[Bug target/100232] [OpenMP][nvptx] Reduction fails with optimization and 'loop'/'for simd' but not with 'for'

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100232

--- Comment #4 from Tom de Vries <vries at gcc dot gnu.org> ---
This commit:
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commit 3af3bec2e4d344bd54a134d8b2263f44d788c3d8
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Mon May 4 21:21:16 2020 +0100

    internal-fn: Avoid dropping the lhs of some calls [PR94941]
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adds:
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   expand_insn (get_multi_vector_move (type, optab), 2, ops);
+  if (!rtx_equal_p (target, ops[0].value))
+    emit_move_insn (target, ops[0].value);
...
in expand_load_lanes_optab_fn and mentions:
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    create_output_operand coerces an output operand to the insn's
    predicates, using a suggested rtx location if convenient.
    But if that rtx location is actually required rather than
    optional, the builder of the insn has to emit a move afterwards.

    (We could instead add a new interface that does this automatically,
    but that's future work.)

    This PR shows that we were failing to emit the move for some of the
    vector load internal functions.  I think there are other routines in
    internal-fn.c that potentially have the same problem, but this patch is
    supposed to be a conservative subset suitable for backporting to GCC 10.
...