Fix linker-plugin-lto only doing thin lto

When rust provides LLVM bitcode files to lld and the bitcode contains
function summaries as used for thin lto, lld defaults to using thin lto.
This prevents some optimizations that are only applied for fat lto.

We solve this by not creating function summaries when fat lto is
enabled. The bitcode for the module is just directly written out.

An alternative solution would be to set the `ThinLTO=0` module flag to
signal lld to do fat lto.
The code in clang that sets this flag is here:
https://github.com/llvm/llvm-project/blob/560149b5e3c891c64899e9912e29467a69dc3a4c/clang/lib/CodeGen/BackendUtil.cpp#L1150
The code in LLVM that queries the flag and defaults to thin lto if not
set is here:
https://github.com/llvm/llvm-project/blob/e258bca9505f35e0a22cb213a305eea9b76d11ea/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4441-L4446

Author: Flakebi

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -827,7 +827,7 @@ pub(crate) unsafe fn codegen(
                         "LLVM_module_codegen_make_bitcode",
                         &*module.name,
                     );
-                    ThinBuffer::new(llmod, config.emit_thin_lto, false)
+                    ThinBuffer::new(llmod, cgcx.lto != Lto::Fat, false)
                 };
                 let data = thin.data();
                 let _timer = cgcx

src/tools/run-make-support/src/external_deps/llvm.rs

@@ -60,6 +60,12 @@ pub fn llvm_pdbutil() -> LlvmPdbutil {
     LlvmPdbutil::new()
 }
 
+/// Construct a new `llvm-as` invocation. This assumes that `llvm-as` is available
+/// at `$LLVM_BIN_DIR/llvm-as`.
+pub fn llvm_as() -> LlvmAs {
+    LlvmAs::new()
+}
+
 /// Construct a new `llvm-dis` invocation. This assumes that `llvm-dis` is available
 /// at `$LLVM_BIN_DIR/llvm-dis`.
 pub fn llvm_dis() -> LlvmDis {
@@ -135,6 +141,13 @@ pub struct LlvmPdbutil {
     cmd: Command,
 }
 
+/// A `llvm-as` invocation builder.
+#[derive(Debug)]
+#[must_use]
+pub struct LlvmAs {
+    cmd: Command,
+}
+
 /// A `llvm-dis` invocation builder.
 #[derive(Debug)]
 #[must_use]
@@ -158,6 +171,7 @@ crate::macros::impl_common_helpers!(LlvmNm);
 crate::macros::impl_common_helpers!(LlvmBcanalyzer);
 crate::macros::impl_common_helpers!(LlvmDwarfdump);
 crate::macros::impl_common_helpers!(LlvmPdbutil);
+crate::macros::impl_common_helpers!(LlvmAs);
 crate::macros::impl_common_helpers!(LlvmDis);
 crate::macros::impl_common_helpers!(LlvmObjcopy);
 
@@ -441,6 +455,22 @@ impl LlvmObjcopy {
     }
 }
 
+impl LlvmAs {
+    /// Construct a new `llvm-as` invocation. This assumes that `llvm-as` is available
+    /// at `$LLVM_BIN_DIR/llvm-as`.
+    pub fn new() -> Self {
+        let llvm_as = llvm_bin_dir().join("llvm-as");
+        let cmd = Command::new(llvm_as);
+        Self { cmd }
+    }
+
+    /// Provide an input file.
+    pub fn input<P: AsRef<Path>>(&mut self, path: P) -> &mut Self {
+        self.cmd.arg(path.as_ref());
+        self
+    }
+}
+
 impl LlvmDis {
     /// Construct a new `llvm-dis` invocation. This assumes that `llvm-dis` is available
     /// at `$LLVM_BIN_DIR/llvm-dis`.

