RFR: 8359965: Enable paired pushp and popp instruction usage for APX enabled CPUs [v2]

[Jatin Bhateja]

On Wed, 2 Jul 2025 23:28:42 GMT, Srinivas Vamsi Parasa <sparasa at openjdk.org> wrote:

>> For a cleaner interface, I think we can also maintain a RAII style APXPushPopPairTracker in the stub snippets using push/pop instruction sequence and wrap the actual assembler call underneath. The idea here is to catch the balancing error upfront as PPX is purely a performance hint. Instructions with this hint have the same functional semantics as those without. PPX hints set by the compiler that violate the balancing rule may turn off the PPX
>> optimization, but they will not affect program semantics..
>> 
>> 
>> class APXPushPopPairTracker {
>>     private:
>>         int _counter;
>>  
>>     public:
>>         APXPushPopPairTracker() _counter(0) {
>>         }
>> 
>>        ~APXPushPopPairTracker() {
>>            assert(_counter == 0, "Push/pop pair mismatch");
>>         }
>>      
>>         void push(Register reg, bool has_matching_pop) {
>>             if (has_matching_pop && VM_Version::supports_apx_f()) {
>>                Assembler::pushp(reg);
>>                incrementCounter();
>>             } else {
>>                Assembler::push(reg);
>>             }
>>         }
>>         void pop(Register reg, bool has_matching_push) {
>>             if (has_matching_push && VM_Version::supports_apx_f()) {
>>                Assembler::popp(reg);
>>                decrementCounter();
>>             } else {
>>                Assembler::pop(reg);
>>             }
>>         }     
>>         void incrementCounter() {
>>           _counter++;
>>         }
>>         void decrementCounter() {
>>            _counter--;
>>         }
>> }
>
> Hi Jatin (@jatin-bhateja) and Vlad (@vpaprotsk),
> 
> There's one more issue to be considered. The C++ PushPopTracker code will be run during the stub generation time. There are code bocks which do a single push onto the stack but due to multiple exit paths, there will be multiple pops as illustrated below. Will this reference counting approach not fail in such a scenario as the stub code is generated all at once during the stub generation phase?
> 
> 
> #begin stack frame
> push(r21)
> 
> #exit condition 1
> pop(r21)
> 
> # exit condition 2
> pop(r21)

There is no one size fits all soution, idea is to be smart whereever possible, by maintaining a fixed stack of registers populated during push operation we can delegate the responsibility of emitting pop instructions in reverse order to tracker, @vamsi-parasa for now I am ok with maintaining existing implimentation.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/25889#discussion_r2204183436