[RFC] Delay definition DIE searching when parse a declaration DIE for record type

Subprojects LLDB
15

Here’s an example of binary built without -gsimple-template-names which could also benefit from this change:

main.cpp

namespace N1 {
  struct S;
}
template<typename T>
struct Foo {};

Foo<N1::S> foo;
int main() {

}

namespace.cpp

namespace N1 {
struct S {};
}

N1::S s;

Compile with clang++ -g main.cpp namespace.cpp and test with lldb a.out -o "log enable dwarf comp" -o "b main" -o "r" -o "p foo" -o "p s"

Current lldb logging:

(lldb) p foo
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x0000000000000040, decl_ctx = 0x000055F3D2C378A0 (die 0x000000000000000c)) DW_TAG_subprogram name = 'main')
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000004f, decl_ctx = 0x000055F3D2C378A0 (die 0x000000000000000c)) DW_TAG_base_type name = 'int')
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000002e, decl_ctx = 0x000055F3D2C378A0 (die 0x000000000000000c)) DW_TAG_structure_type name = 'Foo<N1::S>')
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000003d, decl_ctx = 0x000055F3D2C381E8 (die 0x000000000000003b)) DW_TAG_structure_type name = 'S')
lldb             (x86_64) /tmp/a.out: SymbolFileDWARF(0x000055F3D2C215E0) - 0x000000000000003d: DW_TAG_structure_type type "S" is a forward declaration, trying to find complete type
lldb             (x86_64) /tmp/a.out: SymbolFileDWARF::FindDefinitionTypeForDWARFDeclContext(tag=DW_TAG_structure_type, name='S')
lldb             (x86_64) /tmp/a.out: SymbolFileDWARF::FindDefinitionTypeForDWARFDeclContext(tag=DW_TAG_structure_type, name='S') trying die=0x000000000000007f (N1::S)
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000007f, decl_ctx = 0x000055F3D2C381E8 (die 0x000000000000007d)) DW_TAG_structure_type name = 'S')
lldb             (x86_64) /tmp/a.out: SymbolFileDWARF(0x000055F3D2C215E0) - 0x000000000000003d: DW_TAG_structure_type type "S" is a forward declaration, complete type is 0x0000007f
lldb             (x86_64) /tmp/a.out: 0x0000002e: DW_TAG_structure_type 'Foo<N1::S>' resolving forward declaration...
(Foo<N1::S>)  {}
(lldb) p s
(N1::S)  {}

Notice that it’s trying to find definition DIE for N1::S and create type from it when printing foo whose type is Foo<N1::S>. But it’s not actually being used at this moment. This is extra unnecessary work.
Snippet of the debug info:

0x0000007d:   DW_TAG_namespace
                DW_AT_name      ("N1")

0x0000007f:     DW_TAG_structure_type
                  DW_AT_calling_convention      (DW_CC_pass_by_value)
                  DW_AT_name    ("S")
                  DW_AT_byte_size       (0x01)
                  DW_AT_decl_file       ("/tmp/namespace.cpp")
                  DW_AT_decl_line       (2)

With this change, lldb only search for definition DIE when p s and that’s the time when we want to complete the type N1::S and searching for its definition DIE:

(lldb) p foo
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x0000000000000040, decl_ctx = 0x000055632AA21570 (die 0x000000000000000c)) DW_TAG_subprogram name = 'main')
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000004f, decl_ctx = 0x000055632AA21570 (die 0x000000000000000c)) DW_TAG_base_type name = 'int')
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000002e, decl_ctx = 0x000055632AA21570 (die 0x000000000000000c)) DW_TAG_structure_type name = 'Foo<N1::S>')
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000003d, decl_ctx = 0x000055632AA21EB8 (die 0x000000000000003b)) DW_TAG_structure_type name = 'S')
lldb             (x86_64) /tmp/a.out: 0x0000002e: DW_TAG_structure_type 'Foo<N1::S>' resolving forward declaration...
(Foo<N1::S>)  {}
(lldb) p s
lldb             (x86_64) /tmp/a.out: DWARFASTParserClang::ParseTypeFromDWARF (die = 0x000000000000007f, decl_ctx = 0x000055632AA21EB8 (die 0x000000000000007d)) DW_TAG_structure_type name = 'S')
lldb             (x86_64) /tmp/a.out: 0x0000007f: DW_TAG_structure_type 'S' resolving forward declaration...
(N1::S)  {}