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Lazy deference underlying object for shared/weak/unique_ptr synthetic… (
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llvm#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
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jeffreytan81 authored Sep 21, 2023
1 parent 7421040 commit 9faf8b5
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Showing 2 changed files with 23 additions and 21 deletions.
22 changes: 11 additions & 11 deletions lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -377,9 +377,16 @@ lldb::ValueObjectSP
LibStdcppSharedPtrSyntheticFrontEnd::GetChildAtIndex(size_t idx) {
if (idx == 0)
return m_ptr_obj->GetSP();
if (idx == 1)
return m_obj_obj->GetSP();

if (idx == 1) {
if (m_ptr_obj && !m_obj_obj) {
Status error;
ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
if (error.Success())
m_obj_obj = obj_obj->Clone(ConstString("object")).get();
}
if (m_obj_obj)
return m_obj_obj->GetSP();
}
return lldb::ValueObjectSP();
}

Expand All @@ -397,14 +404,7 @@ bool LibStdcppSharedPtrSyntheticFrontEnd::Update() {
return false;

m_ptr_obj = ptr_obj_sp->Clone(ConstString("pointer")).get();

if (m_ptr_obj) {
Status error;
ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
if (error.Success()) {
m_obj_obj = obj_obj->Clone(ConstString("object")).get();
}
}
m_obj_obj = nullptr;

return false;
}
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22 changes: 12 additions & 10 deletions lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -108,14 +108,7 @@ bool LibStdcppUniquePtrSyntheticFrontEnd::Update() {
if (del_obj)
m_del_obj = del_obj->Clone(ConstString("deleter")).get();
}

if (m_ptr_obj) {
Status error;
ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
if (error.Success()) {
m_obj_obj = obj_obj->Clone(ConstString("object")).get();
}
}
m_obj_obj = nullptr;

return false;
}
Expand All @@ -128,8 +121,17 @@ LibStdcppUniquePtrSyntheticFrontEnd::GetChildAtIndex(size_t idx) {
return m_ptr_obj->GetSP();
if (idx == 1 && m_del_obj)
return m_del_obj->GetSP();
if (idx == 2 && m_obj_obj)
return m_obj_obj->GetSP();
if (idx == 2) {
if (m_ptr_obj && !m_obj_obj) {
Status error;
ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
if (error.Success()) {
m_obj_obj = obj_obj->Clone(ConstString("object")).get();
}
}
if (m_obj_obj)
return m_obj_obj->GetSP();
}
return lldb::ValueObjectSP();
}

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