RFR: 8299162: Refactor shared trampoline emission logic
Xiaolin Zheng
xlinzheng at openjdk.org
Wed Dec 21 03:47:25 UTC 2022
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(), emitting nothing or a 4-byte padding
<..., (B)> <-- multiple relocations at the pc: __ relocate(<the pc here>, trampoline_stub_Relocation::spec())
<...>
<..., (C)>
<shared trampoline stub2:>
align(), emitting nothing or a 4-byte padding
<...> <-- multiple relocations at the pc: __ relocate(<the pc here>, trampoline_stub_Relocation::spec())
<...>
<...>
<shared trampoline stub3:>
align(), emitting nothing or a 4-byte padding
<...> <-- multiple relocations at the pc: __ relocate(<the pc here>, trampoline_stub_Relocation::spec())
<...>
<...>
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
-------------
Commit messages:
- Refactor and cleanup for shared trampolines
Changes: https://git.openjdk.org/jdk/pull/11749/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11749&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8299162
Stats: 104 lines in 9 files changed: 44 ins; 29 del; 31 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
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