configury: enhance short float detection with clang

With clang, short float support requires --rtlib=compiler-rt
to be passed to the linker. In order to avoid a false positive,
it is necessary have the runtime called in the test program.

The test program was hence updated with the function
below that generates a call to the __extendhfsf2() runtime function.

float extend (_Float16 v) {
    return (float)v;
}

Thanks Blaise Bourdin for reporting this.

Refs: #13181

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>

Author: ggouaillardet

config/opal_check_alt_short_float.m4

@@ -1,7 +1,9 @@
 dnl -*- shell-script -*-
 dnl
 dnl Copyright (c) 2018-2021 FUJITSU LIMITED.  All rights reserved.
-dnl Copyright (c) 2020 Cisco Systems, Inc.  All rights reserved.
+dnl Copyright (c) 2020      Cisco Systems, Inc.  All rights reserved.
+dnl Copyright (c) 2025      Research Organization for Information Science
+dnl                         and Technology (RIST).  All rights reserved.
 dnl $COPYRIGHT$
 dnl
 dnl Additional copyrights may follow
@@ -50,8 +52,12 @@ AC_DEFUN([OPAL_CHECK_ALT_SHORT_FLOAT], [
         AS_IF([test $opal_alt_short_float_exists -eq 1],
               [AC_MSG_CHECKING([if compiler supports arithmetic operations on $opal_short_float_type])
                AC_LINK_IFELSE([AC_LANG_PROGRAM([], [[
-static $opal_short_float_type a = 2.5, b = 3.8;
-a += b;]])],
+    static $opal_short_float_type a = 2.5, b = 3.8;
+    a += b;
+}
+
+float extend ($opal_short_float_type v) {
+    return (float)v;]])],
                                  [AC_MSG_RESULT([yes])
                                   opal_enable_short_float=1],
                                  [AC_MSG_RESULT([no])

ompi/mpiext/shortfloat/README.md

@@ -25,11 +25,13 @@ to the Open MPI `configure` script.
 NOTE: The Clang 6.0.x and 7.0.x compilers support the `_Float16` type
 (via software emulation), but require an additional linker flag to
 function properly.  If you wish to enable Clang 6.0.x or 7.0.x's
-software emulation of `_Float16`, use the following CLI options to Open
-MPI configure script:
+software emulation of `_Float16`, or recent Clang, use the
+following CLI options to Open MPI configure script:
 
 ```
 ./configure \
         LDFLAGS=--rtlib=compiler-rt \
         --with-wrapper-ldflags=--rtlib=compiler-rt ...
 ```
+Depending on your `clang++` version/configuration, you might also have to add
+`--stdlib=libc++` and/or `-lunwind`.