[Bug target/80547] New: [6/7/8 Regression] nvptx back end ICE with OpenACC "reduction(OP:x)", "x = [...]"

[tschwinge at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80547

            Bug ID: 80547
           Summary: [6/7/8 Regression] nvptx back end ICE with OpenACC
                    "reduction(OP:x)", "x = [...]"
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: openacc
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tschwinge at gcc dot gnu.org
                CC: cesar at gcc dot gnu.org, jakub at gcc dot gnu.org
  Target Milestone: ---
            Target: nvptx

Created attachment 41280
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41280&action=edit
r.c

As far as I remember, it is permissible to directly write into an
OpenACC/OpenMP reduction variable ("x = [...]", as opposed to modifying it with
some binary operator: "x = x OP [...]").  Looking at the attached test case
(greatly reduced from another test case), that works (and returns the expected
result) for the OpenMP code therein, but runs into a nvptx back end ICE for
OpenACC, if optimizations "-O" are enabled:

    Program received signal SIGSEGV, Segmentation fault.
    gimplify_assign (dst=0x0, src=0x7ffff6883750, seq_p=0x7fffffffcaf8) at
[...]/gcc/gimplify.c:12779
    12779     tree t = build2 (MODIFY_EXPR, TREE_TYPE (dst), dst, src);
    (gdb) bt
    #0  gimplify_assign (dst=0x0, src=0x7ffff6883750, seq_p=0x7fffffffcaf8) at
[...]/gcc/gimplify.c:12779
    #1  0x0000000000d6f384 in nvptx_goacc_reduction_init (call=0x7ffff6957210)
at [...]/gcc/config/nvptx/nvptx.c:5177
    #2  nvptx_goacc_reduction (call=0x7ffff6957210) at
[...]/gcc/config/nvptx/nvptx.c:5309
    #3  0x000000000090ee56 in execute_oacc_device_lower () at
[...]/gcc/omp-offload.c:1537
    #4  (anonymous namespace)::pass_oacc_device_lower::execute (this=<optimized
out>) at [...]/gcc/omp-offload.c:1662
    #5  0x00000000009792ad in execute_one_pass (pass=pass@entry=0x16ec8e0) at
[...]/gcc/passes.c:2465
    #6  0x0000000000979b78 in execute_pass_list_1 (pass=0x16ec8e0) at
[...]/gcc/passes.c:2554
    #7  0x0000000000979bd5 in execute_pass_list (fn=<optimized out>,
pass=<optimized out>) at [...]/gcc/passes.c:2565
    #8  0x000000000062f0c7 in cgraph_node::expand
(this=this@entry=0x7ffff6952000) at [...]/gcc/cgraphunit.c:2042
    #9  0x0000000000630a4c in expand_all_functions () at
[...]/gcc/cgraphunit.c:2178
    #10 symbol_table::compile (this=0x7ffff6888000) at
[...]/gcc/cgraphunit.c:2535
    #11 0x0000000000630f36 in symbol_table::compile (this=<optimized out>) at
[...]/gcc/cgraphunit.c:2568
    #12 0x000000000058f2b2 in lto_main () at [...]/gcc/lto/lto.c:3334
    #13 0x0000000000a4c1af in compile_file () at [...]/gcc/toplev.c:467
    #14 0x000000000055ce4f in do_compile () at [...]/gcc/toplev.c:2000
    #15 toplev::main (this=this@entry=0x7fffffffcf90, argc=argc@entry=20,
argv=0x16c8b40, argv@entry=0x7fffffffd098) at [...]/gcc/toplev.c:2134
    #16 0x000000000055f287 in main (argc=20, argv=0x7fffffffd098) at
[...]/gcc/main.c:39