Commits on Jul 21, 2023

1. Support .comment section like GCC/Clang (!llvm.ident) …

   Both GCC and Clang write by default a `.comment` section with compiler
   information:
   
   ```txt
   $ gcc -c -xc /dev/null && readelf -p '.comment' null.o
   
   String dump of section '.comment':
     [     1]  GCC: (GNU) 11.2.0
   
   $ clang -c -xc /dev/null && readelf -p '.comment' null.o
   
   String dump of section '.comment':
     [     1]  clang version 14.0.1 (https://github.com/llvm/llvm-project.git c62053979489ccb002efe411c3af059addcb5d7d)
   ```
   
   They also implement the `-Qn` flag to avoid doing so:
   
   ```txt
   $ gcc -Qn -c -xc /dev/null && readelf -p '.comment' null.o
   readelf: Warning: Section '.comment' was not dumped because it does not exist!
   
   $ clang -Qn -c -xc /dev/null && readelf -p '.comment' null.o
   readelf: Warning: Section '.comment' was not dumped because it does not exist!
   ```
   
   So far, `rustc` only does it for WebAssembly targets and only
   when debug info is enabled:
   
   ```txt
   $ echo 'fn main(){}' | rustc --target=wasm32-unknown-unknown --emit=llvm-ir -Cdebuginfo=2 - && grep llvm.ident rust_out.ll
   !llvm.ident = !{!27}
   ```
   
   In the RFC part of this PR it was decided to always add
   the information, which gets us closer to other popular compilers.
   An opt-out flag like GCC and Clang may be added later on if deemed
   necessary.
   
   Implementation-wise, this covers both `ModuleLlvm::new()` and
   `ModuleLlvm::new_metadata()` cases by moving the addition to
   `context::create_module` and adds a few test cases.
   
   ThinLTO also sees the `llvm.ident` named metadata duplicated (in
   temporary outputs), so this deduplicates it like it is done for
   `wasm.custom_sections`. The tests also check this duplication does
   not take place.
   
   Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

   

   ojeda committed Jul 21, 2023
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