[CLANG] Clang crashes when target_clones attribute is only on a declaration

[EbinJose2002]

According to the Clang docs , https://clang.llvm.org/docs/AttributeReference.html#target-clones for target_clones attribute:
"This attribute may be attached to a function declaration and causes function multiversioning,"
However, with a simple test, Clang appears to require that the attribute be on the function definition, not just the declaration. When the attribute is only on the declaration and the definition lacks it, Clang crashes in CodeGen with “None multiversion type isn't valid here” and hits an UNREACHABLE.

Clang Version: 21.1.0

__attribute__((target_clones("arch=znver4", "default")))
void foo(int n, double*);


void foo(int n, double* X) {
    for ( int i = 0 ; i < n ; i++ ) {
        X[i] *= 2.0;
    }
}

clang -O3 -c foo.c 

None multiversion type isn't valid here
UNREACHABLE executed at /llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:2089!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: ../llvm_upstream/install_dir/bin/clang -O3 -c foo.c
1.      <eof> parser at end of file
2.      foo.c:4:6: LLVM IR generation of declaration 'foo'
3.      foo.c:4:6: Generating code for declaration 'foo'
 #0 0x00007f70faf8f880 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x1f8880)
 #1 0x00007f70faf8c52f llvm::sys::RunSignalHandlers() (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x1f552f)
 #2 0x00007f70fae7bf98 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007f70fa642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007f70fa6969fc __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #5 0x00007f70fa6969fc __pthread_kill_internal ./nptl/pthread_kill.c:78:10
 #6 0x00007f70fa6969fc pthread_kill ./nptl/pthread_kill.c:89:10
 #7 0x00007f70fa642476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #8 0x00007f70fa6287f3 abort ./stdlib/abort.c:81:7
 #9 0x00007f70fae9917e (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x10217e)
#10 0x00007f70ff3ee8a7 getMangledNameImpl(clang::CodeGen::CodeGenModule&, clang::GlobalDecl, clang::NamedDecl const*, bool) CodeGenModule.cpp:0:0
#11 0x00007f70ff4023f4 clang::CodeGen::CodeGenModule::getMangledName(clang::GlobalDecl) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5983f4)
#12 0x00007f70ff42e8fd clang::CodeGen::CodeGenModule::GetAddrOfFunction(clang::GlobalDecl, llvm::Type*, bool, bool, clang::CodeGen::ForDefinition_t) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c48fd)
#13 0x00007f70ff42ef01 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) /install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c4f01)
#14 0x00007f70ff4320f7 clang::CodeGen::CodeGenModule::EmitMultiVersionFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c80f7)
#15 0x00007f70ff4299c5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5bf9c5)
#16 0x00007f70ff42a790 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c0790)
#17 0x00007f70ff4342a7 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#18 0x00007f70ff4dbdd0 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#19 0x00007f70ff3a07b1 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5367b1)
#20 0x00007f70f7bf8c74 clang::ParseAST(clang::Sema&, bool, bool) (/install_dir/bin/../lib/../lib/libclangParse.so.21.1+0x42c74)
#21 0x00007f70fd80b5c9 clang::FrontendAction::Execute() (/install_dir/bin/../lib/libclangFrontend.so.21.1+0x1455c9)
#22 0x00007f70fd77be05 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/install_dir/bin/../lib/libclangFrontend.so.21.1+0xb5e05)
#23 0x00007f71000a8c08 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/install_dir/bin/../lib/libclangFrontendTool.so.21.1+0x5c08)
#24 0x0000562a543a85fe cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (../llvm_upstream/install_dir/bin/clang+0x175fe)
#25 0x0000562a5439e81a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#26 0x00007f70fd39569d void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#27 0x00007f70fae7c440 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0xe5440)
#28 0x00007f70fd395f3e clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#29 0x00007f70fd34db91 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/install_dir/bin/../lib/libclangDriver.so.21.1+0x79b91)
#30 0x00007f70fd34ecb1 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/install_dir/bin/../lib/libclangDriver.so.21.1+0x7acb1)
#31 0x00007f70fd35fce4 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/install_dir/bin/../lib/libclangDriver.so.21.1+0x8bce4)
#32 0x0000562a543a40d7 clang_main(int, char**, llvm::ToolContext const&) (../llvm_upstream/install_dir/bin/clang+0x130d7)
#33 0x0000562a5439deeb main (../llvm_upstream/install_dir/bin/clang+0xceeb)
#34 0x00007f70fa629d90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#35 0x00007f70fa629e40 call_init ./csu/../csu/libc-start.c:128:20
#36 0x00007f70fa629e40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#37 0x0000562a5439df45 _start (../llvm_upstream/install_dir/bin/clang+0xcf45)
clang: error: clang frontend command failed with exit code 134 (use -v to see invocation)
clang version 21.1.0 (https://github.com/llvm/llvm-project.git 3623fe661ae35c6c80ac221f14d85be76aa870f1)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /install_dir/bin
Build config: +assertions
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/foo-1c1aca.c
clang: note: diagnostic msg: /tmp/foo-1c1aca.sh
clang: note: diagnostic msg: 

