RFR: 8346989: C2: deoptimization and re-compilation cycle with Math.*Exact in case of frequent overflow [v2]

[Vladimir Ivanov]

On Wed, 26 Mar 2025 08:33:58 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> `Math.*Exact` intrinsics can cause many deopt when used repeatedly with problematic arguments.
>> This fix proposes not to rely on intrinsics after `too_many_traps()` has been reached.
>> 
>> Benchmark show that this issue affects every Math.*Exact functions. And this fix improve them all.
>> 
>> tl;dr:
>> - C1: no problem, no change
>> - C2:
>>   - with intrinsics:
>>     - with overflow: clear improvement. Was way worse than C1, now is similar (~4s => ~600ms)
>>     - without overflow: no problem, no change
>>   - without intrinsics: no problem, no change
>> 
>> Before the fix:
>> 
>> Benchmark                                           (SIZE)  Mode  Cnt     Score      Error  Units
>> MathExact.C1_1.loopAddIInBounds                    1000000  avgt    3     1.272 ±    0.048  ms/op
>> MathExact.C1_1.loopAddIOverflow                    1000000  avgt    3   641.917 ±   58.238  ms/op
>> MathExact.C1_1.loopAddLInBounds                    1000000  avgt    3     1.402 ±    0.842  ms/op
>> MathExact.C1_1.loopAddLOverflow                    1000000  avgt    3   671.013 ±  229.425  ms/op
>> MathExact.C1_1.loopDecrementIInBounds              1000000  avgt    3     3.722 ±   22.244  ms/op
>> MathExact.C1_1.loopDecrementIOverflow              1000000  avgt    3   653.341 ±  279.003  ms/op
>> MathExact.C1_1.loopDecrementLInBounds              1000000  avgt    3     2.525 ±    0.810  ms/op
>> MathExact.C1_1.loopDecrementLOverflow              1000000  avgt    3   656.750 ±  141.792  ms/op
>> MathExact.C1_1.loopIncrementIInBounds              1000000  avgt    3     4.621 ±   12.822  ms/op
>> MathExact.C1_1.loopIncrementIOverflow              1000000  avgt    3   651.608 ±  274.396  ms/op
>> MathExact.C1_1.loopIncrementLInBounds              1000000  avgt    3     2.576 ±    3.316  ms/op
>> MathExact.C1_1.loopIncrementLOverflow              1000000  avgt    3   662.216 ±   71.879  ms/op
>> MathExact.C1_1.loopMultiplyIInBounds               1000000  avgt    3     1.402 ±    0.587  ms/op
>> MathExact.C1_1.loopMultiplyIOverflow               1000000  avgt    3   615.836 ±  252.137  ms/op
>> MathExact.C1_1.loopMultiplyLInBounds               1000000  avgt    3     2.906 ±    5.718  ms/op
>> MathExact.C1_1.loopMultiplyLOverflow               1000000  avgt    3   655.576 ±  147.432  ms/op
>> MathExact.C1_1.loopNegateIInBounds                 1000000  avgt    3     2.023 ±    0.027  ms/op
>> MathExact.C1_1.loopNegateIOverflow                 1000000  avgt    3   639.136 ±   30.841  ms/op
>> MathExact.C1_1.loop...
>
> Marc Chevalier has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - Use builtin_throw
>  - Merge branch 'master' into fix/Deoptimization-and-re-compilation-cycle-with-C2-compiled-code
>  - More exhaustive bench
>  - Limit inlining of math Exact operations in case of too many deopts

Thanks, Marc.

It looks a bit too convoluted to me. IMO an unconditional call to `builtin_throw`, plus `too_many_traps` check should do the job. Do I miss something important here?

src/hotspot/share/opto/graphKit.hpp line 279:

> 277:   // The JVMS must allow the bytecode to be re-executed via an uncommon trap.
> 278:   // If `exception_object` is nullptr, the exception to throw will be guessed based on `reason`
> 279:   void builtin_throw(Deoptimization::DeoptReason reason, ciInstance* exception_object = nullptr);

Please, introduce a new overload instead. 

I suggest to extract Deoptimization::DeoptReason -> ciInstance mapping into a helper method and turn `void builtin_throw(Deoptimization::DeoptReason reason)` into a wrapper:

void GraphKit::builtin_throw(Deoptimization::DeoptReason reason) {
   builtin_throw(reason, exception_on_deopt(reason));
}

src/hotspot/share/opto/library_call.cpp line 2035:

> 2033: 
> 2034:     if (use_builtin_throw) {
> 2035:       builtin_throw(Deoptimization::Reason_intrinsic, env()->ArithmeticException_instance());

I suggest to unconditionally call `builtin_throw()`. It should handle `uncommon_trap` case as well.

What makes sense is to ensure that `builtin_throw()` doesn't change deoptimization reason. It can be implemented with an extra argument to new `GraphKit::builtin_throw` overload (e.g., `bool allow_deopt_reason_none`).

src/hotspot/share/opto/library_call.cpp line 2054:

> 2052:     // instead of bailing out on intrinsic or potentially deopting, let's do that!
> 2053:     use_builtin_throw = true;
> 2054:   } else if (too_many_traps(Deoptimization::Reason_intrinsic)) {

Why `too_many_traps(Deoptimization::Reason_intrinsic)` check is not enough here?

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

PR Review: https://git.openjdk.org/jdk/pull/23916#pullrequestreview-2726864135
PR Review Comment: https://git.openjdk.org/jdk/pull/23916#discussion_r2019432922
PR Review Comment: https://git.openjdk.org/jdk/pull/23916#discussion_r2019444895
PR Review Comment: https://git.openjdk.org/jdk/pull/23916#discussion_r2019449996