Rollup merge of rust-lang#58057 - michaelwoerister:stabilize-xlto, r=alexcrichton

Stabilize linker-plugin based LTO (aka cross-language LTO)

This PR stabilizes [linker plugin based LTO](rust-lang#49879), also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects.

As described in the tracking issue, it works by making `rustc` emit LLVM bitcode instead of machine code, the same as `clang` does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules.

The feature has been implemented over a number of pull requests and there are various [codegen](https://github.com/rust-lang/rust/blob/master/src/test/codegen/no-dllimport-w-cross-lang-lto.rs) and [run](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-clang)-[make](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-upstream-rlibs) [tests](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto) that make sure that it keeps working.

It also works for building big projects like [Firefox](https://treeherder.mozilla.org/#/jobs?repo=try&revision=2ce2d5ddcea6fbff790503eac406954e469b2f5d).

The PR makes the feature available under the `-C linker-plugin-lto` flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific. `-C linker-plugin-lto` is descriptive of what it does. If someone has a better name, let me know `:)`

Author: Centril

src/doc/rustc/src/SUMMARY.md

@@ -13,4 +13,5 @@
 - [Targets](targets/index.md)
     - [Built-in Targets](targets/built-in.md)
     - [Custom Targets](targets/custom.md)
-- [Contributing to `rustc`](contributing.md)
+- [Linker-plugin based LTO](linker-plugin-lto.md)
+- [Contributing to `rustc`](contributing.md)

src/doc/rustc/src/linker-plugin-lto.md

@@ -0,0 +1,108 @@
+# Linker-plugin-LTO
+
+The `-C linker-plugin-lto` flag allows for deferring the LTO optimization
+to the actual linking step, which in turn allows for performing
+interprocedural optimizations across programming language boundaries if
+all the object files being linked were created by LLVM based toolchains.
+The prime example here would be linking Rust code together with
+Clang-compiled C/C++ code.
+
+## Usage
+
+There are two main cases how linker plugin based LTO can be used:
+
+ - compiling a Rust `staticlib` that is used as a C ABI dependency
+ - compiling a Rust binary where `rustc` invokes the linker
+
+In both cases the Rust code has to be compiled with `-C linker-plugin-lto` and
+the C/C++ code with `-flto` or `-flto=thin` so that object files are emitted
+as LLVM bitcode.
+
+### Rust `staticlib` as dependency in C/C++ program
+
+In this case the Rust compiler just has to make sure that the object files in
+the `staticlib` are in the right format. For linking, a linker with the
+LLVM plugin must be used (e.g. LLD).
+
+Using `rustc` directly:
+
+```bash
+# Compile the Rust staticlib
+rustc --crate-type=staticlib -Clinker-plugin-lto -Copt-level=2 ./lib.rs
+# Compile the C code with `-flto=thin`
+clang -c -O2 -flto=thin -o main.o ./main.c
+# Link everything, making sure that we use an appropriate linker
+clang -flto=thin -fuse-ld=lld -L . -l"name-of-your-rust-lib" -o main -O2 ./cmain.o
+```
+
+Using `cargo`:
+
+```bash
+# Compile the Rust staticlib
+RUSTFLAGS="-Clinker-plugin-lto" cargo build --release
+# Compile the C code with `-flto=thin`
+clang -c -O2 -flto=thin -o main.o ./main.c
+# Link everything, making sure that we use an appropriate linker
+clang -flto=thin -fuse-ld=lld -L . -l"name-of-your-rust-lib" -o main -O2 ./cmain.o
+```
+
+### C/C++ code as a dependency in Rust
+
+In this case the linker will be invoked by `rustc`. We again have to make sure
+that an appropriate linker is used.
+
+Using `rustc` directly:
+
+```bash
+# Compile C code with `-flto`
+clang ./clib.c -flto=thin -c -o ./clib.o -O2
+# Create a static library from the C code
+ar crus ./libxyz.a ./clib.o
+
+# Invoke `rustc` with the additional arguments
+rustc -Clinker-plugin-lto -L. -Copt-level=2 -Clinker=clang -Clink-arg=-fuse-ld=lld ./main.rs
+```
+
+Using `cargo` directly:
+
+```bash
+# Compile C code with `-flto`
+clang ./clib.c -flto=thin -c -o ./clib.o -O2
+# Create a static library from the C code
+ar crus ./libxyz.a ./clib.o
+
+# Set the linking arguments via RUSTFLAGS
+RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build --release
+```
+
+### Explicitly specifying the linker plugin to be used by `rustc`
+
+If one wants to use a linker other than LLD, the LLVM linker plugin has to be
+specified explicitly. Otherwise the linker cannot read the object files. The
+path to the plugin is passed as an argument to the `-Clinker-plugin-lto`
+option:
+
+```bash
+rustc -Clinker-plugin-lto="/path/to/LLVMgold.so" -L. -Copt-level=2 ./main.rs
+```
+
+
+## Toolchain Compatibility
+
+In order for this kind of LTO to work, the LLVM linker plugin must be able to
+handle the LLVM bitcode produced by both `rustc` and `clang`.
+
+Best results are achieved by using a `rustc` and `clang` that are based on the
+exact same version of LLVM. One can use `rustc -vV` in order to view the LLVM
+used by a given `rustc` version. Note that the version number given
+here is only an approximation as Rust sometimes uses unstable revisions of
+LLVM. However, the approximation is usually reliable.
+
+The following table shows known good combinations of toolchain versions.
+
+|           | Clang 7   | Clang 8   |
+|-----------|-----------|-----------|
+| Rust 1.34 |     ✗     |     ✓     |
+| Rust 1.35 |     ✗     |    ✓(?)   |
+
+Note that the compatibility policy for this feature might change in the future.

