Add documentation about -Clinker-plugin-lto to rustc book.

Author: michaelwoerister

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.