Re: RFA (gimplify): PATCH to implement C++ order of evaluation paper

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Jul 7, 2016 at 9:18 PM, Jason Merrill <jason@redhat.com> wrote:
> On Tue, Jun 28, 2016 at 10:00 AM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Thu, Jun 16, 2016 at 6:15 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>>> On Thu, Jun 16, 2016 at 11:28:48AM -0400, Jason Merrill wrote:
>>>>  gimple_predicate
>>>>  rhs_predicate_for (tree lhs)
>>>>  {
>>>> -  if (is_gimple_reg (lhs))
>>>> +  if (will_be_gimple_reg (lhs))
>>>>      return is_gimple_reg_rhs_or_call;
>>>>    else
>>>>      return is_gimple_mem_rhs_or_call;
>>>> @@ -4778,10 +4811,6 @@ gimplify_modify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
>>>>       that is what we must do here.  */
>>>>    maybe_with_size_expr (from_p);
>>>>
>>>> -  ret = gimplify_expr (to_p, pre_p, post_p, is_gimple_lvalue, fb_lvalue);
>>>> -  if (ret == GS_ERROR)
>>>> -    return ret;
>>>> -
>>>>    /* As a special case, we have to temporarily allow for assignments
>>>>       with a CALL_EXPR on the RHS.  Since in GIMPLE a function call is
>>>>       a toplevel statement, when gimplifying the GENERIC expression
>>>> @@ -4799,6 +4828,10 @@ gimplify_modify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
>>>>    if (ret == GS_ERROR)
>>>>      return ret;
>>>>
>>>> +  ret = gimplify_expr (to_p, pre_p, post_p, is_gimple_lvalue, fb_lvalue);
>>>> +  if (ret == GS_ERROR)
>>>> +    return ret;
>>>> +
>>>>    /* In case of va_arg internal fn wrappped in a WITH_SIZE_EXPR, add the type
>>>>       size as argument to the call.  */
>>>>    if (TREE_CODE (*from_p) == WITH_SIZE_EXPR)
>>>
>>> I wonder if instead of trying to guess early what we'll gimplify into it
>>> wouldn't be better to gimplify *from_p twice, first time with a predicate
>>> that would assume *to_p could be gimplified into is_gimple_ref, but
>>> guarantee there are no side-effects (so that those aren't evaluated
>>> after lhs side-effects), and second time if needed (if *to_p didn't end up
>>> being is_gimple_reg).  So something like a new predicate like:
>>
>> Yes, that is what I was suggesting.
>
> How about this?  I also have a patch to handle assignment order
> entirely in the front end, but my impression has been that you wanted
> to make this change for other reasons as well.

Yes.  Looks good to me.

> In other news, I convinced the committee to drop function arguments
> from the order of evaluation paper, so we don't have to worry about
> that hit on PUSH_ARGS_REVERSED targets.

Good.

Thanks,
Richard.

> Jason