Hoisting built-in call out of loop - LIM pass [tree-ssa-loop-im.c]

[Shubham Narlawar <gsocshubham@gmail.com>]

Hello,

[1] I want to achieve hosting of a custom built-in call out of loop that
loads an immediate value.

    for (int i = 0; i < 4000; i++) {
       _9 = (unsigned int) _1;
       slli_6 = _9 << 8;
       srli_36 = slli_6 >> 8;
       li_37 = __builtin_load_immediate (15);        //I want to hoist this
out of loop which will cause hoisting of add_39 stmt also
       add_39 = srli_36 + li_37;
       _10 = __builtin_foo (add_39, 1, _9);
    }

First 5 instructions in the above loop are being inserted by a custom
gimple plugin whenever it sees a __builtin_foo().

LICM pass () could hoist below instructions out of loop -

       _9 = (unsigned int) _1;
       slli_6 = _9 << 8;
       srli_36 = slli_6 >> 8;

I want to hoist below gimple statement out of loop

li_22 = __builtin_riscv_load_immediate(15);      //BUILTIN_MD

I am generating above builtin call using a custom GCC gimple plugin using -

              tree li_tmp_name = make_temp_ssa_name (integer_type_node,
NULL, "li");
              tree load = build_int_cst (unsigned_type_node, 15);

              gcall *li_stmt = gimple_build_call (decl,
1);                          //where decl is function_decl
              gimple_call_set_lhs (li_stmt, li_tmp_name);
              gimple_call_set_arg (li_stmt, 0, load);
              gsi_insert_after (&gsi, li_stmt, GSI_NEW_STMT);

How do I make a GIMPLE function call as const such that loop invariant code
motion hoist it out of the loop.

[2] I tried below approaches to make it happen -

a. I made load_immediate decl as constant but that did not help
TREE_CONSTANT(decl) = 1

b. I teached licm pass to forcefull move out __builtin_load_immediate() by
introducing various checks but in the end, it alters function cfg and some
other pass like ""tree-ssa-loop-manip.c"" crashes here -

          /* We should have met DEF_BB along the way.  */
          gcc_assert (pred != ENTRY_BLOCK_PTR_FOR_FN (cfun));

From above approaches, making changes in licm pass is proving to be costly
and inappropriate. Is there any other way I can achieve it? I am guessing
making it a constant gimple function call with no side effects will be
hoisted automatically by LICM.

If yes, how can I make a gimple call as constant so that it is hoisted out
by LICM. Also, please suggest any other approaches to achieve it.

Thanks and Regards,
Shubham