********************

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Ebin-McW (EbinJose2002)

According to the Clang docs , [https://clang.llvm.org/docs/AttributeReference.html#target-clones](url) for target_clones attribute: `"This attribute may be attached to a function declaration and causes function multiversioning," ` However, with a simple test, Clang appears to require that the attribute be on the function definition, not just the declaration. When the attribute is only on the declaration and the definition lacks it, Clang crashes in CodeGen with “None multiversion type isn't valid here” and hits an UNREACHABLE.

Clang Version: 21.1.0

__attribute__((target_clones("arch=znver4", "default")))
void foo(int n, double*);


void foo(int n, double* X) {
    for ( int i = 0 ; i < n ; i++ ) {
        X[i] *= 2.0;
    }
}

clang -O3 -c foo.c 

None multiversion type isn't valid here
UNREACHABLE executed at /llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:2089!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: ../llvm_upstream/install_dir/bin/clang -O3 -c foo.c
1.      <eof> parser at end of file
2.      foo.c:4:6: LLVM IR generation of declaration 'foo'
3.      foo.c:4:6: Generating code for declaration 'foo'
 #<!-- -->0 0x00007f70faf8f880 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x1f8880)
 #<!-- -->1 0x00007f70faf8c52f llvm::sys::RunSignalHandlers() (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x1f552f)
 #<!-- -->2 0x00007f70fae7bf98 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x00007f70fa642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #<!-- -->4 0x00007f70fa6969fc __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #<!-- -->5 0x00007f70fa6969fc __pthread_kill_internal ./nptl/pthread_kill.c:78:10
 #<!-- -->6 0x00007f70fa6969fc pthread_kill ./nptl/pthread_kill.c:89:10
 #<!-- -->7 0x00007f70fa642476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #<!-- -->8 0x00007f70fa6287f3 abort ./stdlib/abort.c:81:7
 #<!-- -->9 0x00007f70fae9917e (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x10217e)
#<!-- -->10 0x00007f70ff3ee8a7 getMangledNameImpl(clang::CodeGen::CodeGenModule&amp;, clang::GlobalDecl, clang::NamedDecl const*, bool) CodeGenModule.cpp:0:0
#<!-- -->11 0x00007f70ff4023f4 clang::CodeGen::CodeGenModule::getMangledName(clang::GlobalDecl) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5983f4)
#<!-- -->12 0x00007f70ff42e8fd clang::CodeGen::CodeGenModule::GetAddrOfFunction(clang::GlobalDecl, llvm::Type*, bool, bool, clang::CodeGen::ForDefinition_t) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c48fd)
#<!-- -->13 0x00007f70ff42ef01 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) /install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c4f01)
#<!-- -->14 0x00007f70ff4320f7 clang::CodeGen::CodeGenModule::EmitMultiVersionFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c80f7)
#<!-- -->15 0x00007f70ff4299c5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5bf9c5)
#<!-- -->16 0x00007f70ff42a790 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c0790)
#<!-- -->17 0x00007f70ff4342a7 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#<!-- -->18 0x00007f70ff4dbdd0 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#<!-- -->19 0x00007f70ff3a07b1 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5367b1)
