[ld.lld] understand -r, --relocatable

clang and gcc accept `-r`, which is a `--relocatable` argument of
ld.lld. This diff understands the flag and passes to the linker.

Impact: this makes the Go's `cmd/link` test suite pass[*]. To reproduce,
do in go/src directory (currently patched, hopefully not for long):

```
echo 'exec zig cc "$@"' > zig-cc; chmod a+x zig-cc
CC="./zig-cc" ./run.bash -run cmd/link
```

[*]: at the moment it requires golang/go#52815 to pass

Author: motiejus

src/Compilation.zig

@@ -732,6 +732,7 @@ pub const InitOptions = struct {
     use_clang: ?bool = null,
     use_stage1: ?bool = null,
     single_threaded: ?bool = null,
+    relocatable: bool = false,
     rdynamic: bool = false,
     strip: bool = false,
     function_sections: bool = false,
@@ -1615,6 +1616,7 @@ pub fn create(gpa: Allocator, options: InitOptions) !*Compilation {
             .gc_sections = options.linker_gc_sections,
             .eh_frame_hdr = link_eh_frame_hdr,
             .emit_relocs = options.link_emit_relocs,
+            .relocatable = options.relocatable,
             .rdynamic = options.rdynamic,
             .soname = options.soname,
             .version = options.version,
@@ -2255,7 +2257,7 @@ fn prepareWholeEmitSubPath(arena: Allocator, opt_emit: ?EmitLoc) error{OutOfMemo
 /// to remind the programmer to update multiple related pieces of code that
 /// are in different locations. Bump this number when adding or deleting
 /// anything from the link cache manifest.
-pub const link_hash_implementation_version = 2;
+pub const link_hash_implementation_version = 3;
 
 fn addNonIncrementalStuffToCacheManifest(comp: *Compilation, man: *Cache.Manifest) !void {
     const gpa = comp.gpa;
@@ -2265,7 +2267,7 @@ fn addNonIncrementalStuffToCacheManifest(comp: *Compilation, man: *Cache.Manifes
     defer arena_allocator.deinit();
     const arena = arena_allocator.allocator();
 
-    comptime assert(link_hash_implementation_version == 2);
+    comptime assert(link_hash_implementation_version == 3);
 
     if (comp.bin_file.options.module) |mod| {
         const main_zig_file = try mod.main_pkg.root_src_directory.join(arena, &[_][]const u8{
@@ -2324,6 +2326,7 @@ fn addNonIncrementalStuffToCacheManifest(comp: *Compilation, man: *Cache.Manifes
     man.hash.addOptional(comp.bin_file.options.gc_sections);
     man.hash.add(comp.bin_file.options.eh_frame_hdr);
     man.hash.add(comp.bin_file.options.emit_relocs);
+    man.hash.add(comp.bin_file.options.relocatable);
     man.hash.add(comp.bin_file.options.rdynamic);
     man.hash.addListOfBytes(comp.bin_file.options.lib_dirs);
     man.hash.addListOfBytes(comp.bin_file.options.rpath_list);

src/clang_options_data.zig

@@ -4623,7 +4623,14 @@ flagpd1("print-stats"),
 flagpd1("private_bundle"),
 flagpd1("pthread"),
 flagpd1("pthreads"),
-flagpd1("r"),
+.{
+    .name = "r",
+    .syntax = .flag,
+    .zig_equivalent = .r,
+    .pd1 = true,
+    .pd2 = false,
+    .psl = false,
+},
 .{
     .name = "rdynamic",
     .syntax = .flag,

src/link.zig

@@ -107,6 +107,7 @@ pub const Options = struct {
     function_sections: bool,
     eh_frame_hdr: bool,
     emit_relocs: bool,
+    relocatable: bool,
     rdynamic: bool,
     z_nodelete: bool,
     z_notext: bool,

src/link/Coff.zig

@@ -969,7 +969,7 @@ fn linkWithLLD(self: *Coff, comp: *Compilation, prog_node: *std.Progress.Node) !
         man = comp.cache_parent.obtain();
         self.base.releaseLock();
 
-        comptime assert(Compilation.link_hash_implementation_version == 2);
+        comptime assert(Compilation.link_hash_implementation_version == 3);
 
         for (self.base.options.objects) |obj| {
             _ = try man.addFile(obj.path, null);

src/link/Elf.zig

@@ -1294,7 +1294,7 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
         // We are about to obtain this lock, so here we give other processes a chance first.
         self.base.releaseLock();
 
-        comptime assert(Compilation.link_hash_implementation_version == 2);
+        comptime assert(Compilation.link_hash_implementation_version == 3);
 
         try man.addOptionalFile(self.base.options.linker_script);
         try man.addOptionalFile(self.base.options.version_script);
@@ -1316,6 +1316,7 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
         man.hash.add(gc_sections);
         man.hash.add(self.base.options.eh_frame_hdr);
         man.hash.add(self.base.options.emit_relocs);
+        man.hash.add(self.base.options.relocatable);
         man.hash.add(self.base.options.rdynamic);
         man.hash.addListOfBytes(self.base.options.lib_dirs);
         man.hash.addListOfBytes(self.base.options.rpath_list);
@@ -1472,6 +1473,10 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
             try argv.append("--emit-relocs");
         }
 
