Re: [PATCH 4/7] Class-fy partial_die_info

Simon Marchi
2018-01-31T03:46:35Z

On 2018-01-30 06:39, Pedro Alves wrote:
> On 01/29/2018 01:15 AM, Simon Marchi wrote:
>
>> From what I understand, the only reason to have that private
>> constructor is
>> to construct a temporary partial_die_info object used to search in the
>> htab,
>> is that right? If so, converting that htab_t to an std::unordered_map
>> would
>> remove the need for all this, since you don't need to construct an
>> object
>> to search it. See the diff below that applies on top of this patch.
>>
>> It's not thoroughly tested and I am not sure of the validity of the
>> per_cu->cu->partial_dies.empty () call in find_partial_die, but I
>> think it
>> should work. Plus, it adds some type-safety, which I am a big fan of.
>>
>> But otherwise, the patch is fine with me.
>
> Careful here. This could do with some benchmarking. The DWARF reading
> code
> is performance (both timing and memory) sensitive. This is trading an
> open
> addressing hash table (htab_t), for a node-based closed addressing hash
> table.
> The keys/elements in the map are small so I'd expect this to make
> a difference. Also, this is trading a in-principle cache-friendly
> obstack allocation scheme for the standard new allocator.

Ah, indeed. I thought that unordered_map would be implemented the same
way as htab_t, but I see it's not the case. Doing some quick tests on a
big binary, it increases the time reading the symbols from an average of
37 seconds to an average of 42 seconds.

I understand the different hash table implementation having an impact,
but I don't really understand how the allocation scheme can have a
meaningful impact. The partial_die_info objects are still allocated on
the obstack, aren't they? So it's just the space for the table itself
that isn't on the objstack, but I don't see why that would make a
difference.

If we want to have a data structure with the same kind of performance as
htab_t but with type-safety in the future, is your vision that we'll
have to implement it ourselves? Should we make a wrapper around htab_t?

Simon