formats than the host.  MPFR will be linked in if it is
+available at build time.  If GNU MPFR it is not available, GDB will fall
+back to using host floating-point arithmetic.  The latest version of
+GNU MPFR should be available from `http://www.mpfr.org'.
+
    GDB can be used as a cross-debugger, running on a machine of one
 type while debugging a program running on a machine of another type.
 See below.
@@ -475,6 +483,15 @@ prefer; but you may abbreviate option names if you use `--'.
      have libexpat installed, you can  get the latest version from
      http://expat.sourceforge.net.
 
+`--with-mpfr'
+     Build GDB with the GNU MPFR library.  (Done by default if
+     GNU MPFR is installed and found at configure time.)  This library
+     is used to emulate target floating-point arithmetic during expression
+     evaluation when the target uses different floating-point formats than
+     the host.  If GNU MPFR is not available, GDB will fall back to using
+     host floating-point arithmetic.  If your host does not have GNU MPFR
+     installed, you can get the latest version from http://www.mpfr.org.
+
 `--with-python[=PATH]'
      Build GDB with Python scripting support.  (Done by default if
      libpython is present and found at configure time.)  Python makes