RFR: 8299162: Refactor shared trampoline emission logic [v2]

[Fei Yang]

On Thu, 22 Dec 2022 03:44:11 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

>> After the quick fix [JDK-8297763](https://bugs.openjdk.org/browse/JDK-8297763), shared trampoline logic gets a bit verbose. If we can turn to batch emission of trampoline stubs, pre-calculating the total size, and pre-allocating them, then we can remove the CodeBuffer expansion checks each time and clean up the code around.
>> 
>> 
>> [Stub Code]
>> ...
>> <shared trampoline stub1, (A):>
>>   __ align()  // emit nothing or a 4-byte padding
>>               <-- (B) emit multiple relocations at the pc: __ relocate(<the pc here>, trampoline_stub_Relocation::spec())
>>   __ ldr()
>>   __ br()
>>   __ emit_int64()
>> <shared trampoline stub2, (C):>
>>   __ align()  // emit nothing or a 4-byte padding
>>               <-- emit multiple relocations at the pc: __ relocate(<the pc here>, trampoline_stub_Relocation::spec())
>>   __ ldr()    
>>   __ br()
>>   __ emit_int64()
>> <shared trampoline stub3:>
>>   __ align()  // emit nothing or a 4-byte padding
>>               <-- emit multiple relocations at the pc: __ relocate(<the pc here>, trampoline_stub_Relocation::spec())
>>   __ ldr()
>>   __ br()
>>   __ emit_int64()
>> 
>> 
>> Here, the `pc_at_(C) - pc_at_(B)` is the fixed length `NativeCallTrampolineStub::instruction_size`; but the `pc_at_(B) - pc_at_(A)` may be a 0 or 4, which is not a fixed-length value.
>> 
>> So originally:
>> The logic of the lambda `emit()` inside the `emit_shared_trampolines()` when emitting a shared trampoline:
>> 
>> We are at (A) -> 
>> do an align() -> 
>> We are at (B) -> 
>> emit lots of relocations bound to this shared trampoline at (B) -> 
>> do an emit_trampoline_stub() -> 
>> We are at (C)
>> 
>> 
>> After this patch:
>> 
>> We are at (A) -> 
>> do an emit_trampoline_stub(), which contains an align() already ->
>> We are at (C) directly ->
>> reversely calculate the (B) address, for `pc_at_(C) - pc_at_(B)` is a fixed-length value -> 
>> emit lots of relocations bound to this shared trampoline at (B)
>> 
>> 
>> Theoretically the same. Just a code refactoring and we can remove some checks inside and make the code clean.
>> 
>> Tested AArch64 hotspot tier1~4 with fastdebug build twice; Tested RISC-V hotspot tier1~4 with fastdebug build on hardware once.
>> 
>> 
>> Thanks,
>> Xiaolin
>
> Xiaolin Zheng has updated the pull request incrementally with one additional commit since the last revision:
> 
>   __

Looks good to me. Better to have another reviewer.

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

Marked as reviewed by fyang (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11749