#<!-- -->20 0x00007f70f7bf8c74 clang::ParseAST(clang::Sema&amp;, bool, bool) (/install_dir/bin/../lib/../lib/libclangParse.so.21.1+0x42c74)
#<!-- -->21 0x00007f70fd80b5c9 clang::FrontendAction::Execute() (/install_dir/bin/../lib/libclangFrontend.so.21.1+0x1455c9)
#<!-- -->22 0x00007f70fd77be05 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/install_dir/bin/../lib/libclangFrontend.so.21.1+0xb5e05)
#<!-- -->23 0x00007f71000a8c08 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/install_dir/bin/../lib/libclangFrontendTool.so.21.1+0x5c08)
#<!-- -->24 0x0000562a543a85fe cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (../llvm_upstream/install_dir/bin/clang+0x175fe)
#<!-- -->25 0x0000562a5439e81a ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->26 0x00007f70fd39569d void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const::'lambda'()&gt;(long) Job.cpp:0:0
#<!-- -->27 0x00007f70fae7c440 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0xe5440)
#<!-- -->28 0x00007f70fd395f3e clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const (.part.0) Job.cpp:0:0
#<!-- -->29 0x00007f70fd34db91 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/install_dir/bin/../lib/libclangDriver.so.21.1+0x79b91)
#<!-- -->30 0x00007f70fd34ecb1 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;, bool) const (/install_dir/bin/../lib/libclangDriver.so.21.1+0x7acb1)
#<!-- -->31 0x00007f70fd35fce4 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;) (/install_dir/bin/../lib/libclangDriver.so.21.1+0x8bce4)
#<!-- -->32 0x0000562a543a40d7 clang_main(int, char**, llvm::ToolContext const&amp;) (../llvm_upstream/install_dir/bin/clang+0x130d7)
#<!-- -->33 0x0000562a5439deeb main (../llvm_upstream/install_dir/bin/clang+0xceeb)
#<!-- -->34 0x00007f70fa629d90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#<!-- -->35 0x00007f70fa629e40 call_init ./csu/../csu/libc-start.c:128:20
#<!-- -->36 0x00007f70fa629e40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#<!-- -->37 0x0000562a5439df45 _start (../llvm_upstream/install_dir/bin/clang+0xcf45)
clang: error: clang frontend command failed with exit code 134 (use -v to see invocation)
clang version 21.1.0 (https://github.com/llvm/llvm-project.git 3623fe661ae35c6c80ac221f14d85be76aa870f1)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /install_dir/bin
Build config: +assertions
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/foo-1c1aca.c
clang: note: diagnostic msg: /tmp/foo-1c1aca.sh
clang: note: diagnostic msg: 

********************

[shafik]

This looks like a modified version of #130850 and #129483

CC @erichkeane

[erichkeane]

Hmm.... looks like we're doing a bad job inherit'ing this attribute during redeclaration? Though #130850 looks a little different. I wonder if this broke during various refactors of how that works.

The getMultiVersionKind is checking the attribute, and despite us having set the isMultiVersion appropriately, we're not getting the attribute inherited or something? I would very much expect it to be attached as an inherited version.

I'm going to mark this as 'good-first-issue', since I suspect it is just a case of bad-attribute-merging, and should be reasonably easy to track down? Perhaps we're just opting-out of the normal inherit mechanism by accident?