src/librustc/session/config.rs

@@ -96,18 +96,18 @@ pub enum LtoCli {
 }
 
 #[derive(Clone, PartialEq, Hash)]
-pub enum CrossLangLto {
+pub enum LinkerPluginLto {
     LinkerPlugin(PathBuf),
     LinkerPluginAuto,
     Disabled
 }
 
-impl CrossLangLto {
+impl LinkerPluginLto {
     pub fn enabled(&self) -> bool {
         match *self {
-            CrossLangLto::LinkerPlugin(_) |
-            CrossLangLto::LinkerPluginAuto => true,
-            CrossLangLto::Disabled => false,
+            LinkerPluginLto::LinkerPlugin(_) |
+            LinkerPluginLto::LinkerPluginAuto => true,
+            LinkerPluginLto::Disabled => false,
         }
     }
 }
@@ -812,7 +812,7 @@ macro_rules! options {
         pub const parse_lto: Option<&str> =
             Some("either a boolean (`yes`, `no`, `on`, `off`, etc), `thin`, \
                   `fat`, or omitted");
-        pub const parse_cross_lang_lto: Option<&str> =
+        pub const parse_linker_plugin_lto: Option<&str> =
             Some("either a boolean (`yes`, `no`, `on`, `off`, etc), \
                   or the path to the linker plugin");
         pub const parse_merge_functions: Option<&str> =
@@ -821,7 +821,7 @@ macro_rules! options {
 
     #[allow(dead_code)]
     mod $mod_set {
-        use super::{$struct_name, Passes, Sanitizer, LtoCli, CrossLangLto};
+        use super::{$struct_name, Passes, Sanitizer, LtoCli, LinkerPluginLto};
         use rustc_target::spec::{LinkerFlavor, MergeFunctions, PanicStrategy, RelroLevel};
         use std::path::PathBuf;
         use std::str::FromStr;
@@ -1037,22 +1037,22 @@ macro_rules! options {
             true
         }
 
-        fn parse_cross_lang_lto(slot: &mut CrossLangLto, v: Option<&str>) -> bool {
+        fn parse_linker_plugin_lto(slot: &mut LinkerPluginLto, v: Option<&str>) -> bool {
             if v.is_some() {
                 let mut bool_arg = None;
                 if parse_opt_bool(&mut bool_arg, v) {
                     *slot = if bool_arg.unwrap() {
-                        CrossLangLto::LinkerPluginAuto
+                        LinkerPluginLto::LinkerPluginAuto
                     } else {
-                        CrossLangLto::Disabled
+                        LinkerPluginLto::Disabled
                     };
                     return true
                 }
             }
 
