[PING][PATCH] define auto_vec copy ctor and assignment (PR 90904)

[Martin Sebor]

On 7/20/21 2:08 PM, Jason Merrill wrote:
> On 7/20/21 2:34 PM, Martin Sebor wrote:
>> On 7/14/21 10:23 AM, Jason Merrill wrote:
>>> On 7/14/21 10:46 AM, Martin Sebor wrote:
>>>> On 7/13/21 9:39 PM, Jason Merrill wrote:
>>>>> On 7/13/21 4:02 PM, Martin Sebor wrote:
>>>>>> On 7/13/21 12:37 PM, Jason Merrill wrote:
>>>>>>> On 7/13/21 10:08 AM, Jonathan Wakely wrote:
>>>>>>>> On Mon, 12 Jul 2021 at 12:02, Richard Biener wrote:
>>>>>>>>> Somebody with more C++ knowledge than me needs to approve the
>>>>>>>>> vec.h changes - I don't feel competent to assess all effects of 
>>>>>>>>> the change.
>>>>>>>>
>>>>>>>> They look OK to me except for:
>>>>>>>>
>>>>>>>> -extern vnull vNULL;
>>>>>>>> +static constexpr vnull vNULL{ };
>>>>>>>>
>>>>>>>> Making vNULL have static linkage can make it an ODR violation to 
>>>>>>>> use
>>>>>>>> vNULL in templates and inline functions, because different
>>>>>>>> instantiations will refer to a different "vNULL" in each 
>>>>>>>> translation
>>>>>>>> unit.
>>>>>>>
>>>>>>> The ODR says this is OK because it's a literal constant with the 
>>>>>>> same value (6.2/12.2.1).
>>>>>>>
>>>>>>> But it would be better without the explicit 'static'; then in 
>>>>>>> C++17 it's implicitly inline instead of static.
>>>>>>
>>>>>> I'll remove the static.
>>>>>>
>>>>>>>
>>>>>>> But then, do we really want to keep vNULL at all?  It's a weird 
>>>>>>> blurring of the object/pointer boundary that is also dependent on 
>>>>>>> vec being a thin wrapper around a pointer.  In almost all cases 
>>>>>>> it can be replaced with {}; one exception is == comparison, where 
>>>>>>> it seems to be testing that the embedded pointer is null, which 
>>>>>>> is a weird thing to want to test.
>>>>>>
>>>>>> The one use case I know of for vNULL where I can't think of
>>>>>> an equally good substitute is in passing a vec as an argument by
>>>>>> value.  The only way to do that that I can think of is to name
>>>>>> the full vec type (i.e., the specialization) which is more typing
>>>>>> and less generic than vNULL.  I don't use vNULL myself so I wouldn't
>>>>>> miss this trick if it were to be removed but others might feel
>>>>>> differently.
>>>>>
>>>>> In C++11, it can be replaced by {} in that context as well.
>>>>
>>>> Cool.  I thought I'd tried { } here but I guess not.
>>>>
>>>>>
>>>>>> If not, I'm all for getting rid of vNULL but with over 350 uses
>>>>>> of it left, unless there's some clever trick to make the removal
>>>>>> (mostly) effortless and seamless, I'd much rather do it independently
>>>>>> of this initial change. I also don't know if I can commit to making
>>>>>> all this cleanup.
>>>>>
>>>>> I already have a patch to replace all but one use of vNULL, but 
>>>>> I'll hold off with it until after your patch.
>>>>
>>>> So what's the next step?  The patch only removes a few uses of vNULL
>>>> but doesn't add any.  Is it good to go as is (without the static and
>>>> with the additional const changes Richard suggested)?  This patch is
>>>> attached to my reply to Richard:
>>>> https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575199.html
>>>
>>> As Richard wrote:
>>>
>>>> The pieces where you change vec<> passing to const vec<>& and the few
