stage2: WASM LLD linking

BRANCH_TODO

@@ -1,5 +1,4 @@
  * MachO LLD linking
- * WASM LLD linking
  * audit the CLI options for stage2
  * audit the base cache hash
  * On operating systems that support it, do an execve for `zig test` and `zig run` rather than child process.

lib/std/target.zig

@@ -1486,7 +1486,7 @@ pub const Target = struct {
 
     /// Return whether or not the given host target is capable of executing natively executables
     /// of the other target.
-    pub fn canExecBinariesOf(host_target: std.Target, binary_target: std.Target) bool {
+    pub fn canExecBinariesOf(host_target: Target, binary_target: Target) bool {
         if (host_target.os.tag != binary_target.os.tag)
             return false;
 

src/Compilation.zig

@@ -2418,7 +2418,7 @@ fn buildStaticLibFromZig(comp: *Compilation, src_basename: []const u8, out: *?CR
     const bin_basename = try std.zig.binNameAlloc(comp.gpa, .{
         .root_name = root_name,
         .target = target,
-        .output_mode = .Lib,
+        .output_mode = .Obj,
     });
     defer comp.gpa.free(bin_basename);
 
@@ -2441,7 +2441,7 @@ fn buildStaticLibFromZig(comp: *Compilation, src_basename: []const u8, out: *?CR
         .target = target,
         .root_name = root_name,
         .root_pkg = &root_pkg,
-        .output_mode = .Lib,
+        .output_mode = .Obj,
         .rand = comp.rand,
         .libc_installation = comp.bin_file.options.libc_installation,
         .emit_bin = emit_bin,

src/link/Wasm.zig

@@ -1,10 +1,12 @@
 const Wasm = @This();
 
 const std = @import("std");
+const mem = std.mem;
 const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;
 const fs = std.fs;
 const leb = std.debug.leb;
+const log = std.log.scoped(.link);
 
 const Module = @import("../Module.zig");
 const Compilation = @import("../Compilation.zig");
@@ -12,6 +14,7 @@ const codegen = @import("../codegen/wasm.zig");
 const link = @import("../link.zig");
 const trace = @import("../tracy.zig").trace;
 const build_options = @import("build_options");
+const Cache = @import("../Cache.zig");
 
 /// Various magic numbers defined by the wasm spec
 const spec = struct {
@@ -137,7 +140,7 @@ pub fn freeDecl(self: *Wasm, decl: *Module.Decl) void {
 
 pub fn flush(self: *Wasm, comp: *Compilation) !void {
     if (build_options.have_llvm and self.base.options.use_lld) {
-        return error.WasmLinkingWithLLDUnimplemented;
+        return self.linkWithLLD(comp);
     } else {
         return self.flushModule(comp);
     }
@@ -248,6 +251,211 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
     }
 }
 
