[PATCH, OpenACC] Enable GOMP_MAP_FIRSTPRIVATE_INT for OpenACC

[Julian Brown]

This patch (by Cesar) changes the way that mapping of firstprivate
scalars works for OpenACC. For scalars whose type has a size equal to or
smaller than the size of a pointer, rather than copying the value of
the scalar to the target device and having a separate mapping for a
pointer to the copied value, a single "pointer" is mapped whose bits
are a type-punned representation of the value itself.

This is a performance optimisation: the idea, IIUC, is that it is a
good idea to avoid having all launched compute resources contend for a
single memory location -- the pointed-to cell containing the scalar on
the device, in this case. Cesar talks about speedups obtained here
(for an earlier version of the patch):

https://gcc.gnu.org/ml/gcc-patches/2017-01/msg02171.html

The patch implies an API change for the libgomp plugin, in that it must
now understand that NULL device pointers correspond to host pointers
that are actually type-punned scalars.

Tested with offloading to NVPTX and bootstrapped. OK for mainline?

Julian

ChangeLog

2018-09-20  Cesar Philippidis  <cesar@codesourcery.com>
            Julian Brown  <julian@codesourcery.com>

	gcc/
	* omp-low.c (maybe_lookup_field_in_outer_ctx): New function.
	(convert_to_firstprivate_int): New function.
	(convert_from_firstprivate_int): New function.
	(lower_omp_target): Enable GOMP_MAP_FIRSTPRIVATE_INT in OpenACC.

	libgomp/
	* oacc-parallel.c (GOACC_parallel_keyed): Handle
	GOMP_MAP_FIRSTPRIVATE_INT host addresses.
	* plugin/plugin-nvptx.c (nvptx_exec): Handle
	GOMP_MAP_FIRSTPRIVATE_INT host addresses.
	* testsuite/libgomp.oacc-c++/firstprivate-int.C: New test.
	* testsuite/libgomp.oacc-c-c++-common/firstprivate-int.c: New
	test.
	* testsuite/libgomp.oacc-fortran/firstprivate-int.f90: New test.
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