[ELF] Automatically fall back to ld.bfd or ld.lld if LTO is in use

This is very hacky but highly practical, so I couldn't resist to not
implement this. We should support LTO natively in the future. In the
meantime, this feature should work as a poor-man's replacement.

Fixes rui314#242

elf/main.cc

@@ -89,8 +89,10 @@ void read_file(Context<E> &ctx, MappedFile<Context<E>> *mf) {
     parse_linker_script(ctx, mf);
     return;
   case FileType::LLVM_BITCODE:
-    Fatal(ctx) << mf->name << ": looks like this is an LLVM bitcode, "
-               << "but mold does not support LTO";
+    Warn(ctx) << mf->name << ": looks like this is an LLVM bitcode, "
+              << "but mold does not support LTO";
+    ctx.llvm_lto = true;
+    return;
   default:
     Fatal(ctx) << mf->name << ": unknown file type: " << type;
   }
@@ -436,6 +438,23 @@ static int elf_main(int argc, char **argv) {
   // included to the final output.
   resolve_symbols(ctx);
 
+  // We currently do not natively support LTO.
+  // If LLVM LTO is in use, fallback to the lld linker by invoking
+  // it with the exact same command line arguments as we got.
+  if (ctx.llvm_lto) {
+    Warn(ctx) << "LLVM LTO is detected, so falling back to ld.lld";
+    argv[0] = (char *)"ld.lld";
+    execvp("ld.lld", argv);
+    Fatal(ctx) << "execvp failed: ld.lld: " << errno_string();
+  }
+
+  // Do the same for GCC LTO.
+  if (ctx.gcc_lto) {
+    Warn(ctx) << "GCC LTO is detected, so falling back to ld.bfd";
+    execvp("ld.bfd", argv);
+    Fatal(ctx) << "execvp failed: ld.bfd: " << errno_string();
+  }
+
   // Remove redundant comdat sections (e.g. duplicate inline functions).
   eliminate_comdats(ctx);
 

elf/mold.h

@@ -1305,6 +1305,9 @@ struct Context {
   tbb::task_group tg;
 
   bool has_error = false;
+  bool gcc_lto = false;
+  bool llvm_lto = false;
+
   i64 page_size = -1;
 
   // Symbol table

elf/passes.cc

@@ -167,9 +167,11 @@ void resolve_symbols(Context<E> &ctx) {
     file->resolve_common_symbols(ctx);
   });
 
-  if (Symbol<E> *sym = get_symbol(ctx, "__gnu_lto_slim"); sym->file)
-    Fatal(ctx) << *sym->file << ": looks like this file contains a GCC "
-               << "intermediate code, but mold does not support LTO";
+  if (Symbol<E> *sym = get_symbol(ctx, "__gnu_lto_slim"); sym->file) {
+    Warn(ctx) << *sym->file << ": looks like this file contains a GCC "
+              << "intermediate code, but mold does not support LTO";
+    ctx.gcc_lto = true;
+  }
 }
 
 template <typename E>

test/elf/lto-gcc-error.sh → test/elf/lto-gcc-fallback.sh

@@ -9,10 +9,14 @@ t="$(pwd)/out/test/elf/$testname"
 mkdir -p "$t"
 
 cat <<EOF | gcc -flto -c -o "$t"/a.o -xc -
-int main() {}
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
 EOF
 
-! clang -fuse-ld="$mold" -o "$t"/exe "$t"/a.o &> "$t"/log
-grep -q '.*/a.o: .*mold does not support LTO' "$t"/log
+gcc -B"$(pwd)" -o "$t"/exe "$t"/a.o >& "$t"/log
+grep -q 'falling back' "$t"/log
+"$t"/exe | grep -q 'Hello world'
 
 echo OK

test/elf/lto-llvm-error.sh → test/elf/lto-llvm-fallback.sh

@@ -9,10 +9,14 @@ t="$(pwd)/out/test/elf/$testname"
 mkdir -p "$t"
 
 cat <<EOF | clang -flto -c -o "$t"/a.o -xc -
-int main() {}
+#include <stdio.h>
+int main() {
+  printf("Hello world\n");
+}
 EOF
 
-! clang -fuse-ld="$mold" -o "$t"/exe "$t"/a.o &> "$t"/log
-grep -q '.*/a.o: .*mold does not support LTO' "$t"/log
+clang -fuse-ld="$mold" -o "$t"/exe "$t"/a.o &> "$t"/log
+grep -q 'falling back' "$t"/log
+"$t"/exe | grep -q 'Hello world'
 
 echo OK