Re: [PATCH] Add LANG_HOOKS_EMPTY_RECORD_P for C++ empty class

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Mon, Nov 23, 2015 at 3:53 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Nov 23, 2015 at 1:57 AM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Sat, Nov 21, 2015 at 12:46 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Fri, Nov 20, 2015 at 2:17 PM, Jason Merrill <jason@redhat.com> wrote:
>>>> On 11/20/2015 01:52 PM, H.J. Lu wrote:
>>>>>
>>>>> On Tue, Nov 17, 2015 at 4:22 AM, Richard Biener
>>>>> <richard.guenther@gmail.com> wrote:
>>>>>>
>>>>>> On Tue, Nov 17, 2015 at 12:01 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>>
>>>>>>> Empty record should be returned and passed the same way in C and C++.
>>>>>>> This patch adds LANG_HOOKS_EMPTY_RECORD_P for C++ empty class, which
>>>>>>> defaults to return false.  For C++, LANG_HOOKS_EMPTY_RECORD_P is defined
>>>>>>> to is_really_empty_class, which returns true for C++ empty classes.  For
>>>>>>> LTO, we stream out a bit to indicate if a record is empty and we store
>>>>>>> it in TYPE_LANG_FLAG_0 when streaming in.  get_ref_base_and_extent is
>>>>>>> changed to set bitsize to 0 for empty records.  Middle-end and x86
>>>>>>> backend are updated to ignore empty records for parameter passing and
>>>>>>> function value return.  Other targets may need similar changes.
>>>>>>
>>>>>>
>>>>>> Please avoid a new langhook for this and instead claim a bit in
>>>>>> tree_type_common
>>>>>> like for example restrict_flag (double-check it is unused for
>>>>>> non-pointers).
>>>>>
>>>>>
>>>>> There is no bit in tree_type_common I can overload.  restrict_flag is
>>>>> checked for non-pointers to issue an error when it is used on
>>>>> non-pointers:
>>>>>
>>>>>
>>>>> /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/template/qualttp20.C:19:38:
>>>>> error: â__restrict__â qualifiers cannot be applied to âAS::Lâ
>>>>>     typedef typename T::L __restrict__ r;// { dg-error "'__restrict__'
>>>>> qualifiers cannot" "" }
>>>>
>>>>
>>>> The C++ front end only needs to check TYPE_RESTRICT for this purpose on
>>>> front-end-specific type codes like TEMPLATE_TYPE_PARM; cp_type_quals could
>>>> handle that specifically if you change TYPE_RESTRICT to only apply to
>>>> pointers.
>>>>
>>>
>>> restrict_flag is also checked in this case:
>>>
>>> [hjl@gnu-6 gcc]$ cat x.i
>>> struct dummy { };
>>>
>>> struct dummy
>>> foo (struct dummy __restrict__ i)
>>> {
>>>   return i;
>>> }
>>> [hjl@gnu-6 gcc]$ gcc -S x.i -Wall
>>> x.i:4:13: error: invalid use of ârestrictâ
>>>  foo (struct dummy __restrict__ i)
>>>              ^
>>> x.i:4:13: error: invalid use of ârestrictâ
>>> [hjl@gnu-6 gcc]$
>>>
>>> restrict_flag can't also be used to indicate `i' is an empty record.
>>
>> I'm sure this error can be done during parsing w/o relying on TYPE_RESTRICT.
>>
>> But well, use any other free bit (but do not enlarge
>> tree_type_common).  Eventually
>> you can free up a bit by putting sth into type_lang_specific currently
>> using bits
>> in tree_type_common.
>
> There are no bits in tree_type_common I can move.  Instead,
> this patch overloads side_effects_flag in tree_base.  Tested on
> Linux/x86-64.  OK for trunk?
>

Hi,

Coincidentally a few months ago I was experimenting with making
empty-struct function arguments zero-cost (and thus making them behave
the same way as in GNU C).  My approach (patch attached) was to assign
empty-struct arguments to a virtual register (instead of on the stack
or to a hard register) during RTL call expansion.  These
virtual-register assignments would then be trivially DCE'd later.
This approach seemed to work surprisingly well with minimal code
changes.  I wonder what
your thoughts are on this approach..

