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>

1 files changed, 15 insertions, 0 deletions

diff --git a/tests/codegen/llvm-ident.rs b/tests/codegen/llvm-ident.rs
new file mode 100644
index 00000000000..927f0d602ad
--- /dev/null
+++ b/tests/codegen/llvm-ident.rs
@@ -0,0 +1,15 @@
+// Verifies that the `!llvm.ident` named metadata is emitted.
+//
+// revisions: NONE OPT DEBUG
+//
+// [OPT] compile-flags: -Copt-level=2
+// [DEBUG] compile-flags: -Cdebuginfo=2
+
+// The named metadata should contain a single metadata node (see
+// `LLVMRustPrepareThinLTOImport` for details).
+// CHECK: !llvm.ident = !{![[ID:[0-9]+]]}
+
+// In addition, check that the metadata node has the expected content.
+// CHECK: ![[ID]] = !{!"rustc version 1.{{.*}}"}
+
+fn main() {}