[Bug tree-optimization/66610] New: Compound assignments prevent value-numbering optimization

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66610

            Bug ID: 66610
           Summary: Compound assignments prevent value-numbering
                    optimization
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Created attachment 35818
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35818&action=edit
Minimal testcase demonstrating the issue

libgccjit showed poor performance relative to a LLVM backend within an
experimental JIT-compiler for Lua (https://github.com/dibyendumajumdar/ravi).

On investigation it appears to be due to unions and structs stopping
value-numbering from working.

I'm attaching a minimal reproducer for the issue.

If the code copies the struct and the union within it field-wise, pass_fre
(tree-ssa-pre.c) uses value numbering to eliminate the copy of the loop index
through *arr, and turns loop_using_union_field_assignment into:

  loop_using_union_field_assignment (int num_iters, struct s * arr)
  {
    int i;

    <bb 2>:
    goto <bb 4>;

    <bb 3>:
    arr_6(D)->union_field.int_field = i_1;
    MEM[(struct s *)arr_6(D) + 4B].union_field.int_field = i_1;
    i_11 = i_1 + 1;

    <bb 4>:
    # i_1 = PHI <0(2), i_11(3)>
    if (i_1 < num_iters_5(D))
      goto <bb 3>;
    else
      goto <bb 5>;

    <bb 5>:
    return;
  }

and the loop is eliminated altogether by cddce2.

However, if the code does a compound copy, pass_fre doesn't eliminate the copy
of the loop index and the loop can't be eliminated (with a big performance
loss); in the example, functions "loop_using_struct_assignment" and
"loop_using_union_assignment" fail to have their loops optimized away at -O3.

Hence the libgccjit user has patched things at their end to direct copying the
fields (fwiw their workaround was this commit
https://github.com/dibyendumajumdar/ravi/commit/a5b192cd4f4213cd544e31b08b02eb9082142b20
)

Should compound assignments be optimizable via value-numbering?  Would it make
sense to split out the compound assignments field-wise internally before doing
value-numbering?

This is all with gcc trunk (r224625 aka
e3a904dbdc78cb45b98e8b109e0e49e759315b7c) on x86_64 at -O3.