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Auto merge of #85086 - petrochenkov:linkord2, r=nagisa
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linker: Reorder linker arguments

- Split arguments into order-independent and order-dependent, to define more precisely what (pre-,post-,late-,)link-args mean.
- Add some comments.
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@bors
bors committed Jun 6, 2021
2 parents 86b0baf + 3eab280 commit 5b638c1
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Showing 2 changed files with 164 additions and 165 deletions.
296 changes: 160 additions & 136 deletions compiler/rustc_codegen_ssa/src/back/link.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1058,7 +1058,7 @@ fn link_natively<'a, B: ArchiveBuilder<'a>>(
}
}

fn link_sanitizers(sess: &Session, crate_type: CrateType, linker: &mut dyn Linker) {
fn add_sanitizer_libraries(sess: &Session, crate_type: CrateType, linker: &mut dyn Linker) {
// On macOS the runtimes are distributed as dylibs which should be linked to
// both executables and dynamic shared objects. Everywhere else the runtimes
// are currently distributed as static liraries which should be linked to
Expand Down Expand Up @@ -1680,55 +1680,6 @@ fn add_local_crate_metadata_objects(
}
}

/// Link native libraries corresponding to the current crate and all libraries corresponding to
/// all its dependency crates.
/// FIXME: Consider combining this with the functions above adding object files for the local crate.
fn link_local_crate_native_libs_and_dependent_crate_libs<'a, B: ArchiveBuilder<'a>>(
cmd: &mut dyn Linker,
sess: &'a Session,
crate_type: CrateType,
codegen_results: &CodegenResults,
tmpdir: &Path,
) {
// Take careful note of the ordering of the arguments we pass to the linker
// here. Linkers will assume that things on the left depend on things to the
// right. Things on the right cannot depend on things on the left. This is
// all formally implemented in terms of resolving symbols (libs on the right
// resolve unknown symbols of libs on the left, but not vice versa).
//
// For this reason, we have organized the arguments we pass to the linker as
// such:
//
// 1. The local object that LLVM just generated
// 2. Local native libraries
// 3. Upstream rust libraries
// 4. Upstream native libraries
//
// The rationale behind this ordering is that those items lower down in the
// list can't depend on items higher up in the list. For example nothing can
// depend on what we just generated (e.g., that'd be a circular dependency).
// Upstream rust libraries are not allowed to depend on our local native
// libraries as that would violate the structure of the DAG, in that
// scenario they are required to link to them as well in a shared fashion.
//
// Note that upstream rust libraries may contain native dependencies as
// well, but they also can't depend on what we just started to add to the
// link line. And finally upstream native libraries can't depend on anything
// in this DAG so far because they're only dylibs and dylibs can only depend
// on other dylibs (e.g., other native deps).
//
// If -Zlink-native-libraries=false is set, then the assumption is that an
// external build system already has the native dependencies defined, and it
// will provide them to the linker itself.
if sess.opts.debugging_opts.link_native_libraries {
add_local_native_libraries(cmd, sess, codegen_results);
}
add_upstream_rust_crates::<B>(cmd, sess, codegen_results, crate_type, tmpdir);
if sess.opts.debugging_opts.link_native_libraries {
add_upstream_native_libraries(cmd, sess, codegen_results, crate_type);
}
}

/// Add sysroot and other globally set directories to the directory search list.
fn add_library_search_dirs(cmd: &mut dyn Linker, sess: &Session, self_contained: bool) {
// The default library location, we need this to find the runtime.
Expand Down Expand Up @@ -1787,12 +1738,13 @@ fn add_rpath_args(
}

