[build] Link with lld on Windows when using Clang.

- Make the build deterministic

Bug: https://github.com/dart-lang/sdk/issues/55995
Change-Id: Ic800dd66f23bc402dfede09db3f67f01aa82d29d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373360
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

build/config/BUILDCONFIG.gn

@@ -257,7 +257,10 @@ if (is_win) {
     "//build/config/win:winver",
   ]
   if (is_clang) {
-    _native_compiler_configs += [ "//build/config/win:relative_paths" ]
+    _native_compiler_configs += [
+      "//build/config/win:relative_paths",
+      "//build/config/win:deterministic_builds",
+    ]
   }
 }
 if (is_posix) {

build/config/win/BUILD.gn

@@ -221,3 +221,31 @@ config("relative_paths") {
     "-no-canonical-prefixes",
   ]
 }
+
+if (is_clang) {
+  build_timestamp =
+      exec_script("//tools/make_coff_timestamp.py", [], "trim string")
+}
+
+config("deterministic_builds") {
+  if (is_clang) {
+    # /Brepro lets the compiler not write the mtime field in the .obj output.
+    # link.exe /incremental relies on this field to work correctly, but lld
+    # never looks at this timestamp, so it's safe to pass this flag with
+    # lld and get more deterministic compiler output in return.
+    # In LTO builds, the compiler doesn't write .obj files containing mtimes,
+    # so /Brepro is ignored there.
+    cflags = [ "/Brepro" ]
+
+    # lld defaults to writing the current time in the pe/coff header.
+    # For build reproducibility, pass an explicit timestamp. See
+    # build/compute_build_timestamp.py for how the timestamp is chosen.
+    # (link.exe also writes the current time, but it doesn't have a flag to
+    # override that behavior.)
+    ldflags = [ "/TIMESTAMP:" + build_timestamp ]
+
+    # Use a fake fixed base directory for paths in the pdb to make the pdb
+    # output fully deterministic and independent of the build directory.
+    ldflags += [ "/PDBSourcePath:o:\fake\prefix" ]
+  }
+}

build/toolchain/win/BUILD.gn

@@ -140,9 +140,18 @@ template("msvc_toolchain") {
       ]
     }
 
+    if (is_clang) {
+      prefix = rebase_path("$clang_base_path/bin", root_build_dir)
+      lib = "$prefix/lld-link.exe /lib"
+      link = "$prefix/lld-link.exe"
+    } else {
+      lib = "lib.exe"
+      link = "link.exe"
+    }
+
     tool("alink") {
       rspfile = "{{output}}.rsp"
-      command = "$python_path $tool_wrapper_path link-wrapper $env False lib.exe /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
+      command = "$python_path $tool_wrapper_path link-wrapper $env False $lib /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
       description = "LIB {{output}}"
       outputs = [
         # Ignore {{output_extension}} and always use .lib, there's no reason to
@@ -163,7 +172,7 @@ template("msvc_toolchain") {
                                                                                    # foo.dll.lib
       rspfile = "${dllname}.rsp"
 
-      link_command = "$python_path $tool_wrapper_path link-wrapper $env False link.exe /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
+      link_command = "$python_path $tool_wrapper_path link-wrapper $env False $link /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
 
       # TODO(brettw) support manifests
       #manifest_command = "$python_path $tool_wrapper_path manifest-wrapper $env mt.exe -nologo -manifest $manifests -out:${dllname}.manifest"
@@ -216,7 +225,7 @@ template("msvc_toolchain") {
       rspfile = "$binary_output.rsp"
       pdbfile = "$binary_output.pdb"
 
-      link_command = "$python_path $tool_wrapper_path link-wrapper $env False link.exe /nologo /OUT:$binary_output /PDB:$pdbfile @$rspfile"
+      link_command = "$python_path $tool_wrapper_path link-wrapper $env False $link /nologo /OUT:$binary_output /PDB:$pdbfile @$rspfile"
 
       # TODO(brettw) support manifests
       #manifest_command = "$python_path $tool_wrapper_path manifest-wrapper $env mt.exe -nologo -manifest $manifests -out:{{output}}.manifest"

tools/make_coff_timestamp.py

@@ -0,0 +1,19 @@
+# Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+import subprocess
+import utils
+import sys
+
+
+def Main():
+    p = subprocess.Popen(['git', 'show', '--no-patch', '--format=%ct'],
+                         shell=utils.IsWindows(),
+                         cwd=utils.DART_DIR)
+    p.communicate()
+    return p.wait()
+
+
+if __name__ == '__main__':
+    sys.exit(Main())