+        if (self.base.options.relocatable) {
+            try argv.append("--relocatable");
+        }
+
         if (self.base.options.rdynamic) {
             try argv.append("--export-dynamic");
         }

src/link/MachO.zig

@@ -527,7 +527,7 @@ pub fn flushModule(self: *MachO, comp: *Compilation, prog_node: *std.Progress.No
         // We are about to obtain this lock, so here we give other processes a chance first.
         self.base.releaseLock();
 
-        comptime assert(Compilation.link_hash_implementation_version == 2);
+        comptime assert(Compilation.link_hash_implementation_version == 3);
 
         for (self.base.options.objects) |obj| {
             _ = try man.addFile(obj.path, null);

src/link/Wasm.zig

@@ -2274,7 +2274,7 @@ fn linkWithLLD(self: *Wasm, comp: *Compilation, prog_node: *std.Progress.Node) !
         // We are about to obtain this lock, so here we give other processes a chance first.
         self.base.releaseLock();
 
-        comptime assert(Compilation.link_hash_implementation_version == 2);
+        comptime assert(Compilation.link_hash_implementation_version == 3);
 
         for (self.base.options.objects) |obj| {
             _ = try man.addFile(obj.path, null);
@@ -2290,6 +2290,7 @@ fn linkWithLLD(self: *Wasm, comp: *Compilation, prog_node: *std.Progress.Node) !
         man.hash.add(self.base.options.import_memory);
         man.hash.add(self.base.options.import_table);
         man.hash.add(self.base.options.export_table);
+        man.hash.add(self.base.options.relocatable);
         man.hash.addOptional(self.base.options.initial_memory);
         man.hash.addOptional(self.base.options.max_memory);
         man.hash.add(self.base.options.shared_memory);
@@ -2388,6 +2389,10 @@ fn linkWithLLD(self: *Wasm, comp: *Compilation, prog_node: *std.Progress.Node) !
             try argv.append("-s");
         }
 
+        if (self.base.options.relocatable) {
+            try argv.append("--relocatable");
+        }
+
         if (self.base.options.initial_memory) |initial_memory| {
             const arg = try std.fmt.allocPrint(arena, "--initial-memory={d}", .{initial_memory});
             try argv.append(arg);

src/main.zig

@@ -417,6 +417,7 @@ const usage_build_generic =
     \\  -fno-LLD                       Prevent using LLD as the linker
     \\  -fcompiler-rt                  Always include compiler-rt symbols in output
     \\  -fno-compiler-rt               Prevent including compiler-rt symbols in output
+    \\  -r                             Produce a relocatable object as output. This is also known as partial linking
     \\  -rdynamic                      Add all symbols to the dynamic symbol table
     \\  -rpath [path]                  Add directory to the runtime library search path
     \\  -feach-lib-rpath               Ensure adding rpath for each used dynamic library
@@ -633,6 +634,7 @@ fn buildOutputType(
     var want_valgrind: ?bool = null;
     var want_tsan: ?bool = null;
     var want_compiler_rt: ?bool = null;
+    var r: bool = false;
     var rdynamic: bool = false;
     var linker_script: ?[]const u8 = null;
     var version_script: ?[]const u8 = null;
@@ -1099,6 +1101,8 @@ fn buildOutputType(
                         use_stage1 = true;
                     } else if (mem.eql(u8, arg, "-fno-stage1")) {
                         use_stage1 = false;
+                    } else if (mem.eql(u8, arg, "-r")) {
+                        r = true;
                     } else if (mem.eql(u8, arg, "-rdynamic")) {
                         rdynamic = true;
                     } else if (mem.eql(u8, arg, "-fsoname")) {
@@ -1408,6 +1412,7 @@ fn buildOutputType(
                         link_mode = .Dynamic;
                         is_shared_lib = true;
                     },
+                    .r => r = true,
                     .rdynamic => rdynamic = true,
                     .wl => {
                         var split_it = mem.split(u8, it.only_arg, ",");
@@ -1602,6 +1607,8 @@ fn buildOutputType(
                         fatal("expected linker arg after '{s}'", .{arg});
                     }
                     target_dynamic_linker = linker_args.items[i];
+                } else if (mem.eql(u8, arg, "-r") or mem.eql(u8, arg, "--relocatable")) {
+                    r = true;
                 } else if (mem.eql(u8, arg, "-E") or
                     mem.eql(u8, arg, "--export-dynamic") or
                     mem.eql(u8, arg, "-export-dynamic"))
@@ -2671,6 +2678,7 @@ fn buildOutputType(
         .use_clang = use_clang,
         .use_stage1 = use_stage1,
         .hash_style = hash_style,
+        .relocatable = r,
         .rdynamic = rdynamic,
         .linker_script = linker_script,
         .version_script = version_script,
@@ -4401,6 +4409,7 @@ pub const ClangArgIterator = struct {
         nostdlib,
         nostdlib_cpp,
         shared,
+        r,
         rdynamic,
         wl,
         preprocess_only,

tools/update_clang_options.zig

@@ -142,6 +142,10 @@ const known_options = [_]KnownOpt{
         .name = "shared",
         .ident = "shared",
     },
+    .{
+        .name = "r",
+        .ident = "r",
+    },
     .{
         .name = "rdynamic",
         .ident = "rdynamic",