/// Produce the linker command line containing linker path and arguments.
/// `NO-OPT-OUT` marks the arguments that cannot be removed from the command line
/// by the user without creating a custom target specification.
/// `OBJECT-FILES` specify whether the arguments can add object files.
/// `CUSTOMIZATION-POINT` means that arbitrary arguments defined by the user
/// or by the target spec can be inserted here.
/// `AUDIT-ORDER` - need to figure out whether the option is order-dependent or not.
///
/// When comments in the function say "order-(in)dependent" they mean order-dependence between
/// options and libraries/object files. For example `--whole-archive` (order-dependent) applies
/// to specific libraries passed after it, and `-o` (output file, order-independent) applies
/// to the linking process as a whole.
/// Order-independent options may still override each other in order-dependent fashion,
/// e.g `--foo=yes --foo=no` may be equivalent to `--foo=no`.
fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
path: &Path,
flavor: LinkerFlavor,
Expand All @@ -1810,16 +1762,151 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
let cmd = &mut *codegen_results.linker_info.to_linker(base_cmd, &sess, flavor);
let link_output_kind = link_output_kind(sess, crate_type);

// NO-OPT-OUT, OBJECT-FILES-MAYBE, CUSTOMIZATION-POINT
// ------------ Early order-dependent options ------------

// If we're building something like a dynamic library then some platforms
// need to make sure that all symbols are exported correctly from the
// dynamic library.
// Must be passed before any libraries to prevent the symbols to export from being thrown away,
// at least on some platforms (e.g. windows-gnu).
cmd.export_symbols(tmpdir, crate_type);

// Can be used for adding custom CRT objects or overriding order-dependent options above.
// FIXME: In practice built-in target specs use this for arbitrary order-independent options,
// introduce a target spec option for order-independent linker options and migrate built-in
// specs to it.
add_pre_link_args(cmd, sess, flavor);

// NO-OPT-OUT, OBJECT-FILES-NO
// ------------ Object code and libraries, order-dependent ------------

// Pre-link CRT objects.
add_pre_link_objects(cmd, sess, link_output_kind, crt_objects_fallback);

// Sanitizer libraries.
add_sanitizer_libraries(sess, crate_type, cmd);

// Object code from the current crate.
// Take careful note of the ordering of the arguments we pass to the linker
// here. Linkers will assume that things on the left depend on things to the
// right. Things on the right cannot depend on things on the left. This is
// all formally implemented in terms of resolving symbols (libs on the right
// resolve unknown symbols of libs on the left, but not vice versa).
//
// For this reason, we have organized the arguments we pass to the linker as
// such:
//
// 1. The local object that LLVM just generated
// 2. Local native libraries
// 3. Upstream rust libraries
// 4. Upstream native libraries
//
// The rationale behind this ordering is that those items lower down in the
// list can't depend on items higher up in the list. For example nothing can
// depend on what we just generated (e.g., that'd be a circular dependency).
// Upstream rust libraries are not supposed to depend on our local native
// libraries as that would violate the structure of the DAG, in that
// scenario they are required to link to them as well in a shared fashion.
// (The current implementation still doesn't prevent it though, see the FIXME below.)
//
// Note that upstream rust libraries may contain native dependencies as
// well, but they also can't depend on what we just started to add to the
// link line. And finally upstream native libraries can't depend on anything
// in this DAG so far because they can only depend on other native libraries
// and such dependencies are also required to be specified.
add_local_crate_regular_objects(cmd, codegen_results);
add_local_crate_metadata_objects(cmd, crate_type, codegen_results);
add_local_crate_allocator_objects(cmd, codegen_results);

// Avoid linking to dynamic libraries unless they satisfy some undefined symbols
// at the point at which they are specified on the command line.
// Must be passed before any (dynamic) libraries to have effect on them.
// On Solaris-like systems, `-z ignore` acts as both `--as-needed` and `--gc-sections`
// so it will ignore unreferenced ELF sections from relocatable objects.
// For that reason, we put this flag after metadata objects as they would otherwise be removed.
// FIXME: Support more fine-grained dead code removal on Solaris/illumos
// and move this option back to the top.
cmd.add_as_needed();

// FIXME: Move this below to other native libraries
// (or alternatively link all native libraries after their respective crates).
// This change is somewhat breaking in practice due to local static libraries being linked
// as whole-archive (#85144), so removing whole-archive may be a pre-requisite.
if sess.opts.debugging_opts.link_native_libraries {
add_local_native_libraries(cmd, sess, codegen_results);
}

// Rust libraries.
add_upstream_rust_crates::<B>(cmd, sess, codegen_results, crate_type, tmpdir);

