8000 [RFC] Support `.comment` section like GCC/Clang (`!llvm.ident`) by ojeda · Pull Request #97550 · rust-lang/rust · GitHub
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[RFC] Support .comment section like GCC/Clang (!llvm.ident) #97550

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Merged
merged 1 commit into from
Jul 21, 2023
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[RFC] Support .comment section like GCC/Clang (!llvm.ident) #97550

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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>
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@ojeda
ojeda committed Jul 21, 2023
commit 74b8d324eb77a8f337b35dc68ac91b0c2c06debc
18 changes: 18 additions & 0 deletions compiler/rustc_codegen_llvm/src/context.rs
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Expand Up @@ -33,6 +33,7 @@ use rustc_target::abi::{
use rustc_target::spec::{HasTargetSpec, RelocModel, Target, TlsModel};
use smallvec::SmallVec;

use libc::c_uint;
use std::cell::{Cell, RefCell};
use std::ffi::CStr;
use std::str;
Expand Down Expand Up @@ -349,6 +350,23 @@ pub unsafe fn create_module<'ll>(
);
}

// Insert `llvm.ident` metadata.
//
// On the wasm targets it will get hooked up to the "producer" sections
// `processed-by` information.
let rustc_producer =
format!("rustc version {}", option_env!("CFG_VERSION").expect("CFG_VERSION"));
let name_metadata = llvm::LLVMMDStringInContext(
llcx,
rustc_producer.as_ptr().cast(),
rustc_producer.as_bytes().len() as c_uint,
);
llvm::LLVMAddNamedMetadataOperand(
llmod,
cstr!("llvm.ident").as_ptr(),
llvm::LLVMMDNodeInContext(llcx, &name_metadata, 1),
);

llmod
}

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15 changes: 0 additions & 15 deletions compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
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Expand Up @@ -888,21 +888,6 @@ pub fn build_compile_unit_di_node<'ll, 'tcx>(
llvm::LLVMAddNamedMetadataOperand(debug_context.llmod, llvm_gcov_ident.as_ptr(), val);
}

// Insert `llvm.ident` metadata on the wasm targets since that will
// get hooked up to the "producer" sections `processed-by` information.
if tcx.sess.target.is_like_wasm {
let name_metadata = llvm::LLVMMDStringInContext(
debug_context.llcontext,
rustc_producer.as_ptr().cast(),
rustc_producer.as_bytes().len() as c_uint,
);
llvm::LLVMAddNamedMetadataOperand(
debug_context.llmod,
cstr!("llvm.ident").as_ptr(),
llvm::LLVMMDNodeInContext(debug_context.llcontext, &name_metadata, 1),
);
}

return unit_metadata;
};

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5 changes: 5 additions & 0 deletions compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
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Expand Up @@ -1366,6 +1366,11 @@ LLVMRustPrepareThinLTOImport(const LLVMRustThinLTOData *Data, LLVMModuleRef M,
if (WasmCustomSections)
WasmCustomSections->eraseFromParent();

// `llvm.ident` named metadata also gets duplicated.
auto *llvmIdent = (*MOrErr)->getNamedMetadata("llvm.ident");
if (llvmIdent)
llvmIdent->eraseFromParent();

return MOrErr;
};
bool ClearDSOLocal = clearDSOLocalOnDeclarations(Mod, Target);
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15 changes: 15 additions & 0 deletions tests/codegen/llvm-ident.rs
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@@ -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() {}
15 changes: 15 additions & 0 deletions tests/run-make/comment-section/Makefile
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@@ -0,0 +1,15 @@
include ../tools.mk

# only-linux

all:
echo 'fn main(){}' | $(RUSTC) - --emit=link,obj -Csave-temps --target=$(TARGET)

# Check linked output has a `.comment` section with the expected content.
readelf -p '.comment' $(TMPDIR)/rust_out | $(CGREP) -F 'rustc version 1.'

# Check all object files (including temporary outputs) have a `.comment`
# section with the expected content.
set -e; for f in $(TMPDIR)/*.o; do \
readelf -p '.comment' $$f | $(CGREP) -F 'rustc version 1.'; \
done
19 changes: 19 additions & 0 deletions tests/run-make/llvm-ident/Makefile
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@@ -0,0 +1,19 @@
include ../tools.mk

# only-linux

all:
# `-Ccodegen-units=16 -Copt-level=2` is used here to trigger thin LTO
# across codegen units to test deduplication of the named metadata
# (see `LLVMRustPrepareThinLTOImport` for details).
echo 'fn main(){}' | $(RUSTC) - --emit=link,obj -Csave-temps -Ccodegen-units=16 -Copt-level=2 --target=$(TARGET)

# `llvm-dis` is used here since `--emit=llvm-ir` does not emit LLVM IR
# for temporary outputs.
"$(LLVM_BIN_DIR)"/llvm-dis $(TMPDIR)/*.bc

# Check LLVM IR files (including temporary outputs) have `!llvm.ident`
# named metadata, reusing the related codegen test.
set -e; for f in $(TMPDIR)/*.ll; do \
$(LLVM_FILECHECK) --input-file $$f ../../codegen/llvm-ident.rs; \
done
0