src/tools/run-make-support/src/external_deps/rustc.rs

@@ -171,6 +171,12 @@ impl Rustc {
         self
     }
 
+    /// This flag enables LTO in the specified form.
+    pub fn lto(&mut self, option: &str) -> &mut Self {
+        self.cmd.arg(format!("-Clto={option}"));
+        self
+    }
+
     /// This flag defers LTO optimizations to the linker.
     pub fn linker_plugin_lto(&mut self, option: &str) -> &mut Self {
         self.cmd.arg(format!("-Clinker-plugin-lto={option}"));

src/tools/run-make-support/src/lib.rs

@@ -63,7 +63,7 @@ pub use cargo::cargo;
 pub use clang::{clang, Clang};
 pub use htmldocck::htmldocck;
 pub use llvm::{
-    llvm_ar, llvm_bcanalyzer, llvm_dis, llvm_dwarfdump, llvm_filecheck, llvm_nm, llvm_objcopy,
+    llvm_ar, llvm_bcanalyzer, llvm_as, llvm_dis, llvm_dwarfdump, llvm_filecheck, llvm_nm, llvm_objcopy,
     llvm_objdump, llvm_profdata, llvm_readobj, LlvmAr, LlvmBcanalyzer, LlvmDis, LlvmDwarfdump,
     LlvmFilecheck, LlvmNm, LlvmObjcopy, LlvmObjdump, LlvmProfdata, LlvmReadobj,
 };

tests/run-make/cross-lang-lto-clang/rmake.rs

@@ -28,15 +28,24 @@ static C_NEVER_INLINED_PATTERN: &'static str = "bl.*<c_never_inlined>";
 static C_NEVER_INLINED_PATTERN: &'static str = "call.*c_never_inlined";
 
 fn main() {
+    test_lto(false);
+    test_lto(true);
+}
+
+fn test_lto(fat_lto: bool) {
+    let lto = if fat_lto { "fat" } else { "thin" };
+    let clang_lto = if fat_lto { "full" } else { "thin" };
+
     rustc()
+        .lto(lto)
         .linker_plugin_lto("on")
         .output(static_lib_name("rustlib-xlto"))
         .opt_level("2")
         .codegen_units(1)
         .input("rustlib.rs")
         .run();
     clang()
-        .lto("thin")
+        .lto(clang_lto)
         .use_ld("lld")
         .arg("-lrustlib-xlto")
         .out_exe("cmain")
@@ -57,9 +66,10 @@ fn main() {
         .input("cmain")
         .run()
         .assert_stdout_contains_regex(RUST_NEVER_INLINED_PATTERN);
-    clang().input("clib.c").lto("thin").arg("-c").out_exe("clib.o").arg("-O2").run();
+    clang().input("clib.c").lto(clang_lto).arg("-c").out_exe("clib.o").arg("-O2").run();
     llvm_ar().obj_to_ar().output_input(static_lib_name("xyz"), "clib.o").run();
     rustc()
+        .lto(lto)
         .linker_plugin_lto("on")
         .opt_level("2")
         .linker(&env_var("CLANG"))
@@ -72,9 +82,12 @@ fn main() {
         .input("rsmain")
         .run()
         .assert_stdout_not_contains_regex(C_ALWAYS_INLINED_PATTERN);
-    llvm_objdump()
-        .disassemble()
-        .input("rsmain")
-        .run()
-        .assert_stdout_contains_regex(C_NEVER_INLINED_PATTERN);
+
+    let dump = llvm_objdump().disassemble().input("rsmain").run();
+    if !fat_lto {
+        dump.assert_stdout_contains_regex(C_NEVER_INLINED_PATTERN);
+    } else {
+        // fat lto inlines this anyway
+        dump.assert_stdout_not_contains_regex(C_NEVER_INLINED_PATTERN);
+    }
 }

tests/run-make/fat-then-thin-lto/lib.rs

@@ -0,0 +1,8 @@
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "rlib"]
+
+#[lang = "sized"]
+trait Sized {}
+
+pub fn foo() {}

tests/run-make/fat-then-thin-lto/main.rs

@@ -0,0 +1,11 @@
+#![allow(internal_features)]
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "cdylib"]
+
+extern crate lib;
+
+#[unsafe(no_mangle)]
+pub fn bar() {
+    lib::foo();
+}

