[all-commits] [llvm/llvm-project] 9faf8b: Lazy deference underlying object for shared/weak/u...

[jeffreytan81 via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9faf8b5117758cc6f117465fc6ce995a30864ae7
      https://github.com/llvm/llvm-project/commit/9faf8b5117758cc6f117465fc6ce995a30864ae7
  Author: jeffreytan81 <jeffreytan at fb.com>
  Date:   2023-09-21 (Thu, 21 Sep 2023)

  Changed paths:
    M lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
    M lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp

  Log Message:
  -----------
  Lazy deference underlying object for shared/weak/unique_ptr synthetic… (#67069)

We noticed some performance issue while in lldb-vscode for grabing the
name of the SBValue. Profiling shows SBValue::GetName() can cause
synthetic children provider of shared/unique_ptr to deference underlying
object and complete it type.

This patch lazily moves the dereference from synthetic child provider's
Update() method to GetChildAtIndex() so that SBValue::GetName() won't
trigger the slow code path.

Here is the culprit slow code path:
```
...
frame #59: 0x00007ff4102e0660 liblldb.so.15`SymbolFileDWARF::CompleteType(this=<unavailable>, compiler_type=0x00007ffdd9829450) at SymbolFileDWARF.cpp:1567:25 [opt]
...
frame #67: 0x00007ff40fdf9bd4 liblldb.so.15`lldb_private::ValueObject::Dereference(this=0x0000022bb5dfe980, error=0x00007ffdd9829970) at ValueObject.cpp:2672:41 [opt]
frame #68: 0x00007ff41011bb0a liblldb.so.15`(anonymous namespace)::LibStdcppSharedPtrSyntheticFrontEnd::Update(this=0x000002298fb94380) at LibStdcpp.cpp:403:40 [opt]
frame #69: 0x00007ff41011af9a liblldb.so.15`lldb_private::formatters::LibStdcppSharedPtrSyntheticFrontEndCreator(lldb_private::CXXSyntheticChildren*, std::shared_ptr<lldb_private::ValueObject>) [inlined] (anonymous namespace)::LibStdcppSharedPtrSyntheticFrontEnd::LibStdcppSharedPtrSyntheticFrontEnd(this=0x000002298fb94380, valobj_sp=<unavailable>) at LibStdcpp.cpp:371:5 [opt]
...
frame #78: 0x00007ff40fdf6e42 liblldb.so.15`lldb_private::ValueObject::CalculateSyntheticValue(this=0x000002296c66a500) at ValueObject.cpp:1836:27 [opt]
frame #79: 0x00007ff40fdf1939 liblldb.so.15`lldb_private::ValueObject::GetSyntheticValue(this=<unavailable>) at ValueObject.cpp:1867:3 [opt]
frame #80: 0x00007ff40fc89008 liblldb.so.15`ValueImpl::GetSP(this=0x0000022c71b90de0, stop_locker=0x00007ffdd9829d00, lock=0x00007ffdd9829d08, error=0x00007ffdd9829d18) at SBValue.cpp:141:46 [opt]
frame #81: 0x00007ff40fc7d82a liblldb.so.15`lldb::SBValue::GetSP(ValueLocker&) const [inlined] ValueLocker::GetLockedSP(this=0x00007ffdd9829d00, in_value=<unavailable>) at SBValue.cpp:208:21 [opt]
frame #82: 0x00007ff40fc7d817 liblldb.so.15`lldb::SBValue::GetSP(this=0x00007ffdd9829d90, locker=0x00007ffdd9829d00) const at SBValue.cpp:1047:17 [opt]
frame #83: 0x00007ff40fc7da6f liblldb.so.15`lldb::SBValue::GetName(this=0x00007ffdd9829d90) at SBValue.cpp:294:32 [opt]
...
```

Differential Revision: https://reviews.llvm.org/D159542