+fn linkWithLLD(self: *Wasm, comp: *Compilation) !void {
+    const tracy = trace(@src());
+    defer tracy.end();
+
+    var arena_allocator = std.heap.ArenaAllocator.init(self.base.allocator);
+    defer arena_allocator.deinit();
+    const arena = &arena_allocator.allocator;
+
+    const directory = self.base.options.emit.?.directory; // Just an alias to make it shorter to type.
+
+    // If there is no Zig code to compile, then we should skip flushing the output file because it
+    // will not be part of the linker line anyway.
+    const module_obj_path: ?[]const u8 = if (self.base.options.module) |module| blk: {
+        const use_stage1 = build_options.is_stage1 and self.base.options.use_llvm;
+        if (use_stage1) {
+            const obj_basename = try std.zig.binNameAlloc(arena, .{
+                .root_name = self.base.options.root_name,
+                .target = self.base.options.target,
+                .output_mode = .Obj,
+            });
+            const o_directory = self.base.options.module.?.zig_cache_artifact_directory;
+            const full_obj_path = try o_directory.join(arena, &[_][]const u8{obj_basename});
+            break :blk full_obj_path;
+        }
+
+        try self.flushModule(comp);
+        const obj_basename = self.base.intermediary_basename.?;
+        const full_obj_path = try directory.join(arena, &[_][]const u8{obj_basename});
+        break :blk full_obj_path;
+    } else null;
+
+    const target = self.base.options.target;
+
+    const id_symlink_basename = "lld.id";
+
+    var man: Cache.Manifest = undefined;
+    defer if (!self.base.options.disable_lld_caching) man.deinit();
+
+    var digest: [Cache.hex_digest_len]u8 = undefined;
+
+    if (!self.base.options.disable_lld_caching) {
+        man = comp.cache_parent.obtain();
+
+        // We are about to obtain this lock, so here we give other processes a chance first.
+        self.base.releaseLock();
+
+        try man.addListOfFiles(self.base.options.objects);
+        for (comp.c_object_table.items()) |entry| {
+            _ = try man.addFile(entry.key.status.success.object_path, null);
+        }
+        try man.addOptionalFile(module_obj_path);
+        man.hash.addOptional(self.base.options.stack_size_override);
+        man.hash.addListOfBytes(self.base.options.extra_lld_args);
+
+        // We don't actually care whether it's a cache hit or miss; we just need the digest and the lock.
+        _ = try man.hit();
+        digest = man.final();
+
+        var prev_digest_buf: [digest.len]u8 = undefined;
+        const prev_digest: []u8 = directory.handle.readLink(id_symlink_basename, &prev_digest_buf) catch |err| blk: {
+            log.debug("WASM LLD new_digest={} readlink error: {}", .{ digest, @errorName(err) });
+            // Handle this as a cache miss.
+            break :blk prev_digest_buf[0..0];
+        };
+        if (mem.eql(u8, prev_digest, &digest)) {
+            log.debug("WASM LLD digest={} match - skipping invocation", .{digest});
+            // Hot diggity dog! The output binary is already there.
+            self.base.lock = man.toOwnedLock();
+            return;
+        }
+        log.debug("WASM LLD prev_digest={} new_digest={}", .{ prev_digest, digest });
+
+        // We are about to change the output file to be different, so we invalidate the build hash now.
+        directory.handle.deleteFile(id_symlink_basename) catch |err| switch (err) {
+            error.FileNotFound => {},
+            else => |e| return e,
+        };
+    }
+
+    const is_obj = self.base.options.output_mode == .Obj;
+
+    // Create an LLD command line and invoke it.
+    var argv = std.ArrayList([]const u8).init(self.base.allocator);
+    defer argv.deinit();
+    // Even though we're calling LLD as a library it thinks the first argument is its own exe name.
+    try argv.append("lld");
+    if (is_obj) {
+        try argv.append("-r");
+    }
+
+    try argv.append("-error-limit=0");
+
+    if (self.base.options.output_mode == .Exe) {
+        // Increase the default stack size to a more reasonable value of 1MB instead of
+        // the default of 1 Wasm page being 64KB, unless overriden by the user.
+        try argv.append("-z");
+        const stack_size = self.base.options.stack_size_override orelse 1048576;