// Native libraries linked with `#[link]` attributes at and `-l` command line options.
// If -Zlink-native-libraries=false is set, then the assumption is that an
// external build system already has the native dependencies defined, and it
// will provide them to the linker itself.
if sess.opts.debugging_opts.link_native_libraries {
add_upstream_native_libraries(cmd, sess, codegen_results, crate_type);
}

// Library linking above uses some global state for things like `-Bstatic`/`-Bdynamic` to make
// command line shorter, reset it to default here before adding more libraries.
cmd.reset_per_library_state();

// FIXME: Built-in target specs occasionally use this for linking system libraries,
// eliminate all such uses by migrating them to `#[link]` attributes in `lib(std,c,unwind)`
// and remove the option.
add_late_link_args(cmd, sess, flavor, crate_type, codegen_results);

// ------------ Arbitrary order-independent options ------------

// Add order-independent options determined by rustc from its compiler options,
// target properties and source code.
add_order_independent_options(
cmd,
sess,
link_output_kind,
crt_objects_fallback,
flavor,
crate_type,
codegen_results,
out_filename,
tmpdir,
);

// Can be used for arbitrary order-independent options.
// In practice may also be occasionally used for linking native libraries.
// Passed after compiler-generated options to support manual overriding when necessary.
add_user_defined_link_args(cmd, sess);

// ------------ Object code and libraries, order-dependent ------------

// Post-link CRT objects.
add_post_link_objects(cmd, sess, link_output_kind, crt_objects_fallback);

// ------------ Late order-dependent options ------------

// Doesn't really make sense.
// FIXME: In practice built-in target specs use this for arbitrary order-independent options,
// introduce a target spec option for order-independent linker options, migrate built-in specs
// to it and remove the option.
add_post_link_args(cmd, sess, flavor);

cmd.take_cmd()
}

fn add_order_independent_options(
cmd: &mut dyn Linker,
sess: &Session,
link_output_kind: LinkOutputKind,
crt_objects_fallback: bool,
flavor: LinkerFlavor,
crate_type: CrateType,
codegen_results: &CodegenResults,
out_filename: &Path,
tmpdir: &Path,
) {
add_apple_sdk(cmd, sess, flavor);

// NO-OPT-OUT
add_link_script(cmd, sess, tmpdir, crate_type);

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
if sess.target.is_like_fuchsia && crate_type == CrateType::Executable {
let prefix = if sess.opts.debugging_opts.sanitizer.contains(SanitizerSet::ADDRESS) {
"asan/"
Expand All @@ -1829,36 +1916,17 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
cmd.arg(format!("--dynamic-linker={}ld.so.1", prefix));
}

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
if sess.target.eh_frame_header {
cmd.add_eh_frame_header();
}

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
// Make the binary compatible with data execution prevention schemes.
cmd.add_no_exec();

// OBJECT-FILES-YES
add_local_crate_metadata_objects(cmd, crate_type, codegen_results);

// NO-OPT-OUT, OBJECT-FILES-NO
// Avoid linking to dynamic libraries unless they satisfy some undefined symbols
// at the point at which they are specified on the command line.
// Must be passed before any dynamic libraries.
// On solaris-like systems, this also will ignore unreferenced ELF sections
// from relocatable objects. For that reason, we move the metadata objects
// to before this flag as they would otherwise be removed.
cmd.add_as_needed();

// NO-OPT-OUT, OBJECT-FILES-NO
if crt_objects_fallback {
cmd.no_crt_objects();
}

// NO-OPT-OUT, OBJECT-FILES-YES
add_pre_link_objects(cmd, sess, link_output_kind, crt_objects_fallback);

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
if sess.target.is_like_emscripten {
cmd.arg("-s");
cmd.arg(if sess.panic_strategy() == PanicStrategy::Abort {
Expand All @@ -1868,108 +1936,64 @@ fn linker_with_args<'a, B: ArchiveBuilder<'a>>(
});
}

