RFR: 8351965: [leyden] Skip installing C2 AOT code if C2 precompiled AOT code trapped [v2]

[Vladimir Kozlov]

On Tue, 18 Mar 2025 18:43:08 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> This is clearly visible in compilation logs:
>> 
>> 
>>      43    W0.1    Q8.1    C0.0    293      AP 4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
>> [0.049s][debug  ][deoptimization] cid= 293     level=4 com.sun.tools.javac.util.StringNameTable::fromString(Ljava/lang/String;)Lcom/sun/tools/javac/util/Name; trap_bci=28 unloaded reinterpret pc=0x00007c6bd7e4e7ac relative_pc=0x000000000000068c
>>      49                            293      AP 4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)   made not entrant
>>      49    W0.2    Q0.0    C0.3   1394         2       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
>>      90                           1394         2       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)   made not entrant
>>      90    W0.0    Q0.0    C0.1   1867      A  4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
>> [0.098s][debug  ][deoptimization] cid=1867     level=4 com.sun.tools.javac.util.StringNameTable::fromString(Ljava/lang/String;)Lcom/sun/tools/javac/util/Name; trap_bci=28 unloaded reinterpret pc=0x00007c6bd7ebcb58 relative_pc=0x00000000000005d8
>>      98                           1867      A  4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)   made not entrant
>>     104    W0.0    Q0.0    C0.3   1942         2       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
>>     130                           1942         2       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)   made not entrant
>>     130    W1.0    Q0.7   C13.3   1968         4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
>> 
>> 
>> So the AP4 method was preloaded, then it trapped and got replaced by T2 method, which eventually got to C2, at which point we loaded A4 method. That method trapped _at the same bci_, so we are back at T2, then then to real T4. So we have spent one deopt cycle unnecessarily, and the code was in T2 for twice as long.
>> 
>> I don't think we would be able to fully tame uncommon traps from the preload code, so fixing this gap is valuable.
>> 
>> `decompile_count()` is only updated by C2, so we can just check it directly.
>> 
>> Additional testing:
>>  - [x] Ad-hoc benchmarks
>>  - [x] Linux x86_64 server fastdebug, `runtime/cds`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:
> 
>  - Indenting
>  - Null-check
>  - Count preload decompiles separately
>  - Cleanup
>  - Fix

> Some uncommon traps are endemic to preload code. I suspect the one in the disassembly above could come from places like ciTypeFlow::StateVector::do_new. Regardless, I believe that if preload code traps, then non-preload code would also trap in those conditions, since both versions were compiled in the same environment.

Right. They are also compiled by the same compiler thread one after an other: [output.cpp#L3463](https://github.com/openjdk/leyden/blob/premain/src/hotspot/share/opto/output.cpp#L3463)
Note, `Reason_PrecompileForPreload` is used only for precompilation feature which we are not using for our workflow AFAICS.

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PR Comment: https://git.openjdk.org/leyden/pull/48#issuecomment-2734456062