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

[Xiaolin Zheng]

> 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:

  __

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11749/files
  - new: https://git.openjdk.org/jdk/pull/11749/files/ac93f3c6..aefe74ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11749&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11749&range=00-01

  Stats: 6 lines in 3 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11749.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11749/head:pull/11749

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