[Bug lto/81430] New: nvptx acceleration compilation broken because of running pass_partition_blocks

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81430

            Bug ID: 81430
           Summary: nvptx acceleration compilation broken because of
                    running pass_partition_blocks
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

[ tentatively setting component to lto ]

Atm, when running the libgomp testsuite for an x86_64 compiler with nvptx
acceleration, we run into a number of ICEs in lto1.  I've investigated an ICE
in fix_crossing_unconditional_branches, which occurs for both openmp and
openacc.  The basic problem is that we're running pass_partition_blocks for
nvptx, while 
we don't want to do that.


Note that for standalone compilation for single-thread ptx execution, we don't
attempt to run pass_partition_blocks. This is because for nvptx,
TARGET_HAVE_NAMED_SECTIONS is set to false, and this bit in finish_options
switches off pass_partition_blocks:
...
   /* If the target requested unwind info, then turn off the partitioning       
     optimization with a different message.  Likewise, if the target does not   
     support named sections.  */

  if (opts->x_flag_reorder_blocks_and_partition
      && (!targetm_common.have_named_sections
          || (opts->x_flag_unwind_tables
              && targetm_common.unwind_tables_default
              && (ui_except == UI_SJLJ || ui_except >= UI_TARGET))))
    {
      if (opts_set->x_flag_reorder_blocks_and_partition)
        inform (loc,
                "-freorder-blocks-and-partition does not work "
                "on this architecture");
      opts->x_flag_reorder_blocks_and_partition = 0;
      opts->x_flag_reorder_blocks = 1;
    }
...


But that doesn't seem to work for the offloading setup. The scenario is as
follows:
- testcase is compiled with -O2
- ix86_option_optimization_table enables OPT_freorder_blocks_and_partition at
  -O2
- cc1 writes out the flag as part of DECL_FUNCTION_SPECIFIC_OPTIMIZATION
- lto1 reads in the flag as part of DECL_FUNCTION_SPECIFIC_OPTIMIZATION
- lto1 uses the flag, and runs pass_partition_blocks
- pass_partition_blocks ICEs