| Issue |
164730
|
| Summary |
[clang] Preferred name not generated to declare variables in DWARF when used as template argument in code *only*
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
royitaqi
|
# Problem
Compiling the following code generates an object file whose DWARF doesn't contain the preferred name. This is true when compiling with `-ggdb` and with `-glldb`.
Inspecting the DWARF shows that the variable is declared as `Foo<Foo<int> >`. The preferred name, i.e. `BarInt` (case sensitive), is no where to be found in the DWARF.
Code:
```
template <typename T> struct Foo;
typedef Foo<int> BarInt;
template <typename T>
struct [[clang::preferred_name(BarInt)]] Foo{};
int main() {
// BarInt barInt;
Foo<BarInt> fooBarInt;
return 0;
}
```
DWARF:
```
0x00000033: DW_TAG_variable
DW_AT_location (DW_OP_fbreg +11)
DW_AT_name ("fooBarInt")
DW_AT_decl_file ("/Users/royshi/demo/preferred_name/two.cpp")
DW_AT_decl_line (10)
DW_AT_type (0x00000043 "Foo<Foo<int> >")
```
So lldb cannot print the preferred name:
```
(lldb) p fooBarInt
(Foo<Foo<int> >) {}
```
# Repro
Use the above source code (call it `two.cpp`), then:
```
~/public_llvm/build/bin/clang -O0 -glldb -c two.cpp # or -ggdb
dwarfdump two.o
# Note: For me, the above clang cannot link, so I used xcrun clang++. It shouldn't matter which one you use.
xcrun clang++ two.o
lldb a.out -o "b two.cpp:10" -o "r" -o "p fooBarInt"
```
See compilation, dwarfdump, and lldb output:
- `-ggdb`: https://gist.github.com/royitaqi/cbf7a4d038f5b73dc60e5932bb2b593d
- `-glldb`: https://gist.github.com/royitaqi/46af47f0472976eb94b9725c073668df
I tested these on macOS. Haven't tested on Linux.
# Analysis
If we uncomment the line `// BarInt barInt;`, the compiler can now generate typedef(s) for the preferred name.
For example, with `-ggdb`, we now see this typedef.
```
0x0000004d: DW_TAG_typedef
DW_AT_type (0x00000055 "Foo<int>")
DW_AT_name ("BarInt")
DW_AT_decl_file ("/Users/royshi/demo/preferred_name/two.cpp")
DW_AT_decl_line (3)
```
_Note: FWIW, `-glldb` generates one more typedef, I was told that this is by design._
However, this typedef is only used by the variable `barInt`, _not_ by `fooBarInt`.
```
0x00000032: DW_TAG_variable
DW_AT_location (DW_OP_fbreg +11)
DW_AT_name ("barInt")
DW_AT_decl_file ("/Users/royshi/demo/preferred_name/two.cpp")
DW_AT_decl_line (9)
DW_AT_type (0x0000004d "BarInt")
0x0000003d: DW_TAG_variable
DW_AT_location (DW_OP_fbreg +10)
DW_AT_name ("fooBarInt")
DW_AT_decl_file ("/Users/royshi/demo/preferred_name/two.cpp")
DW_AT_decl_line (10)
DW_AT_type (0x00000062 "Foo<Foo<int> >")
```
See full output:
- `-ggdb`: https://gist.github.com/royitaqi/d2b6c7c9ca56f886f051479bb7edde90
- `-glldb`: https://gist.github.com/royitaqi/030ac6963d7696f4cc34bb826803e098
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