Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Put crate metadata first in the rlib #93816

Merged
merged 1 commit into from
Feb 20, 2022
Merged

Put crate metadata first in the rlib #93816

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
97 changes: 60 additions & 37 deletions compiler/rustc_codegen_ssa/src/back/link.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ use rustc_target::spec::{PanicStrategy, RelocModel, RelroLevel, SanitizerSet, Ta
use super::archive::{find_library, ArchiveBuilder};
use super::command::Command;
use super::linker::{self, Linker};
use super::metadata::create_rmeta_file;
use super::metadata::{create_rmeta_file, MetadataPosition};
use super::rpath::{self, RPathConfig};
use crate::{
looks_like_rust_object_file, CodegenResults, CompiledModule, CrateInfo, NativeLib,
Expand Down Expand Up @@ -267,6 +267,28 @@ fn link_rlib<'a, B: ArchiveBuilder<'a>>(

let mut ab = <B as ArchiveBuilder>::new(sess, out_filename, None);

let trailing_metadata = match flavor {
RlibFlavor::Normal => {
let (metadata, metadata_position) =
create_rmeta_file(sess, codegen_results.metadata.raw_data());
let metadata = emit_metadata(sess, &metadata, tmpdir);
match metadata_position {
MetadataPosition::First => {
// Most of the time metadata in rlib files is wrapped in a "dummy" object
// file for the target platform so the rlib can be processed entirely by
// normal linkers for the platform. Sometimes this is not possible however.
// If it is possible however, placing the metadata object first improves
// performance of getting metadata from rlibs.
ab.add_file(&metadata);
None
}
MetadataPosition::Last => Some(metadata),
}
}

RlibFlavor::StaticlibBase => None,
};

for m in &codegen_results.modules {
if let Some(obj) = m.object.as_ref() {
ab.add_file(obj);
Expand All @@ -277,6 +299,16 @@ fn link_rlib<'a, B: ArchiveBuilder<'a>>(
}
}

match flavor {
RlibFlavor::Normal => {}
RlibFlavor::StaticlibBase => {
let obj = codegen_results.allocator_module.as_ref().and_then(|m| m.object.as_ref());
if let Some(obj) = obj {
ab.add_file(obj);
}
}
}

// Note that in this loop we are ignoring the value of `lib.cfg`. That is,
// we may not be configured to actually include a static library if we're
// adding it here. That's because later when we consume this rlib we'll
Expand Down Expand Up @@ -334,42 +366,33 @@ fn link_rlib<'a, B: ArchiveBuilder<'a>>(
ab.inject_dll_import_lib(&raw_dylib_name, &raw_dylib_imports, tmpdir);
}

// Note that it is important that we add all of our non-object "magical
// files" *after* all of the object files in the archive. The reason for
// this is as follows:
//
// * When performing LTO, this archive will be modified to remove
// objects from above. The reason for this is described below.
//
// * When the system linker looks at an archive, it will attempt to
// determine the architecture of the archive in order to see whether its
// linkable.
//
// The algorithm for this detection is: iterate over the files in the
// archive. Skip magical SYMDEF names. Interpret the first file as an
// object file. Read architecture from the object file.
//
// * As one can probably see, if "metadata" and "foo.bc" were placed
// before all of the objects, then the architecture of this archive would
// not be correctly inferred once 'foo.o' is removed.
//
// Basically, all this means is that this code should not move above the
// code above.
match flavor {
RlibFlavor::Normal => {
// metadata in rlib files is wrapped in a "dummy" object file for
// the target platform so the rlib can be processed entirely by
// normal linkers for the platform.
let metadata = create_rmeta_file(sess, codegen_results.metadata.raw_data());
ab.add_file(&emit_metadata(sess, &metadata, tmpdir));
}

RlibFlavor::StaticlibBase => {
let obj = codegen_results.allocator_module.as_ref().and_then(|m| m.object.as_ref());
if let Some(obj) = obj {
ab.add_file(obj);
}
}
if let Some(trailing_metadata) = trailing_metadata {
// Note that it is important that we add all of our non-object "magical
// files" *after* all of the object files in the archive. The reason for
// this is as follows:
//
// * When performing LTO, this archive will be modified to remove
// objects from above. The reason for this is described below.
//
// * When the system linker looks at an archive, it will attempt to
// determine the architecture of the archive in order to see whether its
// linkable.
//
// The algorithm for this detection is: iterate over the files in the
// archive. Skip magical SYMDEF names. Interpret the first file as an
// object file. Read architecture from the object file.
//
// * As one can probably see, if "metadata" and "foo.bc" were placed
// before all of the objects, then the architecture of this archive would
// not be correctly inferred once 'foo.o' is removed.
//
// * Most of the time metadata in rlib files is wrapped in a "dummy" object
// file for the target platform so the rlib can be processed entirely by
// normal linkers for the platform. Sometimes this is not possible however.
//
// Basically, all this means is that this code should not move above the
// code above.
ab.add_file(&trailing_metadata);
}

return Ok(ab);
Expand Down
11 changes: 8 additions & 3 deletions compiler/rustc_codegen_ssa/src/back/metadata.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -167,6 +167,11 @@ fn create_object_file(sess: &Session) -> Option<write::Object<'static>> {
Some(file)
}

pub enum MetadataPosition {
First,
Last,
}

// For rlibs we "pack" rustc metadata into a dummy object file. When rustc
// creates a dylib crate type it will pass `--whole-archive` (or the
// platform equivalent) to include all object files from an rlib into the
Expand Down Expand Up @@ -199,7 +204,7 @@ fn create_object_file(sess: &Session) -> Option<write::Object<'static>> {
// * ELF - All other targets are similar to Windows in that there's a
// `SHF_EXCLUDE` flag we can set on sections in an object file to get
// automatically removed from the final output.
pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> Vec<u8> {
pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> (Vec<u8>, MetadataPosition) {
let Some(mut file) = create_object_file(sess) else {
// This is used to handle all "other" targets. This includes targets
// in two categories:
Expand All @@ -217,7 +222,7 @@ pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> Vec<u8> {
// WebAssembly and for targets not supported by the `object` crate
// yet it means that work will need to be done in the `object` crate
// to add a case above.
return metadata.to_vec();
return (metadata.to_vec(), MetadataPosition::Last);
};
let section = file.add_section(
file.segment_name(StandardSegment::Debug).to_vec(),
Expand All @@ -236,7 +241,7 @@ pub fn create_rmeta_file(sess: &Session, metadata: &[u8]) -> Vec<u8> {
_ => {}
};
file.append_section_data(section, metadata, 1);
file.write().unwrap()
(file.write().unwrap(), MetadataPosition::First)
}

// Historical note:
Expand Down