>>>> where you change vec<> * to const vec<> * are OK - this should make the
>>>> rest a smaller piece to review.
>>>
>>> Please go ahead and apply those changes and send a new patch with the 
>>> remainder of the changes.
>>
>> I have just pushed r12-2418:
>> https://gcc.gnu.org/pipermail/gcc-cvs/2021-July/350886.html
>>
>>>
>>> A few other comments:
>>>
>>>> -                       omp_declare_simd_clauses);
>>>> +                       *omp_declare_simd_clauses);
>>>
>>> Instead of doing this indirection in all of the callers, let's change 
>>> c_finish_omp_declare_simd to take a pointer as well, and do the 
>>> indirection in initializing a reference variable at the top of the 
>>> function.
>>
>> Okay.
>>
>>>
>>>> +    sched_init_luids (bbs.to_vec ());
>>>> +    haifa_init_h_i_d (bbs.to_vec ());
>>>
>>> Why are these to_vec changes needed when you are also changing the 
>>> functions to take const&?
>>
>> Calling to_vec() here isn't necessary so I've removed it.
>>
>>>
>>>> -  vec<tree> checks = LOOP_VINFO_CHECK_NONZERO (loop_vinfo);
>>>> +  vec<tree> checks = LOOP_VINFO_CHECK_NONZERO (loop_vinfo).to_vec ();
>>>
>>> Why not use a reference here and in other similar spots?
>>
>> Sure, that works too.
>>
>> Attached is what's left of the original changes now that r12-2418
>> has been applied.
> 
>> @@ -3364,7 +3364,8 @@ static void
>>  vect_check_lower_bound (loop_vec_info loop_vinfo, tree expr, bool 
>> unsigned_p,
>>              poly_uint64 min_value)
>>  {
>> -  vec<vec_lower_bound> lower_bounds = LOOP_VINFO_LOWER_BOUNDS 
>> (loop_vinfo);
>> +  vec<vec_lower_bound> lower_bounds
>> +    = LOOP_VINFO_LOWER_BOUNDS (loop_vinfo).to_vec ();
>>    for (unsigned int i = 0; i < lower_bounds.length (); ++i)
>>      if (operand_equal_p (lower_bounds[i].expr, expr, 0))
>>        {
>> @@ -3466,7 +3467,7 @@ vect_prune_runtime_alias_test_list 
>> (loop_vec_info loop_vinfo)
>>    typedef pair_hash <tree_operand_hash, tree_operand_hash> 
>> tree_pair_hash;
>>    hash_set <tree_pair_hash> compared_objects;
>>
>> -  vec<ddr_p> may_alias_ddrs = LOOP_VINFO_MAY_ALIAS_DDRS (loop_vinfo);
>> +  vec<ddr_p> may_alias_ddrs = LOOP_VINFO_MAY_ALIAS_DDRS 
>> (loop_vinfo).to_vec ();
> 
> These could also be references.

Even const references it turns out for some of them.

> That leaves this as the only remaining use of to_vec:
> 
>>    ipa_call_arg_values (ipa_auto_call_arg_values *aavals)
>> -    : m_known_vals (aavals->m_known_vals),
>> -      m_known_contexts (aavals->m_known_contexts),
>> -      m_known_aggs (aavals->m_known_aggs),
>> -      m_known_value_ranges (aavals->m_known_value_ranges)
>> +    : m_known_vals (aavals->m_known_vals.to_vec ()),
>> +      m_known_contexts (aavals->m_known_contexts.to_vec ()),
>> +      m_known_aggs (aavals->m_known_aggs.to_vec ()),
>> +      m_known_value_ranges (aavals->m_known_value_ranges.to_vec ())
> 
> I think we could handle this by deriving ipa_auto_call_arg_values from 
> ipa_call_arg_values like auto_vec is derived from vec, but perhaps 
> dealing with the IPA datastructures could be saved for the next stage of 
> overhauling vec.  Maybe for now just change the name to_vec to something 
> clearer that new code shouldn't use it, e.g. to_vec_legacy.

Done in the attached patch.

Martin
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