From 8eb52639992ad0f6e5482783604f362bcc04d230 Mon Sep 17 00:00:00 2001
From: Patrick Palka <patrick@parcs.ath.cx>
Date: Mon, 23 Nov 2015 21:02:09 -0500
Subject: [PATCH] zero-cost structs

---
 gcc/calls.c         | 15 +++++++++++++++
 gcc/tree-tailcall.c |  7 ++++++-
 gcc/tree.c          | 17 +++++++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/gcc/calls.c b/gcc/calls.c
index b56556a..4ca668c 100644
--- a/gcc/calls.c
+++ b/gcc/calls.c
@@ -1394,6 +1394,21 @@ initialize_argument_information (int num_actuals ATTRIBUTE_UNUSED,
       args[i].reg = targetm.calls.function_arg (args_so_far, mode, type,
 						argpos < n_named_args);
 
+      bool empty_record_or_union_type_p (const_tree);
+
+      if (type != NULL_TREE
+#if 0
+	  /* ??? This condition was necessary to fix a C regression whose
+	     details I have forgot about.  In GNU C the mode of an empty struct is BLKmode
+	     (and TYPE_SIZE 0) so this condition makes it so that we don't mess
+	     with the codegen of empty structs in C.  In C++ the mode of the empty struct
+	     is QImode and TYPE_SIZE_UNIT 1.  Maybe it's not necessary anymore?   */
+	  && mode != BLKmode
+#endif
+	  && args[i].reg == NULL_RTX
+	  && empty_record_or_union_type_p (type))
+	args[i].reg = gen_reg_rtx (mode);
+
       if (args[i].reg && CONST_INT_P (args[i].reg))
 	{
 	  args[i].special_slot = args[i].reg;
diff --git a/gcc/tree-tailcall.c b/gcc/tree-tailcall.c
index bbd1b29..fa8f66a 100644
--- a/gcc/tree-tailcall.c
+++ b/gcc/tree-tailcall.c
@@ -497,6 +497,8 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
 	tail_recursion = true;
     }
 
+  bool empty_record_or_union_type_p (const_tree);
+
   /* Make sure the tail invocation of this function does not refer
      to local variables.  */
   FOR_EACH_LOCAL_DECL (cfun, idx, var)
@@ -504,7 +506,10 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
       if (TREE_CODE (var) != PARM_DECL
 	  && auto_var_in_fn_p (var, cfun->decl)
 	  && (ref_maybe_used_by_stmt_p (call, var)
-	      || call_may_clobber_ref_p (call, var)))
+	      || call_may_clobber_ref_p (call, var))
+	  /* This change does the same thing as your aliasing change, to allow
+	     tail calling of functions taking by argument empty structs.  */
+	  && !empty_record_or_union_type_p (TREE_TYPE (var)))
 	return;
     }
 
diff --git a/gcc/tree.c b/gcc/tree.c
index 779fe93..f710d15 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -9069,6 +9069,23 @@ auto_var_in_fn_p (const_tree var, const_tree fn)
 	      || TREE_CODE (var) == RESULT_DECL));
 }
 
+/* Return true if if type TYPE is an empty record or union type.  */
+
+/* This predicate is inferior to your TYPE_EMPTY_RECORD-flag approach.  */
+
+bool
+empty_record_or_union_type_p (const_tree type)
+{
+  if (!RECORD_OR_UNION_TYPE_P (type))
+    return false;
+
+  for (tree field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
+    if (TREE_CODE (field) == FIELD_DECL)
+      return false;
+
+  return true;
+}
+
 /* Subprogram of following function.  Called by walk_tree.
 
    Return *TP if it is an automatic variable or parameter of the
-- 
2.6.3.424.g74c917e.dirty