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Auto merge of #83640 - bjorn3:shared_metadata_reader, r=nagisa

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Use the object crate for metadata reading

This allows sharing the metadata reader between cg_llvm, cg_clif and other codegen backends.

This is not currently useful for rlib reading with cg_spirv ([rust-gpu](https://github.com/EmbarkStudios/rust-gpu/)) as it uses tar rather than ar as .rlib format, but it is useful for dylib reading required for loading proc macros. (cc `@eddyb)`

The object crate is already trusted as dependency of libstd through backtrace. As far as I know it supports reading all object file formats used by targets for which we support rust dylibs with crate metadata, but I am not certain. If this happens to not be the case, I could keep using LLVM for reading dylib metadata.

Marked as WIP for a perf run and as it is based on #83637.
This commit is contained in:
bors 2021-05-14 12:58:58 +00:00
commit a5da29cf8f
2 changed files with 2 additions and 66 deletions
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2
src/lib.rs
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@ -165,7 +165,7 @@ fn init(&self, sess: &Session) {
}
fn metadata_loader(&self) -> Box<dyn MetadataLoader + Sync> {
Box::new(crate::metadata::CraneliftMetadataLoader)
Box::new(rustc_codegen_ssa::back::metadata::DefaultMetadataLoader)
}
fn provide(&self, _providers: &mut Providers) {}

66
src/metadata.rs
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@ -1,73 +1,9 @@
//! Reading and writing of the rustc metadata for rlibs and dylibs
//! Writing of the rustc metadata for dylibs
use std::fs::File;
use std::path::Path;
use rustc_codegen_ssa::METADATA_FILENAME;
use rustc_data_structures::memmap::Mmap;
use rustc_data_structures::owning_ref::OwningRef;
use rustc_data_structures::rustc_erase_owner;
use rustc_data_structures::sync::MetadataRef;
use rustc_middle::middle::cstore::MetadataLoader;
use rustc_middle::ty::TyCtxt;
use rustc_target::spec::Target;
use crate::backend::WriteMetadata;
/// The metadata loader used by cg_clif.
///
/// The metadata is stored in the same format as cg_llvm.
///
/// # Metadata location
///
/// <dl>
/// <dt>rlib</dt>
/// <dd>The metadata can be found in the `lib.rmeta` file inside of the ar archive.</dd>
/// <dt>dylib</dt>
/// <dd>The metadata can be found in the `.rustc` section of the shared library.</dd>
/// </dl>
pub(crate) struct CraneliftMetadataLoader;
fn load_metadata_with(
path: &Path,
f: impl for<'a> FnOnce(&'a [u8]) -> Result<&'a [u8], String>,
) -> Result<MetadataRef, String> {
let file = File::open(path).map_err(|e| format!("{:?}", e))?;
let data = unsafe { Mmap::map(file) }.map_err(|e| format!("{:?}", e))?;
let metadata = OwningRef::new(data).try_map(f)?;
return Ok(rustc_erase_owner!(metadata.map_owner_box()));
}
impl MetadataLoader for CraneliftMetadataLoader {
fn get_rlib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
load_metadata_with(path, |data| {
let archive = object::read::archive::ArchiveFile::parse(&*data)
.map_err(|e| format!("{:?}", e))?;
for entry_result in archive.members() {
let entry = entry_result.map_err(|e| format!("{:?}", e))?;
if entry.name() == METADATA_FILENAME.as_bytes() {
return Ok(entry.data());
}
}
Err("couldn't find metadata entry".to_string())
})
}
fn get_dylib_metadata(&self, _target: &Target, path: &Path) -> Result<MetadataRef, String> {
use object::{Object, ObjectSection};
load_metadata_with(path, |data| {
let file = object::File::parse(&data).map_err(|e| format!("parse: {:?}", e))?;
file.section_by_name(".rustc")
.ok_or("no .rustc section")?
.data()
.map_err(|e| format!("failed to read .rustc section: {:?}", e))
})
}
}
// Adapted from https://github.com/rust-lang/rust/blob/da573206f87b5510de4b0ee1a9c044127e409bd3/src/librustc_codegen_llvm/base.rs#L47-L112
pub(crate) fn write_metadata<O: WriteMetadata>(tcx: TyCtxt<'_>, object: &mut O) {
use snap::write::FrameEncoder;