Allow LTO for dylibs

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -32,8 +32,8 @@ pub const THIN_LTO_KEYS_INCR_COMP_FILE_NAME: &str = "thin-lto-past-keys.bin";
 
 pub fn crate_type_allows_lto(crate_type: CrateType) -> bool {
     match crate_type {
-        CrateType::Executable | CrateType::Staticlib | CrateType::Cdylib => true,
+        CrateType::Executable | CrateType::Dylib | CrateType::Staticlib | CrateType::Cdylib => true,
-        CrateType::Dylib | CrateType::Rlib | CrateType::ProcMacro => false,
+        CrateType::Rlib | CrateType::ProcMacro => false,
     }
 }
 
@@ -73,17 +73,6 @@ fn prepare_lto(
     // with either fat or thin LTO
     let mut upstream_modules = Vec::new();
     if cgcx.lto != Lto::ThinLocal {
-        if cgcx.opts.cg.prefer_dynamic {
-            diag_handler
-                .struct_err("cannot prefer dynamic linking when performing LTO")
-                .note(
-                    "only 'staticlib', 'bin', and 'cdylib' outputs are \
-                               supported with LTO",
-                )
-                .emit();
-            return Err(FatalError);
-        }
-
         // Make sure we actually can run LTO
         for crate_type in cgcx.crate_types.iter() {
             if !crate_type_allows_lto(*crate_type) {
@@ -92,6 +81,8 @@ fn prepare_lto(
                                             static library outputs",
                 );
                 return Err(e);
+            } else if *crate_type == CrateType::Dylib {
+                diag_handler.warn("LTO with dylibs may not be as effective");
             }
         }
 

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -11,7 +11,7 @@ use rustc_metadata::find_native_static_library;
 use rustc_metadata::fs::{emit_metadata, METADATA_FILENAME};
 use rustc_middle::middle::dependency_format::Linkage;
 use rustc_middle::middle::exported_symbols::SymbolExportKind;
-use rustc_session::config::{self, CFGuard, CrateType, DebugInfo, LdImpl, Strip};
+use rustc_session::config::{self, CFGuard, CrateType, DebugInfo, LdImpl, Lto, Strip};
 use rustc_session::config::{OutputFilenames, OutputType, PrintRequest, SplitDwarfKind};
 use rustc_session::cstore::DllImport;
 use rustc_session::output::{check_file_is_writeable, invalid_output_for_target, out_filename};
@@ -49,6 +49,7 @@ use std::ops::Deref;
 use std::path::{Path, PathBuf};
 use std::process::{ExitStatus, Output, Stdio};
 use std::{env, fmt, fs, io, mem, str};
+use itertools::Itertools;
 
 pub fn ensure_removed(diag_handler: &Handler, path: &Path) {
     if let Err(e) = fs::remove_file(path) {
@@ -208,11 +209,24 @@ pub fn link_binary<'a>(
 }
 
 pub fn each_linked_rlib(
+    sess: &Session,
     info: &CrateInfo,
     f: &mut dyn FnMut(CrateNum, &Path),
 ) -> Result<(), errors::LinkRlibError> {
     let crates = info.used_crates.iter();
     let mut fmts = None;
+
+    let lto_active = matches!(sess.lto(), Lto::Fat | Lto::Thin);
+    if lto_active {
+        for combination in info.dependency_formats.iter().combinations(2) {
+            let (ty1, list1) = combination[0];
+            let (ty2, list2) = combination[1];
+            if list1 != list2 {
+                return Err(format!("{ty1:?} and {ty2:?} do not have equivalent dependency formats (`{list1:?}` vs `{list2:?}`)"));
+            }
+        }
+    }
+
     for (ty, list) in info.dependency_formats.iter() {
         match ty {
             CrateType::Executable
@@ -222,6 +236,10 @@ pub fn each_linked_rlib(
                 fmts = Some(list);
                 break;
             }
+            CrateType::Dylib if lto_active => {
+                fmts = Some(list);
+                break;
+            }
             _ => {}
         }
     }
@@ -490,7 +508,7 @@ fn link_staticlib<'a>(
     )?;
     let mut all_native_libs = vec![];
 
-    let res = each_linked_rlib(&codegen_results.crate_info, &mut |cnum, path| {
+    let res = each_linked_rlib(sess, &codegen_results.crate_info, &mut |cnum, path| {
         let name = codegen_results.crate_info.crate_name[&cnum];
         let native_libs = &codegen_results.crate_info.native_libraries[&cnum];
 

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -999,6 +999,14 @@ fn start_executing_work<B: ExtraBackendMethods>(
     let coordinator_send = tx_to_llvm_workers;
     let sess = tcx.sess;
 
+    let mut each_linked_rlib_for_lto = Vec::new();
+    drop(link::each_linked_rlib(sess, crate_info, &mut |cnum, path| {
+        if link::ignored_for_lto(sess, crate_info, cnum) {
+            return;
+        }
+        each_linked_rlib_for_lto.push((cnum, path.to_path_buf()));
+    }));
+
     // Compute the set of symbols we need to retain when doing LTO (if we need to)
     let exported_symbols = {
         let mut exported_symbols = FxHashMap::default();
@@ -1020,7 +1028,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
             }
             Lto::Fat | Lto::Thin => {
                 exported_symbols.insert(LOCAL_CRATE, copy_symbols(LOCAL_CRATE));
-                for &cnum in tcx.crates(()).iter() {
+                for &(cnum, ref _path) in &each_linked_rlib_for_lto {
                     exported_symbols.insert(cnum, copy_symbols(cnum));
                 }
                 Some(Arc::new(exported_symbols))
@@ -1040,14 +1048,6 @@ fn start_executing_work<B: ExtraBackendMethods>(
         })
         .expect("failed to spawn helper thread");
 
-    let mut each_linked_rlib_for_lto = Vec::new();
-    drop(link::each_linked_rlib(crate_info, &mut |cnum, path| {
-        if link::ignored_for_lto(sess, crate_info, cnum) {
-            return;
-        }
-        each_linked_rlib_for_lto.push((cnum, path.to_path_buf()));
-    }));
-
     let ol =
         if tcx.sess.opts.unstable_opts.no_codegen || !tcx.sess.opts.output_types.should_codegen() {
             // If we know that we won’t be doing codegen, create target machines without optimisation.