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Re: update address taken: don't drop clobbers


On Thu, 10 Jul 2014, Richard Biener wrote:

--- gcc/tree-into-ssa.c (revision 212109)
+++ gcc/tree-into-ssa.c (working copy)
@@ -1831,26 +1831,38 @@ maybe_register_def (def_operand_p def_p,
 {
   tree def = DEF_FROM_PTR (def_p);
   tree sym = DECL_P (def) ? def : SSA_NAME_VAR (def);

   /* If DEF is a naked symbol that needs renaming, create a new
      name for it.  */
   if (marked_for_renaming (sym))
     {
       if (DECL_P (def))
        {
-         tree tracked_var;
-
-         def = make_ssa_name (def, stmt);
+         if (gimple_clobber_p (stmt) && is_gimple_reg (sym))

sym should always be a gimple reg here (it's marked for renaming).

+           {
+             /* Replace clobber stmts with a default def.  Create a new
+                variable so we don't later think we must coalesce, which
would
+                fail with some ada abnormal PHIs.  Still, we try to keep a
+                similar name so error messages make sense.  */
+             unlink_stmt_vdef (stmt);

I think that's redundant with gsi_replace (note that using gsi_replace
looks dangerous here as it calls update_stmt during SSA rewrite...
that might open a can of worms).

IIRC it was failing without unlink_stmt_vdef (maybe that was in a different version of the patch not using gsi_replace, but I don't think so). I was hoping that a clobber had little enough effects that update_stmt was unlikely to break anything. Anyway it doesn't matter if I use your suggestion below.

+             gsi_replace (&gsi, gimple_build_nop (), true);
+             tree id = DECL_NAME (sym);
+             const char* name = id ? IDENTIFIER_POINTER (id) : 0;
+             tree newvar = create_tmp_var (TREE_TYPE (sym), name);
+             def = get_or_create_ssa_default_def (cfun, newvar);

So - can't you simply do

   gimple_assign_set_rhs_from_tree (&gsi,
get_or_create_dda_default_def (cfun, sym));

?  Thus replace x = CLOBBER; with x_3 = x_2(D);

+           }
+         else

and of course still rewrite the DEF then.  IMHO the copy-propagation
you do is premature optimization.

I'll try that. I was trying to remain as close as possible to what you wrote in rewrite_stmt:

        if (gimple_clobber_p (stmt)
            && is_gimple_reg (var))
          {
            /* If we rewrite a DECL into SSA form then drop its
               clobber stmts and replace uses with a new default def.  */
            gcc_checking_assert (TREE_CODE (var) == VAR_DECL
                                 && !gimple_vdef (stmt));
            gsi_replace (si, gimple_build_nop (), true);
            register_new_def (get_or_create_ssa_default_def (cfun, var), var);
            break;
          }


I'll be away next week, but I'll re-read all the replies carefully when I come back.

--
Marc Glisse


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