Re: [PATCH v2] ipa-inline: Add target info into fn summary [PR102059]

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Martin,

on 2021/9/17 下午7:26, Martin Jambor wrote:
> Hi,
> 
> On Fri, Sep 17 2021, Kewen.Lin wrote:
>> on 2021/9/16 下午9:19, Martin Jambor wrote:
>>> On Thu, Sep 16 2021, Kewen.Lin wrote:
>>>> on 2021/9/15 下午8:51, Martin Jambor wrote:
>>>>> On Wed, Sep 08 2021, Kewen.Lin wrote:
>>>>>>
>>>>>
>>>>> [...]
>>>>>
>>>>>> diff --git a/gcc/ipa-fnsummary.h b/gcc/ipa-fnsummary.h
>>>>>> index 78399b0b9bb..300b8da4507 100644
>>>>>> --- a/gcc/ipa-fnsummary.h
>>>>>> +++ b/gcc/ipa-fnsummary.h
>>>>>> @@ -193,6 +194,9 @@ public:
>>>>>>    vec<ipa_freqcounting_predicate, va_gc> *loop_strides;
>>>>>>    /* Parameters tested by builtin_constant_p.  */
>>>>>>    vec<int, va_heap, vl_ptr> GTY((skip)) builtin_constant_p_parms;
>>>>>> +  /* Like fp_expressions, but it's to hold some target specific information,
>>>>>> +     such as some target specific isa flags.  */
>>>>>> +  auto_vec<HOST_WIDE_INT> GTY((skip)) target_info;
>>>>>>    /* Estimated growth for inlining all copies of the function before start
>>>>>>       of small functions inlining.
>>>>>>       This value will get out of date as the callers are duplicated, but
>>>>>
>>>>> Segher already wrote in the first thread that a vector of HOST_WIDE_INTs
>>>>> is an overkill and I agree.  So at least make the new field just a
>>>>> HOST_WIDE_INT or better yet, an unsigned int.  But I would even go
>>>>> further and make target_info only a 16-bit bit-field, place it after the
>>>>> other bit-fields in class ipa_fn_summary and pass it to the hooks as
>>>>> uint16_t.  Unless you have plans which require more space, I think we
>>>>> should be conservative here.
>>>>>
>>>>
>>>> OK, yeah, the consideration is mainly for the scenario that target has
>>>> a few bits to care about.  I just realized that to avoid inefficient
>>>> bitwise operation for mapping target info bits to isa_flag bits, target
>>>> can rearrange the sparse bits in isa_flag, so it's not a deal.
>>>> Thanks for re-raising this!  I'll use the 16 bits bit-field in v3 as you
>>>> suggested, if you don't mind, I will put it before the existing bit-fields
>>>> to have a good alignment.
>>>
>>> All right.
>>>
>>
>> Sorry that I failed to use 16 bit-fields for this, I figured out that
>> the bit-fields can not be address-taken or passed as non-const reference.
>> The gentype also failed to recognize uint16_t if I used uint16_t directly
>> in ipa-fnsummary.h.  Finally I used unsigned int instead.
>>
> 
> well, you could have used:
> 
>   unsigned int target_info : 16;
> 
> for the field (and uint16_t when passed to hooks).
> 
> But I am not sure if it is that crucial.
> 

I may miss something, specifically I tried with:

1)

  unsigned int target_info : 16;
  unsigned inlinable : 1;
  ...

  update_ipa_fn_target_info (uint16_t &, const gimple *)

2)

  unsigned int target_info : 16;
  unsigned inlinable : 1;
  ...

  update_ipa_fn_target_info (uint16_t *, const gimple *)

The above two ways failed due to:

"Because bit fields do not necessarily begin at the beginning of a byte,
address of a bit field cannot be taken. Pointers and non-const references
to bit fields are not possible." as [1].

Although we can change the hook prototype to

  bool update_ipa_fn_target_info (const uint16_t, const gimple*, uint16_t&)

or

  uint16_t update_ipa_fn_target_info (const uint16_t, const gimple*, bool&)

to workaround bit field limitation, it looks weird and inefficient.

3)

  ...
  unsigned int fp_expressions : 1;
  uint16_t target_info;

  update_ipa_fn_target_info (uint16_t &, const gimple *)

it fails due to gengtype erroring:

gcc/ipa-fnsummary.h:171: undefined type `uint16_t'
gengtype: didn't write state file tmp-gtype.state after errors


Then I gave up and guessed it's not so crucial like you said, 
and used unsigned int instead. :)

[1] https://en.cppreference.com/w/cpp/language/bit_field

BR,
Kewen