[lldb] lldb can't invoke function when param or return value with anonymous namespaces class.

[zhiqiangzz]

I found that if a function definition with anonymous namespaces class in the parameter or return value.
While debugging with lldb, these functions can't be invoked with errors like

error: error: function 'xxx' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage

See the following example:

namespace {
class A {
  int a;

public:
  explicit A(int a) : a{a} {}
  int func() { return a + 1; }
};

} // namespace

void *param(A /*a*/) { return nullptr; }
A *returnvalue() { return nullptr; }

int main() {
  A a = A{1};

  // avoid eliminated by compiler optimization
  (void)a.func();
  (void)param(a);
  (void)returnvalue();

  return 0;
}

Debug above program by cli lldb -- bin/lldbAnonymousVariable

(lldb) target create "bin/lldbAnonymousVariable"
Current executable set to '/Users/zhiqiangz/llvm/llvm-demo/bin/lldbAnonymousVariable' (arm64).
(lldb) b 23
Breakpoint 1: where = lldbAnonymousVariable`main + 76 at lldbAnonymousVariable.cpp:23:3, address = 0x0000000100003efc
(lldb) r
Process 33550 launched: '/Users/zhiqiangz/llvm/llvm-demo/bin/lldbAnonymousVariable' (arm64)
Process 33550 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100003efc lldbAnonymousVariable`main at lldbAnonymousVariable.cpp:23:3
   20  	  (void)param(a);
   21  	  (void)returnvalue();
   22
-> 23  	  return 0;
   24  	}
(lldb) e param(a)
error: error: function 'param' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
<user expression 0>:1:1: used here
    1 | param(a)
      | ^

(lldb) e returnvalue()
error: error: function 'returnvalue' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
<user expression 1>:1:1: used here
    1 | returnvalue()
      | ^

definitely the exec program contain corresponding symbols, verified by follow command.

nm bin/lldbAnonymousVariable | c++filt | rg "param"
0000000100003f58 t param((anonymous namespace)::A)

• machine info

uname -a
Darwin zhiqiangz-mlt 23.3.0 Darwin Kernel Version 23.3.0: Wed Dec 20 21:30:59 PST 2023; root:xnu-10002.81.5~7/RELEASE_ARM64_T6030 arm64

• llvm version

llvm-config --version
19.0.0git

[zhiqiangzz]

@Michael137 @adrian-prantl Could you please have a look? I found you resolved similar problems recently. Many thanks!

[llvmbot]

@llvm/issue-subscribers-lldb

Author: None (GithubXxz)

I found that if a function definition with anonymous namespaces class in the parameter or return value. While debugging with lldb, these functions can't be invoked with errors like ```shell error: error: function 'xxx' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage ```

See the following example:

namespace {
class A {
  int a;

public:
  explicit A(int a) : a{a} {}
  int func() { return a + 1; }
};

} // namespace

void *param(A /*a*/) { return nullptr; }
A *returnvalue() { return nullptr; }

int main() {
  A a = A{1};

  // avoid eliminated by compiler optimization
  (void)a.func();
  (void)param(a);
  (void)returnvalue();

  return 0;
}

Debug above program by cli lldb -- bin/lldbAnonymousVariable

(lldb) target create "bin/lldbAnonymousVariable"
Current executable set to '/Users/zhiqiangz/llvm/llvm-demo/bin/lldbAnonymousVariable' (arm64).
(lldb) b 23
Breakpoint 1: where = lldbAnonymousVariable`main + 76 at lldbAnonymousVariable.cpp:23:3, address = 0x0000000100003efc
(lldb) r
Process 33550 launched: '/Users/zhiqiangz/llvm/llvm-demo/bin/lldbAnonymousVariable' (arm64)
Process 33550 stopped
* thread #<!-- -->1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #<!-- -->0: 0x0000000100003efc lldbAnonymousVariable`main at lldbAnonymousVariable.cpp:23:3
   20  	  (void)param(a);
   21  	  (void)returnvalue();
   22
-&gt; 23  	  return 0;
   24  	}
(lldb) e param(a)
error: error: function 'param' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
&lt;user expression 0&gt;:1:1: used here
    1 | param(a)
      | ^

(lldb) e returnvalue()
error: error: function 'returnvalue' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
&lt;user expression 1&gt;:1:1: used here
    1 | returnvalue()
      | ^

definitely the exec program contain corresponding symbols, verified by follow command.

nm bin/lldbAnonymousVariable | c++filt | rg "param"
0000000100003f58 t param((anonymous namespace)::A)

• machine info

uname -a
Darwin zhiqiangz-mlt 23.3.0 Darwin Kernel Version 23.3.0: Wed Dec 20 21:30:59 PST 2023; root:xnu-10002.81.5~7/RELEASE_ARM64_T6030 arm64

• llvm version

llvm-config --version
19.0.0git

[Michael137]

Interesting, the error comes from Sema. It looks like func for such cases is marked with UniqueExternal linkage, meaning mightHaveNonExternalLinkage is true for this, and clang inserts it into the UndefinedButUsed set. Which it then diagnoses and errors out.

The reason it thinks the function is undefined is that it doesn't have a function body at the point we parse it (i.e., FuncionDecl::getBody() == nullptr. Usually we resolve function references through linkage names.

But Sema does give us the opportunity to reconcile the UndefinedButUsed set with ExternalASTSource::ReadUndefinedButUsed API, which LLDB currently doesn't implement. But we can definitely do so which should resolve this issue

[zhiqiangzz]

Thanks for your reply, Michael!
Ok I got it, expecting the implementation for this. I come across this problem while debugging the MemorySSA in llvm project, a container(SmallVector) with anonymous object in it can't be accessed by operator[] and begin() which mean I can't even get any element in the container.