             *slot = match v {
-                None => CrossLangLto::LinkerPluginAuto,
-                Some(path) => CrossLangLto::LinkerPlugin(PathBuf::from(path)),
+                None => LinkerPluginLto::LinkerPluginAuto,
+                Some(path) => LinkerPluginLto::LinkerPlugin(PathBuf::from(path)),
             };
             true
         }
@@ -1145,6 +1145,10 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "allow the linker to link its default libraries"),
     linker_flavor: Option<LinkerFlavor> = (None, parse_linker_flavor, [UNTRACKED],
                                            "Linker flavor"),
+    linker_plugin_lto: LinkerPluginLto = (LinkerPluginLto::Disabled,
+        parse_linker_plugin_lto, [TRACKED],
+        "generate build artifacts that are compatible with linker-based LTO."),
+
 }
 
 options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
@@ -1383,8 +1387,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "make the current crate share its generic instantiations"),
     chalk: bool = (false, parse_bool, [TRACKED],
         "enable the experimental Chalk-based trait solving engine"),
-    cross_lang_lto: CrossLangLto = (CrossLangLto::Disabled, parse_cross_lang_lto, [TRACKED],
-        "generate build artifacts that are compatible with linker-based LTO."),
     no_parallel_llvm: bool = (false, parse_bool, [UNTRACKED],
         "don't run LLVM in parallel (while keeping codegen-units and ThinLTO)"),
     no_leak_check: bool = (false, parse_bool, [UNTRACKED],
@@ -2440,7 +2442,7 @@ mod dep_tracking {
     use std::path::PathBuf;
     use std::collections::hash_map::DefaultHasher;
     use super::{CrateType, DebugInfo, ErrorOutputType, OptLevel, OutputTypes,
-                Passes, Sanitizer, LtoCli, CrossLangLto};
+                Passes, Sanitizer, LtoCli, LinkerPluginLto};
     use syntax::feature_gate::UnstableFeatures;
     use rustc_target::spec::{MergeFunctions, PanicStrategy, RelroLevel, TargetTriple};
     use syntax::edition::Edition;
@@ -2507,7 +2509,7 @@ mod dep_tracking {
     impl_dep_tracking_hash_via_hash!(Option<Sanitizer>);
     impl_dep_tracking_hash_via_hash!(TargetTriple);
     impl_dep_tracking_hash_via_hash!(Edition);
-    impl_dep_tracking_hash_via_hash!(CrossLangLto);
+    impl_dep_tracking_hash_via_hash!(LinkerPluginLto);
 
     impl_dep_tracking_hash_for_sortable_vec_of!(String);
     impl_dep_tracking_hash_for_sortable_vec_of!(PathBuf);
@@ -2572,7 +2574,7 @@ mod tests {
     use crate::lint;
     use crate::middle::cstore;
     use crate::session::config::{build_configuration, build_session_options_and_crate_config};
-    use crate::session::config::{LtoCli, CrossLangLto};
+    use crate::session::config::{LtoCli, LinkerPluginLto};
     use crate::session::build_session;
     use crate::session::search_paths::SearchPath;
     use std::collections::{BTreeMap, BTreeSet};
@@ -3105,6 +3107,10 @@ mod tests {
         opts = reference.clone();
         opts.cg.panic = Some(PanicStrategy::Abort);
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
+
+        opts = reference.clone();
+        opts.cg.linker_plugin_lto = LinkerPluginLto::LinkerPluginAuto;
+        assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
     }
 
     #[test]
@@ -3231,10 +3237,6 @@ mod tests {
         opts.debugging_opts.relro_level = Some(RelroLevel::Full);
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
 
