[PATCH][version 3]add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

Qing Zhao qing.zhao@oracle.com
Thu Jun 3 20:14:57 GMT 2021


Hi, Richard,

For the following, I need more clarification:



+/* Expand the IFN_DEFERRED_INIT function according to its second
argument.  */
+static void
+expand_DEFERRED_INIT (internal_fn, gcall *stmt)
+{
+  tree var = gimple_call_lhs (stmt);
+  tree init = NULL_TREE;
+  enum auto_init_type init_type
+    = (enum auto_init_type) TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));
+
+  switch (init_type)
+    {
+    default:
+      gcc_unreachable ();
+    case AUTO_INIT_PATTERN:
+      init = build_pattern_cst_for_auto_init (TREE_TYPE (var));
+      expand_assignment (var, init, false);
+      break;
+    case AUTO_INIT_ZERO:
+      init = build_zero_cst (TREE_TYPE (var));
+      expand_assignment (var, init, false);
+      break;
+    }

I think actually building build_pattern_cst_for_auto_init can generate
massive garbage and for big auto vars code size is also a concern and
ideally on x86 you'd produce rep movq.  So I don't think going
via expand_assignment is good.  Instead you possibly want to lower
.DEFERRED_INIT to MEMs following expand_builtin_memset and
eventually enhance that to allow storing pieces larger than a byte.


I will lower .DEFFERED_INIT to MEMS following expand_builtin_memset for “AUTO_INIT_PATTERN”.
My question is:
Do I need to do the same for “AUTO_INIT_ZERO”?

Thanks.

Qing



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