improvements to build-lib use case of WebAssembly

* build-exe does include the startup code that supplies _start for the
   wasm32-freestanding target. Previously this did not occur because
   of logic excluding "freestanding".
 * build-lib for wasm32-freestanding target gets linked by LLD. To avoid
   infinite recursion, compiler_rt and zig libc are built as objects
   rather than libraries.
   - no "lib" prefix and ".wasm" extension instead of ".a". Rather than
   build-lib foo.zig producing "libfoo.a", now it produces "foo.wasm".
 * go back to using `.o` extension for webassembly objects
 * zig libc only provides _start symbol for wasm when linking libc.

src/codegen.cpp

@@ -8548,7 +8548,7 @@ static void gen_root_source(CodeGen *g) {
     }
     report_errors_and_maybe_exit(g);
 
-    if (!g->is_test_build && g->zig_target->os != OsFreestanding &&
+    if (!g->is_test_build && (g->zig_target->os != OsFreestanding || target_is_wasm(g->zig_target)) &&
         g->zig_target->os != OsUefi &&
         !g->have_c_main && !g->have_winmain && !g->have_winmain_crt_startup &&
         ((g->have_pub_main && g->out_type == OutTypeObj) || g->out_type == OutTypeExe))

src/link.cpp

@@ -800,14 +800,6 @@ static Buf *build_a_raw(CodeGen *parent_gen, const char *aname, Buf *full_path,
     return &child_gen->output_file_path;
 }
 
-static Buf *build_a(CodeGen *parent_gen, const char *aname) {
-    Buf *source_basename = buf_sprintf("%s.zig", aname);
-    Buf *full_path = buf_alloc();
-    os_path_join(parent_gen->zig_std_special_dir, source_basename, full_path);
-
-    return build_a_raw(parent_gen, aname, full_path, OutTypeLib);
-}
-
 static Buf *build_compiler_rt(CodeGen *parent_gen, OutType child_out_type) {
     Buf *full_path = buf_alloc();
     os_path_join(parent_gen->zig_std_special_dir, buf_create_from_str("compiler_rt.zig"), full_path);
@@ -815,6 +807,13 @@ static Buf *build_compiler_rt(CodeGen *parent_gen, OutType child_out_type) {
     return build_a_raw(parent_gen, "compiler_rt", full_path, child_out_type);
 }
 
+static Buf *build_c(CodeGen *parent_gen, OutType child_out_type) {
+    Buf *full_path = buf_alloc();
+    os_path_join(parent_gen->zig_std_special_dir, buf_create_from_str("c.zig"), full_path);
+
+    return build_a_raw(parent_gen, "c", full_path, child_out_type);
+}
+
 static const char *get_darwin_arch_string(const ZigTarget *t) {
     switch (t->arch) {
         case ZigLLVM_aarch64:
@@ -1003,7 +1002,7 @@ static void construct_linker_job_elf(LinkJob *lj) {
 
     if (!g->is_dummy_so && (g->out_type == OutTypeExe || is_dyn_lib)) {
         if (g->libc_link_lib == nullptr) {
-            Buf *libc_a_path = build_a(g, "c");
+            Buf *libc_a_path = build_c(g, OutTypeLib);
             lj->args.append(buf_ptr(libc_a_path));
         }
 
@@ -1118,10 +1117,10 @@ static void construct_linker_job_wasm(LinkJob *lj) {
     }
 
     if (g->out_type != OutTypeObj) {
-        Buf *libc_a_path = build_a(g, "c");
-        lj->args.append(buf_ptr(libc_a_path));
+        Buf *libc_o_path = build_c(g, OutTypeObj);
+        lj->args.append(buf_ptr(libc_o_path));
 
-        Buf *compiler_rt_o_path = build_compiler_rt(g, OutTypeLib);
+        Buf *compiler_rt_o_path = build_compiler_rt(g, OutTypeObj);
         lj->args.append(buf_ptr(compiler_rt_o_path));
     }
 }
@@ -1360,7 +1359,7 @@ static void construct_linker_job_coff(LinkJob *lj) {
 
     if (g->out_type == OutTypeExe || (g->out_type == OutTypeLib && g->is_dynamic)) {
         if (g->libc_link_lib == nullptr && !g->is_dummy_so) {
-            Buf *libc_a_path = build_a(g, "c");
+            Buf *libc_a_path = build_c(g, OutTypeLib);
             lj->args.append(buf_ptr(libc_a_path));
         }
 
@@ -1687,7 +1686,7 @@ void codegen_link(CodeGen *g) {
         lj.args.append("-r");
     }
 
-    if (g->out_type == OutTypeLib && !g->is_dynamic) {
+    if (g->out_type == OutTypeLib && !g->is_dynamic && !target_is_wasm(g->zig_target)) {
         ZigList<const char *> file_names = {};
         for (size_t i = 0; i < g->link_objects.length; i += 1) {
             file_names.append(buf_ptr(g->link_objects.at(i)));

src/target.cpp

@@ -947,8 +947,6 @@ bool target_allows_addr_zero(const ZigTarget *target) {
 const char *target_o_file_ext(const ZigTarget *target) {
     if (target->abi == ZigLLVM_MSVC || target->os == OsWindows || target->os == OsUefi) {
         return ".obj";
-    } else if (target_is_wasm(target)) {
-        return ".wasm";
     } else {
         return ".o";
     }
@@ -975,7 +973,7 @@ const char *target_exe_file_ext(const ZigTarget *target) {
 }
 
 const char *target_lib_file_prefix(const ZigTarget *target) {
-    if (target->os == OsWindows || target->os == OsUefi) {
+    if (target->os == OsWindows || target->os == OsUefi || target_is_wasm(target)) {
         return "";
     } else {
         return "lib";
@@ -985,6 +983,9 @@ const char *target_lib_file_prefix(const ZigTarget *target) {
 const char *target_lib_file_ext(const ZigTarget *target, bool is_static,
         size_t version_major, size_t version_minor, size_t version_patch)
 {
+    if (target_is_wasm(target)) {
+        return ".wasm";
+    }
     if (target->os == OsWindows || target->os == OsUefi) {
         if (is_static) {
             return ".lib";

std/special/bootstrap.zig

@@ -25,21 +25,25 @@ nakedcc fn _start() noreturn {
     }
 
     switch (builtin.arch) {
-        builtin.Arch.x86_64 => {
+        .x86_64 => {
             argc_ptr = asm ("lea (%%rsp), %[argc]"
                 : [argc] "=r" (-> [*]usize)
             );
         },
-        builtin.Arch.i386 => {
+        .i386 => {
             argc_ptr = asm ("lea (%%esp), %[argc]"
                 : [argc] "=r" (-> [*]usize)
             );
         },
-        builtin.Arch.aarch64, builtin.Arch.aarch64_be => {
+        .aarch64, .aarch64_be => {
             argc_ptr = asm ("mov %[argc], sp"
                 : [argc] "=r" (-> [*]usize)
             );
         },
+        .wasm32, .wasm64 => {
+            _ = callMain();
+            while (true) {}
+        },
         else => @compileError("unsupported arch"),
     }
     // If LLVM inlines stack variables into _start, they will overwrite

std/special/c.zig

@@ -11,7 +11,7 @@ const maxInt = std.math.maxInt;
 const is_wasm = switch (builtin.arch) { .wasm32, .wasm64 => true, else => false};
 const is_freestanding = switch (builtin.os) { .freestanding => true, else => false };
 comptime {
-    if (is_freestanding and is_wasm) {
+    if (is_freestanding and is_wasm and builtin.link_libc) {
         @export("_start", wasm_start, .Strong);
     }
 }