+        const arg = try std.fmt.allocPrint(arena, "stack-size={d}", .{stack_size});
+        try argv.append(arg);
+
+        // Put stack before globals so that stack overflow results in segfault immediately
+        // before corrupting globals. See https://github.com/ziglang/zig/issues/4496
+        try argv.append("--stack-first");
+    } else {
+        try argv.append("--no-entry"); // So lld doesn't look for _start.
+        try argv.append("--export-all");
+    }
+    try argv.appendSlice(&[_][]const u8{
+        "--allow-undefined",
+        "-o",
+        try directory.join(arena, &[_][]const u8{self.base.options.emit.?.sub_path}),
+    });
+
+    // Positional arguments to the linker such as object files.
+    try argv.appendSlice(self.base.options.objects);
+
+    for (comp.c_object_table.items()) |entry| {
+        try argv.append(entry.key.status.success.object_path);
+    }
+    if (module_obj_path) |p| {
+        try argv.append(p);
+    }
+
+    if (self.base.options.output_mode == .Exe and !self.base.options.is_compiler_rt_or_libc) {
+        if (!self.base.options.link_libc) {
+            try argv.append(comp.libc_static_lib.?.full_object_path);
+        }
+        try argv.append(comp.compiler_rt_static_lib.?.full_object_path);
+    }
+
+    if (self.base.options.verbose_link) {
+        Compilation.dump_argv(argv.items);
+    }
+
+    // TODO allocSentinel crashed stage1 so this is working around it.
+    const new_argv_with_sentinel = try arena.alloc(?[*:0]const u8, argv.items.len + 1);
+    new_argv_with_sentinel[argv.items.len] = null;
+    const new_argv = new_argv_with_sentinel[0..argv.items.len :null];
+    for (argv.items) |arg, i| {
+        new_argv[i] = try arena.dupeZ(u8, arg);
+    }
+
+    var stderr_context: LLDContext = .{
+        .wasm = self,
+        .data = std.ArrayList(u8).init(self.base.allocator),
+    };
+    defer stderr_context.data.deinit();
+    var stdout_context: LLDContext = .{
+        .wasm = self,
+        .data = std.ArrayList(u8).init(self.base.allocator),
+    };
+    defer stdout_context.data.deinit();
+    const llvm = @import("../llvm.zig");
+    const ok = llvm.Link(
+        .Wasm,
+        new_argv.ptr,
+        new_argv.len,
+        append_diagnostic,
+        @ptrToInt(&stdout_context),
+        @ptrToInt(&stderr_context),
+    );
+    if (stderr_context.oom or stdout_context.oom) return error.OutOfMemory;
+    if (stdout_context.data.items.len != 0) {
+        std.log.warn("unexpected LLD stdout: {}", .{stdout_context.data.items});
+    }
+    if (!ok) {
+        // TODO parse this output and surface with the Compilation API rather than
+        // directly outputting to stderr here.
+        std.debug.print("{}", .{stderr_context.data.items});
+        return error.LLDReportedFailure;
+    }
+    if (stderr_context.data.items.len != 0) {
+        std.log.warn("unexpected LLD stderr: {}", .{stderr_context.data.items});
+    }
+
+    if (!self.base.options.disable_lld_caching) {
+        // Update the dangling symlink with the digest. If it fails we can continue; it only
+        // means that the next invocation will have an unnecessary cache miss.
+        directory.handle.symLink(&digest, id_symlink_basename, .{}) catch |err| {
+            std.log.warn("failed to save linking hash digest symlink: {}", .{@errorName(err)});
+        };
+        // Again failure here only means an unnecessary cache miss.
+        man.writeManifest() catch |err| {
+            std.log.warn("failed to write cache manifest when linking: {}", .{@errorName(err)});
+        };
+        // We hang on to this lock so that the output file path can be used without
+        // other processes clobbering it.
+        self.base.lock = man.toOwnedLock();
+    }
+}
+
+const LLDContext = struct {
+    data: std.ArrayList(u8),
+    wasm: *Wasm,
+    oom: bool = false,
+};
+
+fn append_diagnostic(context: usize, ptr: [*]const u8, len: usize) callconv(.C) void {
+    const lld_context = @intToPtr(*LLDContext, context);
+    const msg = ptr[0..len];
+    lld_context.data.appendSlice(msg) catch |err| switch (err) {
+        error.OutOfMemory => lld_context.oom = true,
+    };
+}
+
 /// Get the current index of a given Decl in the function list
 /// TODO: we could maintain a hash map to potentially make this
 fn getFuncidx(self: Wasm, decl: *Module.Decl) ?u32 {