Weak alias to static function not working

[llvmbot]

Bugzilla Link	17775
Version	trunk
OS	Linux
Blocks	#44654
Attachments	Minimal reproducer for weak-alias problem
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@slacka,@tstellar

Extended Description

Weak aliases to unused static functions fail with an error message. It looks like the compiler is first removing the unused static function then complaining that it can't create a weak alias to a function that no longer exists:

$ clang++ --version | head -1
clang version 3.4 (trunk)
$ clang++ -c aliasing.cpp 
aliasing.cpp:8:56: error: alias must point to a defined variable or function
  const char* do_something (void) __attribute__((weak, alias("do_somethi...
                                                       ^
1 error generated.
$ g++ --version | head -1
g++ (GCC) 4.7.2
$ g++ -c aliasing.cpp 

I'm seeing this problem with r193799, but I recall it working in r190315. This seems to be a C++ issue; analogous C code made it through clang with no problems.

Defining do_something_original() as non-static in the attached file makes the problem go away, but in my real code I'd prefer to have the function declared static.

-- Scott

[tstellar]

This is still an issue in clang 10.0.0.

[tstellar]

Relevant mailing list post: http://lists.llvm.org/pipermail/cfe-dev/2018-May/057870.html

[tstellar]

openhpi 3.8.0 hits this bug: https://github.com/open-hpi/openhpi/releases/tag/3.8.0

[tstellar]

mentioned in issue #44654

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [17775](https://llvm.org/bz17775) | | Version | trunk | | OS | Linux | | Blocks | llvm/llvm-project#44654 | | Attachments | [Minimal reproducer for weak-alias problem](https://user-images.githubusercontent.com/60944935/143748396-238a161f-2f82-4631-94ce-f7fd6f7e0f0c.gz) | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@slacka,@tstellar |

Extended Description

Weak aliases to unused static functions fail with an error message. It looks like the compiler is first removing the unused static function then complaining that it can't create a weak alias to a function that no longer exists:

$ clang++ --version | head -1
clang version 3.4 (trunk)
$ clang++ -c aliasing.cpp
aliasing.cpp:8:56: error: alias must point to a defined variable or function
const char* do_something (void) attribute((weak, alias("do_somethi...
^
1 error generated.
$ g++ --version | head -1
g++ (GCC) 4.7.2
$ g++ -c aliasing.cpp

I'm seeing this problem with r193799, but I recall it working in r190315. This seems to be a C++ issue; analogous C code made it through clang with no problems.

Defining do_something_original() as non-static in the attached file makes the problem go away, but in my real code I'd prefer to have the function declared static.

-- Scott

[MaskRay]

The current behavior matches GCC and is desired. alias needs to refer to the mangled name for C++.

$ cat > a.cc <<e
static const char* do_something_original (void) { return 0; }
const char* do_something (void) __attribute__((weak, alias("do_something_original")));
e
$ gcc -c a.cc
a.cc:2:13: error: ‘const char* do_something()’ aliased to undefined symbol ‘do_something_original’
    2 | const char* do_something (void) __attribute__((weak, alias("do_something_original")));
      |             ^~~~~~~~~~~~
$ gcc -c -xc a.cc
$ clang -c a.cc
a.cc:2:54: error: alias must point to a defined variable or function
    2 | const char* do_something (void) __attribute__((weak, alias("do_something_original")));
      |                                                      ^
a.cc:2:54: note: the function or variable specified in an alias must refer to its mangled name
a.cc:2:54: note: function by that name is mangled as "_ZL21do_something_originalv"
    2 | const char* do_something (void) __attribute__((weak, alias("do_something_original")));
      |                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                      alias("_ZL21do_something_originalv")
1 error generated.
$ clang -c -xc a.cc
$ exit