optimize performance of data copy · Pull Request !840 · OpenHarmony/third_party_llvm-project

1. optimize performance of data copy
---
### Issue
【lldb】optimize performance of data copy between arkts and native in lldb
https://gitee.com/openharmony/third_party_llvm-project/issues/ICLMZ0

### Reason (Purpose, Solved Problem)
Improve the efficiency of fetching ArkTS expression results into LLDB.
Avoid the performance overhead of chunked memory reads and multiple passes over data when copying string results
### Description
This patch improves data transfer performance by introducing a compact binary protocol between ArkTS and LLDB:

ArkTS now prefixes expression results with an 8-byte ASCII header that encodes the payload length (e.g., "00000123").

LLDB reads this fixed-size header once, converts it to an integer, and uses it to perform a single GetPointeeData() read for the full payload.

A null-terminator is appended for safety in downstream string usage.

The new method avoids repeated reads and parsing, significantly reducing the time spent in ExecuteAction.

### Test (Local test)
Verified that local variables, backtrace, and debug message evaluation still function correctly.

Measured latency reduction in ExecuteAction via LLDB logs.

Global variable resolution is unchanged and unaffected by this optimization.

### Need to cherry-pick to release (eg：monthly_20221018, OpenHarmony-3.2-Beta5 ...) branch？

- [ ] yes，branch list:
- [x] no

why:

###  Compatibility impact assessment, please indicate (Y/N)

类型	指派人员	状态
审查	@Eric 由 CODEOWNERS 文件第 1 行指派为审查者	进行中 (0/1人)
审查	hhj Pavel Kosov MrLop	进行中 (0/1人)
测试	openharmony_ci public_ARKCIAccount	进行中 (0/2人)

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