-        opts = reference.clone();
-        opts.debugging_opts.cross_lang_lto = CrossLangLto::LinkerPluginAuto;
-        assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
-
         opts = reference.clone();
         opts.debugging_opts.merge_functions = Some(MergeFunctions::Disabled);
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());

src/librustc/session/mod.rs

@@ -1267,7 +1267,7 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
     // bitcode during ThinLTO. Therefore we disallow dynamic linking on MSVC
     // when compiling for LLD ThinLTO. This way we can validly just not generate
     // the `dllimport` attributes and `__imp_` symbols in that case.
-    if sess.opts.debugging_opts.cross_lang_lto.enabled() &&
+    if sess.opts.cg.linker_plugin_lto.enabled() &&
        sess.opts.cg.prefer_dynamic &&
        sess.target.target.options.is_like_msvc {
         sess.err("Linker plugin based LTO is not supported together with \

src/librustc_codegen_llvm/back/link.rs

@@ -857,7 +857,7 @@ fn link_args(cmd: &mut dyn Linker,
              codegen_results: &CodegenResults) {
 
     // Linker plugins should be specified early in the list of arguments
-    cmd.cross_lang_lto();
+    cmd.linker_plugin_lto();
 
     // The default library location, we need this to find the runtime.
     // The location of crates will be determined as needed.
@@ -1491,7 +1491,7 @@ fn are_upstream_rust_objects_already_included(sess: &Session) -> bool {
         Lto::Thin => {
             // If we defer LTO to the linker, we haven't run LTO ourselves, so
             // any upstream object files have not been copied yet.
-            !sess.opts.debugging_opts.cross_lang_lto.enabled()
+            !sess.opts.cg.linker_plugin_lto.enabled()
         }
         Lto::No |
         Lto::ThinLocal => false,

src/librustc_codegen_llvm/back/lto.rs

@@ -159,7 +159,7 @@ pub(crate) fn run_thin(cgcx: &CodegenContext<LlvmCodegenBackend>,
     let symbol_white_list = symbol_white_list.iter()
                                              .map(|c| c.as_ptr())
                                              .collect::<Vec<_>>();
-    if cgcx.opts.debugging_opts.cross_lang_lto.enabled() {
+    if cgcx.opts.cg.linker_plugin_lto.enabled() {
         unreachable!("We should never reach this case if the LTO step \
                       is deferred to the linker");
     }

src/librustc_codegen_llvm/back/write.rs

@@ -366,7 +366,7 @@ pub(crate) unsafe fn optimize(cgcx: &CodegenContext<LlvmCodegenBackend>,
                 let opt_level = config.opt_level.map(|x| to_llvm_opt_settings(x).0)
                     .unwrap_or(llvm::CodeGenOptLevel::None);
                 let prepare_for_thin_lto = cgcx.lto == Lto::Thin || cgcx.lto == Lto::ThinLocal ||
-                    (cgcx.lto != Lto::Fat && cgcx.opts.debugging_opts.cross_lang_lto.enabled());
+                    (cgcx.lto != Lto::Fat && cgcx.opts.cg.linker_plugin_lto.enabled());
                 with_llvm_pmb(llmod, &config, opt_level, prepare_for_thin_lto, &mut |b| {
                     llvm::LLVMPassManagerBuilderPopulateFunctionPassManager(b, fpm);
                     llvm::LLVMPassManagerBuilderPopulateModulePassManager(b, mpm);

src/librustc_codegen_llvm/consts.rs

@@ -275,12 +275,12 @@ impl CodegenCx<'ll, 'tcx> {
                 self.use_dll_storage_attrs && !self.tcx.is_foreign_item(def_id) &&
                 // ThinLTO can't handle this workaround in all cases, so we don't
                 // emit the attrs. Instead we make them unnecessary by disallowing
-                // dynamic linking when cross-language LTO is enabled.
-                !self.tcx.sess.opts.debugging_opts.cross_lang_lto.enabled();
+                // dynamic linking when linker plugin based LTO is enabled.
+                !self.tcx.sess.opts.cg.linker_plugin_lto.enabled();
 
             // If this assertion triggers, there's something wrong with commandline
             // argument validation.
-            debug_assert!(!(self.tcx.sess.opts.debugging_opts.cross_lang_lto.enabled() &&
+            debug_assert!(!(self.tcx.sess.opts.cg.linker_plugin_lto.enabled() &&
                             self.tcx.sess.target.target.options.is_like_msvc &&
                             self.tcx.sess.opts.cg.prefer_dynamic));