Even if the compiler supports an "alternate" short float type (e.g.,
_Float16), check to make sure that the compiler will correctly link
applications that perform mathematical operations on that type.
Carefully choose the mathematical test in the configure check to
ensure the mathematical operation is not removed by compiler
optimization (when setting CFLAGS=-O1 or higher).
Out of the box, clang 6.0.x and 7.0.x will fail to link applications
that try to perform addition (and other mathematical operations) on
_Float16 variables (an additional CLI flag is required to enable
software emulation of _Float16). If we detect a situation where the
type is supported by a sample program fails to link and the basename
of $CC is "clang", emit a warning and point the user to a relevant
README.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@fujitsu.com>
The type `short float`, which is proposed in ISO/IEC JTC 1/SC 22 WG 14
(C WG), is not supported by most compilers yet. But some compilers
(including gcc 7 for AArch64 and clang 6) support `_Float16`, which
is defined in ISO/IEC TS 18661-3:2015 (ISO/IEC JTC 1/SC 22/WG 14 N1945)
as an extensions for C. If it is detected in `configure`, it is used
as an alternate type of `short float` in Open MPI internal code.
This commit adds a `configure` option `--enable-alt-short-float=TYPE`.
It can be used to specify a type other than `short float` and `_Float16`
as the alternate type.
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
