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Re: [PATCH 3/3] Add a simple array for CU abbreviations with low codes
- From: Josh Stone <jistone at redhat dot com>
- To: elfutils-devel at lists dot fedorahosted dot org
- Date: Thu, 12 Dec 2013 11:16:08 -0800
- Subject: Re: [PATCH 3/3] Add a simple array for CU abbreviations with low codes
On 12/12/2013 09:07 AM, Petr Machata wrote:
> Josh Stone <firstname.lastname@example.org> writes:
>> struct Dwarf_CU
>> @@ -283,7 +285,9 @@ struct Dwarf_CU
>> size_t type_offset;
>> uint64_t type_sig8;
>> - /* Hash table for the abbreviations. */
>> + /* Simple array for the abbreviations with low codes. */
>> + Dwarf_Abbrev *abbrev_array[CU_ABBREV_ARRAY_SIZE];
> This blows up Dwarf_CU from 100odd bytes to something like half the
> page, I'm not entirely fond of that. Especially since the hash table
> that follows is an on-demand-growing structure, having half a page
> reserved (possibly on stack) just in case seems wasteful.
> I'm looking into some debuginfo files. libc has an average of 28
> abbreviations per CU (with 108 being the most), libstdc++ an average of
> 77 (with 108 the most), gcc 88 (142), libbost_python 206 (290), vmlinux
> 112 (185). So reserving space for 256 seems overly generous. I'd be
> fine with a growable heap-allocated array capped at 256. Hopefully that
> would still be helpful performance-wise.
OK, I'll experiment with less generous static sizes, on the heap, as
well as dynamic/realloced size (still capped though).
I also had the idea that maybe lookup can sometimes skip the modulus,
which was by far the hotspot instruction in lookup() by ~72%. Just
basically moving my "is-it-a-small-code" branch into hash lookup. So
everything would stay in the hash table, hopefully just a bit faster.
More things to try, anyway...