RFR: 8356946: x86: Optimize interpreter profile updates

[Aleksey Shipilev]

On Wed, 14 May 2025 16:24:23 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Noticed two awkward things in current x86 interpreter profiling code.
>> 
>> First, we carry the implementation for counter decrements without using them. This is dead code, and can be purged.
>> 
>> Second, we care about overflows for 64-bit for some reason. I think this is a reminiscent of 32-bit x86 support, where we can plausibly have 32-bit counter overflow in a reasonable timeframe. But for 64-bit counter, we need tens of years of constantly bashing the counter to get it to overflow. No other profile counter update code, e.g. in C1, cares about this.
>> 
>> So we can save a few instructions / memory accesses on this path. 
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `tier1`
>>  - [ ] Linux x86_64 server fastdebug, `all`
>
> src/hotspot/cpu/x86/interp_masm_x86.hpp line 217:
> 
>> 215:   void increment_mdp_data_at(Address data, bool decrement = false);
>> 216:   void increment_mdp_data_at(Register mdp_in, int constant,
>> 217:                              bool decrement = false);
> 
> `decrement` is never used?

Yes, decrement is never used, it's a dead code.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25223#discussion_r2090616558