Classfile API support for JSR and RET instructions

[Remi Forax]

> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "Adam Sotona" <adam.sotona at oracle.com>, "classfile-api-dev"
> <classfile-api-dev at openjdk.org>
> Sent: Wednesday, April 19, 2023 5:21:52 PM
> Subject: Re: Classfile API support for JSR and RET instructions

> Overall this is a good plan. There's a small gap regarding control of stack map
> generation.

> The stack map attribute was introduced in classfile version 50; JSR and RET were
> eliminated in classfile version 51. So there's a small window where stackmaps
> and JSR/RET can co-exist.

> Our strategy with stackmaps is to generate based on an option, whose default is
> to generate. You are right that we should refine this so that we never generate
> stack maps for classfiles < 50 because the stackmap attribute wasn't defined
> until then.
Why ? StackMapTable is an attribute, it can be present in class file version < 50 and it will be ignored by the VM. 
Generating stack map is quite costly so generating them beforehand even if the class file version is < 50 is a trick to make agents that upgrade the class file version at runtime faster (because for most transformations, patching the stack maps is linear). 

Rémi 

> On 4/19/2023 9:29 AM, Adam Sotona wrote:

>> Hi,

>> Work on javap conversion from ASM to Classfile API and recent bug ( [
>> https://bugs.openjdk.org/browse/JDK-8305990 | JDK-8305990 ] ) discovered in
>> StripJavaDebugAttributesPlugin brought attention to missing JSR and RET
>> instructions support in Classfile API.

>> I’m proposing following changes to Classfile API:

>>    * Drop Opcode.Kind.UNSUPPORTED and add Opcode.Kind.DISCONTINUED_JSR and
>>     DISCONTINUED_RET
>>    * Add DiscontinuedInstruction interface with inner JsrInstruction and
>>     RetInstruction with standard factory methods
>>    * CodeImpl will parse and stream these instructions when present in the Code
>>     attribute
>>    * Do not add any new conveniency methods to CodeBuilder, the only way to build a
>>    code with these instructions will be cob.with(DiscontinuedInstruction.
>>     JsrInstruction.of(…)) and similar for RetInstruction
>>    * Fix DirectCodeBuilder so it invokes StackMapGenerator only for class file
>>     version >= 51.0 (BufWriterImpl must hold class version for that purpose)
>>    * Fix StackMapGenerator error message when these instruction appear in during
>>     generation
>>    * Implement fallback to jump target inflation in CodeImpl when the StackMapTable
>>     attribute is missing and class file version is < 51.0
>>    * Extend tests to verify JSR and RET instructions are correctly generated,
>>     transformed and reported as error (based on class file version).

>> Please review and comment this proposal under following pull request:

>> [ https://github.com/openjdk/jdk/pull/13478 |
>> https://github.com/openjdk/jdk/pull/13478 ]

>> Next step is to implement simple maxStack and maxLocals counter into
>> DirectCodeBuilder for the cases where StackMapGenerator is not involved. Actual
>> default to maxStack = maxLocals = 255 is not correct.

>> Thanks,

>> Adam
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