Either way, I'll help whoever starts it as best i can.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Ebin-McW (EbinJose2002)

According to the Clang docs , [https://clang.llvm.org/docs/AttributeReference.html#target-clones](url) for target_clones attribute: `"This attribute may be attached to a function declaration and causes function multiversioning," ` However, with a simple test, Clang appears to require that the attribute be on the function definition, not just the declaration. When the attribute is only on the declaration and the definition lacks it, Clang crashes in CodeGen with “None multiversion type isn't valid here” and hits an UNREACHABLE.

Clang Version: 21.1.0

__attribute__((target_clones("arch=znver4", "default")))
void foo(int n, double*);


void foo(int n, double* X) {
    for ( int i = 0 ; i < n ; i++ ) {
        X[i] *= 2.0;
    }
}

clang -O3 -c foo.c 

None multiversion type isn't valid here
UNREACHABLE executed at /llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:2089!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: ../llvm_upstream/install_dir/bin/clang -O3 -c foo.c
1.      <eof> parser at end of file
2.      foo.c:4:6: LLVM IR generation of declaration 'foo'
3.      foo.c:4:6: Generating code for declaration 'foo'
 #<!-- -->0 0x00007f70faf8f880 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x1f8880)
 #<!-- -->1 0x00007f70faf8c52f llvm::sys::RunSignalHandlers() (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x1f552f)
 #<!-- -->2 0x00007f70fae7bf98 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x00007f70fa642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #<!-- -->4 0x00007f70fa6969fc __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #<!-- -->5 0x00007f70fa6969fc __pthread_kill_internal ./nptl/pthread_kill.c:78:10
 #<!-- -->6 0x00007f70fa6969fc pthread_kill ./nptl/pthread_kill.c:89:10
 #<!-- -->7 0x00007f70fa642476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #<!-- -->8 0x00007f70fa6287f3 abort ./stdlib/abort.c:81:7
 #<!-- -->9 0x00007f70fae9917e (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0x10217e)
#<!-- -->10 0x00007f70ff3ee8a7 getMangledNameImpl(clang::CodeGen::CodeGenModule&amp;, clang::GlobalDecl, clang::NamedDecl const*, bool) CodeGenModule.cpp:0:0
#<!-- -->11 0x00007f70ff4023f4 clang::CodeGen::CodeGenModule::getMangledName(clang::GlobalDecl) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5983f4)
#<!-- -->12 0x00007f70ff42e8fd clang::CodeGen::CodeGenModule::GetAddrOfFunction(clang::GlobalDecl, llvm::Type*, bool, bool, clang::CodeGen::ForDefinition_t) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c48fd)
#<!-- -->13 0x00007f70ff42ef01 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) /install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c4f01)
#<!-- -->14 0x00007f70ff4320f7 clang::CodeGen::CodeGenModule::EmitMultiVersionFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c80f7)
#<!-- -->15 0x00007f70ff4299c5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5bf9c5)
#<!-- -->16 0x00007f70ff42a790 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5c0790)
#<!-- -->17 0x00007f70ff4342a7 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#<!-- -->18 0x00007f70ff4dbdd0 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#<!-- -->19 0x00007f70ff3a07b1 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/install_dir/bin/../lib/libclangCodeGen.so.21.1+0x5367b1)
#<!-- -->20 0x00007f70f7bf8c74 clang::ParseAST(clang::Sema&amp;, bool, bool) (/install_dir/bin/../lib/../lib/libclangParse.so.21.1+0x42c74)
#<!-- -->21 0x00007f70fd80b5c9 clang::FrontendAction::Execute() (/install_dir/bin/../lib/libclangFrontend.so.21.1+0x1455c9)
#<!-- -->22 0x00007f70fd77be05 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/install_dir/bin/../lib/libclangFrontend.so.21.1+0xb5e05)
#<!-- -->23 0x00007f71000a8c08 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/install_dir/bin/../lib/libclangFrontendTool.so.21.1+0x5c08)
#<!-- -->24 0x0000562a543a85fe cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (../llvm_upstream/install_dir/bin/clang+0x175fe)
#<!-- -->25 0x0000562a5439e81a ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->26 0x00007f70fd39569d void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const::'lambda'()&gt;(long) Job.cpp:0:0
#<!-- -->27 0x00007f70fae7c440 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/install_dir/bin/../lib/libLLVMSupport.so.21.1+0xe5440)
#<!-- -->28 0x00007f70fd395f3e clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const (.part.0) Job.cpp:0:0
#<!-- -->29 0x00007f70fd34db91 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/install_dir/bin/../lib/libclangDriver.so.21.1+0x79b91)
#<!-- -->30 0x00007f70fd34ecb1 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;, bool) const (/install_dir/bin/../lib/libclangDriver.so.21.1+0x7acb1)
#<!-- -->31 0x00007f70fd35fce4 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;) (/install_dir/bin/../lib/libclangDriver.so.21.1+0x8bce4)
#<!-- -->32 0x0000562a543a40d7 clang_main(int, char**, llvm::ToolContext const&amp;) (../llvm_upstream/install_dir/bin/clang+0x130d7)
#<!-- -->33 0x0000562a5439deeb main (../llvm_upstream/install_dir/bin/clang+0xceeb)
#<!-- -->34 0x00007f70fa629d90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#<!-- -->35 0x00007f70fa629e40 call_init ./csu/../csu/libc-start.c:128:20
#<!-- -->36 0x00007f70fa629e40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#<!-- -->37 0x0000562a5439df45 _start (../llvm_upstream/install_dir/bin/clang+0xcf45)
clang: error: clang frontend command failed with exit code 134 (use -v to see invocation)
clang version 21.1.0 (https://github.com/llvm/llvm-project.git 3623fe661ae35c6c80ac221f14d85be76aa870f1)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /install_dir/bin
Build config: +assertions
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/foo-1c1aca.c
clang: note: diagnostic msg: /tmp/foo-1c1aca.sh
clang: note: diagnostic msg: 

