Re: [PATCH v3] bpf: remove huge memory waste with string allocation.

[Cupertino Miranda <cupertino.miranda@oracle.com>]

David Faust writes:

> Hi Cupertino,
>
> On 4/18/24 13:58, Cupertino Miranda wrote:
>> Hi David, everyone,
>>
>> Following Davids last review I decided to properly detect error cases,
>> as suggested.
>> The error however should be reported earlier in compilation in
>> pack_enum_valud function, where all the errors are reported.
>>
>> Thanks for the quick and detailed reviews.
>>
>> Regards,
>> Cupertino
>
> Thanks for taking the time on this.
> This version is nice, just one little comment:
>
>>
>> The BPF backend was allocating an unnecessarily large string when
>> constructing CO-RE relocations for enum types.
>> This patch further verifies if an enumerator is valid for CO-RE
>> representability and returns an error in those cases.
>
> The second sentence is a little awkward and seems to imply the error is
> returned when the enumerator is valid :)
> Perhaps "...verifies that an enumerator is valid for CO-RE, and returns
> an error if it is not" or similar would be more clear?
Thanks for all the suggestions.
>
> Otherwise, OK.
> Thanks!
Pushed!

>
>
>>
>> gcc/ChangeLog:
>> 	* config/bpf/core-builtins.cc (get_index_for_enum_value): Create
>> 	function.
>> 	(pack_enum_value): Check for enumerator and error out.
>> 	(process_enum_value): Correct string allocation.
>> ---
>>  gcc/config/bpf/core-builtins.cc | 57 ++++++++++++++++++++++-----------
>>  1 file changed, 38 insertions(+), 19 deletions(-)
>>
>> diff --git a/gcc/config/bpf/core-builtins.cc b/gcc/config/bpf/core-builtins.cc
>> index e03e986e2c1..829acea98f7 100644
>> --- a/gcc/config/bpf/core-builtins.cc
>> +++ b/gcc/config/bpf/core-builtins.cc
>> @@ -795,6 +795,23 @@ process_field_expr (struct cr_builtins *data)
>>  static GTY(()) hash_map<tree, tree> *bpf_enum_mappings;
>>  tree enum_value_type = NULL_TREE;
>>
>> +static int
>> +get_index_for_enum_value (tree type, tree expr)
>> +{
>> +  gcc_assert (TREE_CODE (expr) == CONST_DECL
>> +	      && TREE_CODE (type) == ENUMERAL_TYPE);
>> +
>> +  unsigned int index = 0;
>> +  for (tree l = TYPE_VALUES (type); l; l = TREE_CHAIN (l))
>> +    {
>> +      gcc_assert (index < (1 << 16));
>> +      if (TREE_VALUE (l) == expr)
>> +	return index;
>> +      index++;
>> +    }
>> +  return -1;
>> +}
>> +
>>  /* Pack helper for the __builtin_preserve_enum_value.  */
>>
>>  static struct cr_local
>> @@ -846,6 +863,16 @@ pack_enum_value_fail:
>>  	ret.reloc_data.default_value = integer_one_node;
>>      }
>>
>> +  if (ret.fail == false )
>> +    {
>> +      int index = get_index_for_enum_value (type, tmp);
>> +      if (index == -1 || index >= (1 << 16))
>> +	{
>> +	  bpf_error ("enum value in CO-RE builtin cannot be represented");
>> +	  ret.fail = true;
>> +	}
>> +    }
>> +
>>    ret.reloc_data.type = type;
>>    ret.reloc_data.kind = kind;
>>    return ret;
>> @@ -864,25 +891,17 @@ process_enum_value (struct cr_builtins *data)
>>
>>    struct cr_final ret = { NULL, type, data->kind };
>>
>> -  if (TREE_CODE (expr) == CONST_DECL
>> -     && TREE_CODE (type) == ENUMERAL_TYPE)
>> -    {
>> -      unsigned int index = 0;
>> -      for (tree l = TYPE_VALUES (type); l; l = TREE_CHAIN (l))
>> -	{
>> -	  if (TREE_VALUE (l) == expr)
>> -	    {
>> -	      char *tmp = (char *) ggc_alloc_atomic ((index / 10) + 1);
>> -	      sprintf (tmp, "%d", index);
>> -	      ret.str = (const char *) tmp;
>> -
>> -	      break;
>> -	    }
>> -	  index++;
>> -	}
>> -    }
>> -  else
>> -    gcc_unreachable ();
>> +  gcc_assert (TREE_CODE (expr) == CONST_DECL
>> +	      && TREE_CODE (type) == ENUMERAL_TYPE);
>> +
>> +  int index = get_index_for_enum_value (type, expr);
>> +  gcc_assert (index != -1 && index < (1 << 16));
>> +
>> +  /* Index can only be a value up to 2^16.  Should always fit
>> +     in 6 chars.  */
>> +  char tmp[6];
>> +  sprintf (tmp, "%u", index);
>> +  ret.str = CONST_CAST (char *, ggc_strdup(tmp));
>>
>>    return ret;
>>  }