RFR: 8287555: Tighten G1 G1BlockOffsetTable::block_start() code

[Thomas Schatzl]

On Fri, 24 Jun 2022 06:50:32 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1BlockOffsetTable.hpp line 166:
>> 
>>> 164:   // Returns the address of the start of the block containing "addr", assuming that
>>> 165:   // the given address is card-aligned.
>>> 166:   inline HeapWord* block_start_aligned(const void* addr) const;
>> 
>> I am not sure introducing another API is really that necessary. What I have in mind is sth like:
>> 
>> 
>> void block_start(const void* addr) {
>>   if (card_aligned(addr)) {
>>     fast_path
>>   } else {
>>     slow_path
>>   }
>>   assert_post-cond
>> }
>> 
>> 
>> I don't think the cost of if-check will be observable.
>
> I had something like that, but the "not observable" part was not that clear in my results (i.e. during garbage collection; "not clear" as in it seems to be slower) so I only optimized the part where this can be statically determined for now.

CPU resources like branch prediction/speculative execution aren't free either, and this is a very performance sensitive area of code.
I'm planning to revisit this again for the card scanning code which in 99% of cases takes the aligned path.

-------------

PR: https://git.openjdk.org/jdk/pull/9059