RFR: 8351965: [leyden] Skip installing C2 AOT code if C2 precompiled AOT code trapped [v4]

[Aleksey Shipilev]

On Tue, 18 Mar 2025 19:42:03 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 incrementally with one additional commit since the last revision:
> 
>   Handle things on trapping path

For the reference, this is how sample dynamics look like now. After the trap from AP4 code, no attempt to load A4 code is made, and only a single T2 compilation happens (used to be two), followed by T4 compilation.


$ grep com.sun.tools.javac.util.StringNameTable::fromString print-compilation.log 
     43    W0.0    Q8.8    C0.1    333      AP 4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
[0.047s][debug  ][deoptimization] cid= 333     level=4 com.sun.tools.javac.util.StringNameTable::fromString(Ljava/lang/String;)Lcom/sun/tools/javac/util/Name; trap_bci=28 unloaded reinterpret pc=0x000071945803a2c0 relative_pc=0x00000000000003c0
     47                            333      AP 4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)   made not entrant: uncommon trap
     47    W0.1    Q0.0    C0.6   1379         2       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)
     99                           1379         2       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)   made not entrant: not used
     99    W2.7    Q0.0    C8.5   1839         4       com.sun.tools.javac.util.StringNameTable::fromString (50 bytes)

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

PR Comment: https://git.openjdk.org/leyden/pull/48#issuecomment-2734542631