tests/run-make/fat-then-thin-lto/rmake.rs

@@ -0,0 +1,25 @@
+// Compile a library with lto=fat, then compile a binary with lto=thin
+// and check that lto is applied with the library.
+// The goal is to mimic the standard library being build with lto=fat
+// and allowing users to build with lto=thin.
+
+//@ only-x86_64-unknown-linux-gnu
+
+use run_make_support::{dynamic_lib_name, llvm_objdump, rustc};
+
+fn main() {
+    rustc().input("lib.rs").opt_level("3").lto("fat").run();
+    rustc().input("main.rs").panic("abort").opt_level("3").lto("thin").run();
+
+    llvm_objdump()
+        .input(dynamic_lib_name("main"))
+        .arg("--disassemble-symbols=bar")
+        .run()
+        // The called function should be inlined.
+        // Check that we have a ret (to detect tail
+        // calls with a jmp) and no call.
+        .assert_stdout_contains("bar")
+        .assert_stdout_contains("ret")
+        .assert_stdout_not_contains("foo")
+        .assert_stdout_not_contains("call");
+}

tests/run-make/issue-84395-lto-embed-bitcode/rmake.rs

@@ -5,7 +5,7 @@
 //! `-lto-embed-bitcode=optimized` to lld when linking rust code via `linker-plugin-lto` doesn't
 //! produce the expected result.
 //!
-//! See PR <https://github.com/rust-lang/rust/pull/98162> which initially introduced this test.
+//! -Clto=fat needs to explicitly requested to produce the correct, raw bitcode format for lld.
 
 //@ needs-force-clang-based-tests
 
@@ -14,11 +14,11 @@ use run_make_support::{env_var, llvm_dis, llvm_objcopy, rustc};
 fn main() {
     rustc()
         .input("test.rs")
-        .arg("-Clink-arg=-fuse-ld=lld")
-        .arg("-Clinker-plugin-lto")
+        .lto("fat")
+        .linker_plugin_lto("on")
+        .link_arg("-fuse-ld=lld")
         .arg(format!("-Clinker={}", env_var("CLANG")))
-        .arg("-Clink-arg=-Wl,--plugin-opt=-lto-embed-bitcode=optimized")
-        .arg("-Zemit-thin-lto=no")
+        .link_arg("-Wl,--plugin-opt=-lto-embed-bitcode=optimized")
         .run();
 
     llvm_objcopy().dump_section(".llvmbc", "test.bc").arg("test").run();

tests/run-make/linker-plugin-lto-fat/ir.ll

@@ -0,0 +1,6 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @ir_callee() {
+  ret void
+}

tests/run-make/linker-plugin-lto-fat/main.rs

@@ -0,0 +1,17 @@
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "cdylib"]
+
+#[lang = "sized"]
+trait Sized {}
+
+extern "C" {
+    fn ir_callee();
+}
+
+#[no_mangle]
+extern "C" fn rs_foo() {
+    unsafe {
+        ir_callee();
+    }
+}

tests/run-make/linker-plugin-lto-fat/rmake.rs

@@ -0,0 +1,32 @@
+// Check that -C lto=fat with -C linker-plugin-lto actually works and can inline functions.
+// A library is created from LLVM IR, defining a single function. Then a dylib is compiled,
+// linking to the library and calling the function from the library.
+// The function from the library should end up inlined and disappear from the output.
+
+//@ only-x86_64-unknown-linux-gnu
+//@ needs-rust-lld
+
+use run_make_support::{dynamic_lib_name, llvm_as, llvm_objdump, rustc};
+
+fn main() {
+    llvm_as().input("ir.ll").run();
+    rustc()
+        .input("main.rs")
+        .opt_level("3")
+        .lto("fat")
+        .linker_plugin_lto("on")
+        .link_arg("ir.bc")
+        .arg("-Zlinker-features=+lld")
+        .run();
+
+    llvm_objdump()
+        .input(dynamic_lib_name("main"))
+        .arg("--disassemble-symbols=rs_foo")
+        .run()
+        // The called function should be inlined.
+        // Check that we have a ret (to detect tail
+        // calls with a jmp) and no call.
+        .assert_stdout_contains("foo")
+        .assert_stdout_contains("ret")
+        .assert_stdout_not_contains("call");
+}