Re: [PATCH] New attribute to create target clones

[Jeff Law <law@redhat.com>]

On 10/08/2015 02:01 PM, Evgeny Stupachenko wrote:
> On Thu, Oct 8, 2015 at 10:00 PM, Jeff Law <law@redhat.com> wrote:
>> On 09/24/2015 04:28 PM, Evgeny Stupachenko wrote:
>>>
>>> I've fixed ICE and review issues.
>>> x86 make check and bootstrap passed.
>>>
>>> Thanks,
>>> Evgeny
>>>
>>> ChangeLog
>>>
>>> 2015-09-25  Evgeny Stupachenko<evstupac@gmail.com>
>>>
>>> gcc/
>>>           * Makefile.in (OBJS): Add multiple_target.o.
>>>           * multiple_target.c (make_attribute): New.
>>>           (create_dispatcher_calls): Ditto.
>>>           (expand_target_clones): Ditto.
>>>           (ipa_target_clone): Ditto.
>>>           * passes.def (pass_target_clone): New ipa pass.
>>>           * tree-pass.h (make_pass_target_clone): Ditto.
>>>
>>> gcc/c-family
>>>           * c-common.c (handle_target_clones_attribute): New.
>>>           * (c_common_attribute_table): Add handle_target_clones_attribute.
>>>           * (handle_always_inline_attribute): Add check on target_clones
>>>           attribute.
>>>           * (handle_target_attribute): Ditto.
>>>
>>> gcc/testsuite
>>>           * gcc.dg/mvc1.c: New test for multiple targets cloning.
>>>           * gcc.dg/mvc2.c: Ditto.
>>>           * gcc.dg/mvc3.c: Ditto.
>>>           * gcc.dg/mvc4.c: Ditto.
>>>           * gcc.dg/mvc5.c: Ditto.
>>>           * gcc.dg/mvc6.c: Ditto.
>>>           * gcc.dg/mvc7.c: Ditto.
>>>           * g++.dg/ext/mvc1.C: Ditto.
>>>           * g++.dg/ext/mvc2.C: Ditto.
>>>           * g++.dg/ext/mvc3.C: Ditto.
>>>
>>> gcc/doc
>>>           * doc/extend.texi (target_clones): New attribute description.
>>>
>>>
>>
>>>
>>> target_clones.patch
>>
>> Sorry this has taken so long to come back to...  As I mentioned a couple
>> months ago, I'd hoped Jan would chime in on the IPA/symtab requirements.
>> But that didn't happen.
>>
>>
>> SO I went back and reviewed the discussion between Jan, Ilya & myself WRT
>> some of the rules around aliases, clones, etc.  I think the key question for
>> this patch is whether or not the clones have the same assembler name or not.
>>  From looking at expand_target_clones, I'm confident the answer is the clones
>> have different assembler names.  In fact, the assembler names are munged
>> with the options used for that specific clone of the original function.
>>
>>
>>>
>>> +/* Makes a function attribute of the form NAME(ARG_NAME) and chains
>>> +   it to CHAIN.  */
>>> +
>>> +static tree
>>> +make_attribute (const char *name, const char *arg_name, tree chain)
>>> +{
>>> +  tree attr_name;
>>> +  tree attr_arg_name;
>>> +  tree attr_args;
>>> +  tree attr;
>>> +
>>> +  attr_name = get_identifier (name);
>>> +  attr_arg_name = build_string (strlen (arg_name), arg_name);
>>> +  attr_args = tree_cons (NULL_TREE, attr_arg_name, NULL_TREE);
>>> +  attr = tree_cons (attr_name, attr_args, chain);
>>> +  return attr;
>>> +}
>>
>> This seems generic enough that I'd prefer it in attribs.c.  I was rather
>> surprised when I looked and didn't find an existing routine to do this.
>>
>>
>>> +
>>> +/* If the call in NODE has multiple target attribute with multiple
>>> fields,
>>> +   replace it with dispatcher call and create dispatcher (once).  */
>>> +
>>> +static void
>>> +create_dispatcher_calls (struct cgraph_node *node)
>>> +{
>>> +  cgraph_edge *e;
>>> +  cgraph_edge *e_next;
>>> +  for (e = node->callers; e ;e = (e == NULL) ? e_next : e->next_caller)
>>
>> That's a rather strange way to write the loop increment.  If I follow the
>> loop logic correctly, it seems that we always end up using e->next_caller,
>> it's just obscured.
>>
>> For the test if we're calling a versioned function, we just "continue".  e
>> will be non-null and thus we use e->next_caller to set e for the next
>> iteration.
>>
>> If the test for calling a versioned function falls, we set e_next to
>> e->next_caller, then later set e to NULL.  That results in using e_next to
>> set e for the next iteration.  But e_next was initialized to e->next_caller.
>>
>> So why not just write the loop increment as e = e->next-caller?
>
> Because of this:
>        e->redirect_callee (inode);
> It modifies next_caller field.
> The other way is to remember all what we want to redirect and create
> one more loop through the modifications to apply them.
Seems like a comment would be wise.

>>>
>> I'm slightly concerned with using the pretty printer to build up the new
>> name.  Is there precedent for this anywhere else in GCC?
> I don't remember where it exactly came from. However it's not a big
> deal to simplify this to std functions.
Thanks.  Not sure why, but I'd appreciate that change.


>>
>> When creating the munged name, don't you also have to make sure that other
>> symbols that aren't supported for names don't sneak through?  I see that you
>> replace = and -, but you'd need to replace any symbol that could possibly be
>> used in an option, but which isn't allowed in a function name at the
>> assembler level.  I'd be worried about anything that might possibly be seen
>> as an operator by the assembler, '.', and possibly others.
> This restriction comes from "targetm.target_option.valid_attribute_p"
> and it is the same for current implementation of function
> multiversioning.
> It exits with error: "attribute(target("...")) is unknown".
> It looks reasonable to put the check before symtab changes.
Right, but there's nothing inherently that says that a option couldn't 
have other operators such as '+' in the option string.  So I'm concerned 
that if this code were used on a target that had such an option that 
we'd end up generating invalid assembly.

I think all you have to do is map a fuller set of invalid characters to 
a valid character.



Jeff