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Re: Unnecessary check on phi node in tree if-conversion?


On Wed, Apr 6, 2016 at 5:07 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> Hi,
> Function if_convertible_phi_p has below check on virtual PHI nodes:
>
>
>   if (any_mask_load_store)
>     return true;
>
>   /* When there were no if-convertible stores, check
>      that there are no memory writes in the branches of the loop to be
>      if-converted.  */
>   if (virtual_operand_p (gimple_phi_result (phi)))
>     {
>       imm_use_iterator imm_iter;
>       use_operand_p use_p;
>
>       if (bb != loop->header)
>     {
>       if (dump_file && (dump_flags & TDF_DETAILS))
>         fprintf (dump_file, "Virtual phi not on loop->header.\n");
>       return false;
>     }
>
>       FOR_EACH_IMM_USE_FAST (use_p, imm_iter, gimple_phi_result (phi))
>     {
>       if (gimple_code (USE_STMT (use_p)) == GIMPLE_PHI
>           && USE_STMT (use_p) != phi)
>         {
>           if (dump_file && (dump_flags & TDF_DETAILS))
>         fprintf (dump_file, "Difficult to handle this virtual phi.\n");
>           return false;
>         }
>     }
>     }
>
> Since the check is short-cut by any_mask_load_store, when the check is
> triggered, it means there is virtual phi node but no conditional
> memory stores?  Is this possible?  Plus, any_mask_load_store is set by
> below code in if_convertible_gimple_assign_stmt_p:
>       if (ifcvt_can_use_mask_load_store (stmt))
>     {
>       gimple_set_plf (stmt, GF_PLF_2, true);
>       *any_mask_load_store = true;
>       return true;
>     }
>
> So in theory it's possible to skip aforementioned check when only mask
> load is encountered.
>
> Any ideas?
It's possible to have a loop like:

<bb header>
  .MEM_2232 = PHI <.MEM_574(179), .MEM_1247(183)>
  ...
  if (cond)
    goto <bb 1>
  else
    goto <bb2>

<bb 1>:  //empty
<bb2>:
  .MEM_1247 = PHI <.MEM_2232(180), .MEM_2232(181)>
  if (cond2)
    goto <bb exit>
  else
    goto <bb latch>

<bb latch>:
  goto <bb header>

So can we handle the PHI which can be degenerated in if-cvt?

Thanks,
bin


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