On external_word_Relocation and StubCodeDesc

[Dean Long]

How many unique external addresses do we typically have?  If we really 
want to conserve footprint, would it make sense to store them all in a 
table, and not just stub addresses?

However, aren't external_word relocations rare?  It may not be worth it 
to encode the address as an index, or even to encode a 64-bit address as 
something smaller using deltas.  Do you have any statistics on the 
effect of external_word relocations on footprint?

dl

On 2/15/2016 7:37 AM, Vladimir Ivanov wrote:
> While working on JDK-8138922 [1], I noticed that 
> StubCodeDesc::desc_for is called for every external address embedded 
> in the code [2]. It is done to save some space on relocations (record 
> an index in the stub list instead of full address).
>
> StubCodeDesc::desc_for does linear search over the list and there are 
> around 100 stubs registered on x64.
>
> The problem is that the hit rate is very low. My experiments (on 
> Octane [3]) show that there are thousands of requests with 0% hit rate.
>
> So, instead of speeding up the search (switch to array, sort it and do 
> binary search to compute the index or lookup the address by index), I 
> suggest to completely remove that logic and always record the address 
> for external_word relocations.
>
> What's your take on that?
>
> Best regards,
> Vladimir Ivanov
>
> PS: there are some prospects in compacting some "well-known" addresses 
> [4], but it seems stubs aren't good candidates for that.
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8138922
>
> http://mail.openjdk.java.net/pipermail/hotspot-dev/2016-February/021702.html 
>
>
> [2] hotspot/src/share/vm/code/relocInfo.cpp:
> void external_word_Relocation::pack_data_to(CodeSection* dest) {
>   short* p = (short*) dest->locs_end();
>   int32_t index = runtime_address_to_index(_target);
>
> where runtime_address_to_index looks for the _target among stubs 
> registered in StubCodeDescs::_list:
>
> int32_t Relocation::runtime_address_to_index(address runtime_address) {
>   assert(!is_reloc_index((intptr_t)runtime_address), "must not look 
> like an index");
>
>   if (runtime_address == NULL)  return 0;
>
>   StubCodeDesc* p = StubCodeDesc::desc_for(runtime_address);
>   if (p != NULL && p->begin() == runtime_address) {
>     assert(is_reloc_index(p->index()), "there must not be too many 
> stubs");
>     return (int32_t)p->index();
>
> [3] perf counters:
> sun.ci.stubCodeDesc::total_reqs=11788
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=4117
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=3270
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=5183
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=7001
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=2909
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=10335
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=4698
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=5059
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=2763
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=3035
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> sun.ci.stubCodeDesc::total_reqs=7244
> sun.ci.stubCodeDesc::total_reqs_hit=0
>
> [4] Box2D external addresses stats:
> 5628 0x0000000102601396
>   16 0x00000001027cba58
>   20 0x00000001027cba60
>   85 0x00000001027cc198
>  110 0x00000001027d82a2
>   55 0x00000001027ee720
>  351 0x00000001027eea80
>    9 0x0000000102814380
>   41 0x00000001028143a0
>    2 0x0000000103246460
> 3812 0x0000000103246480