On external_word_Relocation and StubCodeDesc

[Vladimir Kozlov]

Actually you could optimize it in case of segmented CodeCache (default 
with tiered) by checking if PC is in Non-nmethods segment.

Also stub's address should not be external since stubs are located in 
CodeCache. External addresses are used for runtime (or C-heap) addresses.

Thanks,
Vladimir K

On 2/15/16 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