// OBJECT-FILES-YES, AUDIT-ORDER
link_sanitizers(sess, crate_type, cmd);
if flavor == LinkerFlavor::PtxLinker {
// Provide the linker with fallback to internal `target-cpu`.
cmd.arg("--fallback-arch");
cmd.arg(&codegen_results.linker_info.target_cpu);
} else if flavor == LinkerFlavor::BpfLinker {
cmd.arg("--cpu");
cmd.arg(&codegen_results.linker_info.target_cpu);
cmd.arg("--cpu-features");
cmd.arg(match &sess.opts.cg.target_feature {
feat if !feat.is_empty() => feat,
_ => &sess.target.options.features,
});
}

// OBJECT-FILES-NO, AUDIT-ORDER
// Linker plugins should be specified early in the list of arguments
// FIXME: How "early" exactly?
cmd.linker_plugin_lto();

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
// FIXME: Order-dependent, at least relatively to other args adding searh directories.
add_library_search_dirs(cmd, sess, crt_objects_fallback);

// OBJECT-FILES-YES
add_local_crate_regular_objects(cmd, codegen_results);

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
cmd.output_filename(out_filename);

// OBJECT-FILES-NO, AUDIT-ORDER
if crate_type == CrateType::Executable && sess.target.is_like_windows {
if let Some(ref s) = codegen_results.windows_subsystem {
cmd.subsystem(s);
}
}

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
// If we're building something like a dynamic library then some platforms
// need to make sure that all symbols are exported correctly from the
// dynamic library.
cmd.export_symbols(tmpdir, crate_type);

// OBJECT-FILES-YES
add_local_crate_allocator_objects(cmd, codegen_results);

// OBJECT-FILES-NO, AUDIT-ORDER
// FIXME: Order dependent, applies to the following objects. Where should it be placed?
// Try to strip as much out of the generated object by removing unused
// sections if possible. See more comments in linker.rs
if !sess.link_dead_code() {
let keep_metadata = crate_type == CrateType::Dylib;
cmd.gc_sections(keep_metadata);
}

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
cmd.set_output_kind(link_output_kind, out_filename);

// OBJECT-FILES-NO, AUDIT-ORDER
add_relro_args(cmd, sess);

// OBJECT-FILES-NO, AUDIT-ORDER
// Pass optimization flags down to the linker.
cmd.optimize();

// OBJECT-FILES-NO, AUDIT-ORDER
// Pass debuginfo and strip flags down to the linker.
cmd.debuginfo(sess.opts.debugging_opts.strip);

// OBJECT-FILES-NO, AUDIT-ORDER
// We want to prevent the compiler from accidentally leaking in any system libraries,
// so by default we tell linkers not to link to any default libraries.
if !sess.opts.cg.default_linker_libraries && sess.target.no_default_libraries {
cmd.no_default_libraries();
}

// OBJECT-FILES-YES
link_local_crate_native_libs_and_dependent_crate_libs::<B>(
cmd,
sess,
crate_type,
codegen_results,
tmpdir,
);

// OBJECT-FILES-NO, AUDIT-ORDER
if sess.opts.cg.profile_generate.enabled() || sess.instrument_coverage() {
cmd.pgo_gen();
}

// OBJECT-FILES-NO, AUDIT-ORDER
if sess.opts.cg.control_flow_guard != CFGuard::Disabled {
cmd.control_flow_guard();
}

// OBJECT-FILES-NO, AUDIT-ORDER
add_rpath_args(cmd, sess, codegen_results, out_filename);

// OBJECT-FILES-MAYBE, CUSTOMIZATION-POINT
add_user_defined_link_args(cmd, sess);

// NO-OPT-OUT, OBJECT-FILES-NO, AUDIT-ORDER
cmd.finalize();

// NO-OPT-OUT, OBJECT-FILES-MAYBE, CUSTOMIZATION-POINT
add_late_link_args(cmd, sess, flavor, crate_type, codegen_results);

// NO-OPT-OUT, OBJECT-FILES-YES
add_post_link_objects(cmd, sess, link_output_kind, crt_objects_fallback);

// NO-OPT-OUT, OBJECT-FILES-MAYBE, CUSTOMIZATION-POINT
add_post_link_args(cmd, sess, flavor);

cmd.take_cmd()
}

/// # Native library linking
Expand Down
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