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Re: DSE calls to builtins (memset, etc)


On Tue, 19 Aug 2014, Richard Biener wrote:

 /* Return true whether REF may refer to global memory.  */

 bool
-ref_may_alias_global_p (tree ref)
+ref_may_alias_global_p (ao_ref *ref)
 {
-  tree base = get_base_address (ref);
+  tree base = ao_ref_base (ref);
   if (DECL_P (base))
     return is_global_var (base);
   else if (TREE_CODE (base) == MEM_REF
           || TREE_CODE (base) == TARGET_MEM_REF)
     return ptr_deref_may_alias_global_p (TREE_OPERAND (base, 0));
   return true;
 }

+bool
+ref_may_alias_global_p (tree ref)
+{
+  ao_ref r;
+  ao_ref_init (&r, ref);
+  return ref_may_alias_global_p (&r);
+}
+

I'd do two wrapers then since get_base_address is way cheaper
than doing ao_ref_base.  Thus, split out the code doing the
actual job into a static ref_may_alias_global_1?

Ok, for this function the extra precision in ao_ref_base is indeed
useless.

+  /* We know we have virtual definitions.  We can handle assignments and
+     some builtin calls.  */
+  if (is_gimple_call (stmt))

 if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))

which also does argument verification

Good point, then I can remove the test 'nargs < 2' below.

+    {
+      tree callee = gimple_call_fndecl (stmt);
+      if (!callee || DECL_BUILT_IN_CLASS (callee) != BUILT_IN_NORMAL)
+       return;
+      switch (DECL_FUNCTION_CODE (callee))
+       {
+         case BUILT_IN_MEMCPY:
+         case BUILT_IN_MEMMOVE:
+         case BUILT_IN_MEMSET:
+           {
+             gimple use_stmt;
+             ao_ref ref;
+             tree size = NULL_TREE;
+             int nargs = gimple_call_num_args (stmt);
+             if (nargs < 2)
+               return;
+             if (nargs == 3)
+               size = gimple_call_arg (stmt, 2);
+             tree ptr = gimple_call_arg (stmt, 0);
+             ao_ref_init_from_ptr_and_size (&ref, ptr, size);
+             if (!dse_possible_dead_store_p (&ref, stmt, &use_stmt))
+               return;
+
+             if (dump_file && (dump_flags & TDF_DETAILS))
+               {
+                 fprintf (dump_file, "  Deleted dead store '");

dead call

+                 print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags,
0);
+                 fprintf (dump_file, "'\n");
+               }
+
+             tree lhs = gimple_call_lhs (stmt);
+             if (lhs)
+               {
+                 gimple new_stmt = gimple_build_assign (lhs, ptr);
+                 unlink_stmt_vdef (stmt);
+                 gsi_replace (gsi, new_stmt, true);

You may need eh cleanup here as well.

Cleanup is one of the most mysterious parts of gcc, I am never sure what
to release/update/unlink/purge/etc :-(

Bonus points if you make gsi_replace return whether
maybe_clean_or_replace_eh_stmt returned true.

Ok, that seems trivial.

-      basic_block bb;
-
-      /* If use_stmt is or might be a nop assignment, e.g. for
-          struct { ... } S a, b, *p; ...
-          b = a; b = b;
-        or
-          b = a; b = *p; where p might be &b,
-        or
-           *p = a; *p = b; where p might be &b,
-        or
-           *p = *u; *p = *v; where p might be v, then USE_STMT
-         acts as a use as well as definition, so store in STMT
-         is not dead.  */
-      if (stmt != use_stmt
-         && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
-       return;
-

I don't see how you can remove this code?

dse_possible_dead_store_p already tests ref_maybe_used_by_stmt_p and
thus cannot return true with such a use_stmt, as far as I can see. As I
said, bootstrap+testsuite with an assert instead of 'return' didn't turn
up anything. I could keep it as a gcc_checking_assert (with a slight
update to the comment) if you like. Or did I miss a path in
dse_possible_dead_store_p?

Thanks,

--
Marc Glisse


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