Fix default float_t definition (bug 20855).

The default (top-level) version of bits/mathdef.h defines float_t to
double.  It is used on ColdFire, MicroBlaze, Nios II and SH3, all of
which define FLT_EVAL_METHOD to 0, so float_t should be float (and C11
requires a certain correspondence between these typedefs and
FLT_EVAL_METHOD values).

I proposed fixing this default in
<https://sourceware.org/ml/libc-alpha/2015-01/msg00499.html>, with no
objections from architecture maintainers, and this patch makes that
fix.  As noted in the NEWS entry added, this might affect the ABIs of
non-glibc libraries (ImageMagick has been mentioned in gcc-patches
discussion of the S/390 case - which is unaffected by this patch), but
as noted in my previous message, affected libraries would have
problems with -mfpmath=sse anyway on 32-bit x86.

A (compilation) testcase is added to verify the required
correspondence of typedefs to FLT_EVAL_METHOD values.  This test is
built with -fexcess-precision=standard to avoid any issues with GCC 7
on S/390 providing a more accurate FLT_EVAL_METHOD definition in the
default (no excess precision) mode.  (This will also be usable to test
a fix for the recently reported bug about these typedefs on x86_64
-mfpmath=387, as architecture-specific tests can be added that

It is entirely possible that the fixed default makes some
architecture-specific versions of bits/mathdef.h semantically
equivalent to the default version and so no longer required.  I don't
intend to investigate that separately from the refactoring I proposed
in <https://sourceware.org/ml/libc-alpha/2016-11/msg00745.html>, which
will create as few header variants as possible for each group of
definitions.

Tested (compilation only) with build-many-glibcs.py.

	[BZ #20855]
	* bits/mathdef.h (float_t): Define to float.
	* math/test-flt-eval-method.c: New file.
	* math/Makefile (tests): Add test-flt-eval-method.
	(CFLAGS-test-flt-eval-method.c): New variable.

2 files changed, 67 insertions, 1 deletions

diff --git a/math/Makefile b/math/Makefile
index a5b11e9..fe910ef 100644
--- a/math/Makefile
+++ b/math/Makefile
@@ -158,7 +158,7 @@ tests = test-matherr test-fenv atest-exp atest-sincos atest-exp2 basic-test \
 	test-fexcept test-fexcept-traps test-fesetexcept \
 	test-fesetexcept-traps test-fetestexceptflag test-femode \
 	test-femode-traps test-iszero-excess-precision \
-	test-iseqsig-excess-precision $(tests-static)
+	test-iseqsig-excess-precision test-flt-eval-method $(tests-static)
 tests-static = test-fpucw-static test-fpucw-ieee-static \
 	       test-signgam-uchar-static test-signgam-uchar-init-static \
 	       test-signgam-uint-static test-signgam-uint-init-static \
@@ -271,6 +271,7 @@ CFLAGS-test-math-iszero.cc = -std=gnu++11
 
 CFLAGS-test-iszero-excess-precision.c = -fexcess-precision=standard
 CFLAGS-test-iseqsig-excess-precision.c = -fexcess-precision=standard
+CFLAGS-test-flt-eval-method.c = -fexcess-precision=standard
 
 # The -lieee module sets the _LIB_VERSION_ switch to IEEE mode
 # for error handling in the -lm functions.
diff --git a/math/test-flt-eval-method.c b/math/test-flt-eval-method.c
new file mode 100644
index 0000000..84706a5
--- /dev/null
+++ b/math/test-flt-eval-method.c
@@ -0,0 +1,65 @@
+/* Test evaluation method typedefs match FLT_EVAL_METHOD.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <float.h>
+#include <math.h>
+
+#if FLT_EVAL_METHOD == -1
+/* Cannot test.  */
+typedef float_t my_float_t;
+typedef double_t my_double_t;
+#elif FLT_EVAL_METHOD == 0 || FLT_EVAL_METHOD == 16
+typedef float my_float_t;
+typedef double my_double_t;
+#elif FLT_EVAL_METHOD == 1
+typedef double my_float_t;
+typedef double my_double_t;
+#elif FLT_EVAL_METHOD == 2
+typedef long double my_float_t;
+typedef long double my_double_t;
+#elif FLT_EVAL_METHOD == 32
+typedef _Float32 my_float_t;
+typedef double my_double_t;
+#elif FLT_EVAL_METHOD == 33
+typedef _Float32x my_float_t;
+typedef _Float32x my_double_t;
+#elif FLT_EVAL_METHOD == 64
+typedef _Float64 my_float_t;
+typedef _Float64 my_double_t;
+#elif FLT_EVAL_METHOD == 65
+typedef _Float64x my_float_t;
+typedef _Float64x my_double_t;
+#elif FLT_EVAL_METHOD == 128
+typedef _Float128 my_float_t;
+typedef _Float128 my_double_t;
+#elif FLT_EVAL_METHOD == 129
+typedef _Float128x my_float_t;
+typedef _Float128x my_double_t;
+#else
+# error "Unknown FLT_EVAL_METHOD"
+#endif
+
+extern float_t test_float_t;
+extern my_float_t test_float_t;
+
+extern double_t test_double_t;
+extern my_double_t test_double_t;
+
+/* This is a compilation test.  */
+#define TEST_FUNCTION 0
+#include "../test-skeleton.c"