[PATCH] D69933: [ASTImporter] Limit imports of structs

[Jaroslav Sevcik via Phabricator via cfe-commits]

jarin created this revision.
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This is a work in progress patch for discussion. The goal is to improve performance of LLDB's expression evaluator.

During my investigations, I noticed that the AST importer is very eager to import classes/structs that were already completed on the 'From' side, even if they are not needed. This is best illustrated with an example:

  struct C0 { int x = 0; };
  struct C1 { int x = 1; C0* c0 = 0; };
  struct C2 { int x = 2; C1* c1 = 0; };
  
  int main() {
    C0 c0;
    C1 c1;
    C2 c2;
  
    return 0;  // break here
  }

When we evaluate “c2.x” in LLDB, AST importer completes and imports only class C2. This is working as intended. Similarly, evaluating “c1.x” imports just C1 and “c0.x” imports C0. However, if we evaluate “c2.x” after evaluating “c1.x” and “c0.x”, the importer suddenly imports both C1 and C0 (in addition to C2). See a log from a lldb session at the end of this email for illustration.

I believe the culprit here is the following code at the end of the ASTNodeImporter::VisitRecordDecl method:

  if (D->isCompleteDefinition())
    if (Error Err = ImportDefinition(D, D2, IDK_Default))
      return std::move(Err);

This will import a definition of a class from LLDB if LLDB already happens to have a complete definition from before. For large programs, this can lead to importing very large chunks of ASTs even if they are not needed. I have tried to remove the code above from the AST importer and test performance on several expressions in an Unreal engine sample - preliminary results show this cuts down evaluation time by roughly 50%.

This is work in progress,  couple of lldb tests are failing (but hopefully fixable). What would the experts here think? Is this a plausible direction?

  —— lldb session illustrating the unnecessary imports —-
  This shows that evaluation of “c2.x” after evaluation “c1.x” and “c0.x” calls to LayoutRecordType for C2, C1 and C0.
  
  $ lldb a.out
  (lldb) b h.cc:10
  Breakpoint 1: where = a.out`main + 44 at h.cc:10:3, address = ...
  (lldb) r
  ... Process stopped ...
  (lldb) log enable lldb expr
  (lldb) p c2.x
  ...
  LayoutRecordType[6] ... for (RecordDecl*)0x... [name = 'C2']
  ...
  (lldb) p c1.x
  ...
  LayoutRecordType[7] ... for (RecordDecl*)0x... [name = 'C1']
  ...
  (lldb) p c0.x
  ...
  LayoutRecordType[8] ... for (RecordDecl*)0x... [name = 'C0']
  ...
  (lldb) p c2.x
  ...
  LayoutRecordType[9] ... for (RecordDecl*)0x... [name = 'C2']
  LayoutRecordType[10] ... for (RecordDecl*)0x... [name = 'C1']
  LayoutRecordType[11] ... for (RecordDecl*)0x... [name = 'C0']
  ...


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D69933

Files:
  clang/lib/AST/ASTImporter.cpp


Index: clang/lib/AST/ASTImporter.cpp
===================================================================
--- clang/lib/AST/ASTImporter.cpp
+++ clang/lib/AST/ASTImporter.cpp
@@ -2801,7 +2801,7 @@
   if (D->isAnonymousStructOrUnion())
     D2->setAnonymousStructOrUnion(true);
 
-  if (D->isCompleteDefinition())
+  if (!Importer.isMinimalImport() && D->isCompleteDefinition())
     if (Error Err = ImportDefinition(D, D2, IDK_Default))
       return std::move(Err);
 
@@ -3440,6 +3440,9 @@
   if (ToInitializer)
     ToField->setInClassInitializer(ToInitializer);
   ToField->setImplicit(D->isImplicit());
+  if (CXXRecordDecl *FieldType = D->getType()->getAsCXXRecordDecl())
+    if (Error Err = ImportDefinitionIfNeeded(FieldType))
+      return std::move(Err);
   LexicalDC->addDeclInternal(ToField);
   return ToField;
 }
@@ -5323,7 +5326,7 @@
 
   D2->setTemplateSpecializationKind(D->getTemplateSpecializationKind());
 
-  if (D->isCompleteDefinition())
+  if (!Importer.isMinimalImport() && D->isCompleteDefinition())
     if (Error Err = ImportDefinition(D, D2))
       return std::move(Err);
 


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