********************

[henrybw]

@erichkeane Hi! I would like to work on this.

Looks like we should probably be handling this somewhere in Sema? I’m not sure what the normal inherit mechanism here is supposed to be, though.

[henrybw]

Hmm.... looks like we're doing a bad job inherit'ing this attribute during redeclaration? Though #130850 looks a little different. I wonder if this broke during various refactors of how that works.

The getMultiVersionKind is checking the attribute, and despite us having set the isMultiVersion appropriately, we're not getting the attribute inherited or something? I would very much expect it to be attached as an inherited version.

@erichkeane I tried the repro from this issue on both 9deab60 ("Implement target_clones multiversioning") and fc53eb6 ("Reapply 'Implement target_clones multiversioning'"), and it still hits the same llvm_unreachable call, so I don't think this is a regression.

It appears that, when we set the multiversion-ness of the second function declaration (i.e. the function definition) in CheckMultiVersionAdditionalDecl, we also set Redeclaration to false, which seems incorrect. This prevents Sema::MergeFunctionDecl from being called, which prevents the attribute from being inherited.

(More specifically, in CheckMultiVersionAdditionalDecl, we're reaching the last case at the bottom of the function that sets Redeclaration to false, but also sets isMultiVersion on the new function declaration and clears the previous declaration.)

I think we can fix this by modifying this case near the end of CheckMultiVersionAdditionalDecl to also accept MultiVersionKind::None redeclarations, since we've already asserted that the old declaration was MultiVersionKind::TargetClones.

In other words, something like this:

--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -12005,7 +12005,7 @@ static bool CheckMultiVersionAdditionalDecl(
   }
 
   // Permit forward declarations in the case where these two are compatible.
-  if (!OldFD->isMultiVersion()) {
+  if (!OldFD->isMultiVersion() || NewMVKind == MultiVersionKind::None) {
     OldFD->setIsMultiVersion();
     NewFD->setIsMultiVersion();
     Redeclaration = true;

Does this make sense and seem reasonable to you?