[gn build] replace llvm_allow_tardy_revision with llvm_append_vc_rev

Previously, the gn build would create VCSRevision.h / VCSVersion.h
files with some LLD_REVISION / LLVM_REVISION / CLANG_REVISION but
by default wouldn't add a dependency on .git/logs/HEAD so that
the step doesn't rerun after every branch switch or every pull.

That's bad for deterministic builds, and having --version print
some arbitrarily old revision isn't great either.

Instead, move to the model that the cmake build (now) uses fairly
consistently: If llvm_append_vc_rev is set, include the revision,
else don't.

Since the GN build is focused on developers, set llvm_append_vc_rev
to false instead of true by default (different from the cmake build),
so that things don't rebuild after every branch switch and every
pull.

While here, also remove some pre-monorepo code.

Differential Revision: https://reviews.llvm.org/D72859

Author: nico

llvm/utils/gn/build/write_vcsrevision.gni

@@ -9,11 +9,10 @@
 #       Defaults to [ "LLVM" ]
 
 declare_args() {
-  # If this set to false, VCSRevision.h is updated after every git commit.
+  # If this is set to true, VCSRevision.h is updated after every git commit.
   # That's technically correct, but results in rebuilds after every commit.
-  # If it's true (default), VCSRevision.h will usually be somewhat
-  # out-of-date, but builds will be faster.
-  llvm_allow_tardy_revision = true
+  # If it's false (default), VCSRevision.h will not contain a revision.
+  llvm_append_vc_rev = false
 }
 
 template("write_vcsrevision") {
@@ -29,9 +28,10 @@ template("write_vcsrevision") {
     }
 
     args = [ rebase_path(header, root_build_dir) ]
-    if (!llvm_allow_tardy_revision) {
+    if (llvm_append_vc_rev) {
       depfile = "$header.d"
       args += [
+        "--write-git-rev",
         "-d",
         rebase_path(depfile, root_build_dir),
       ]

llvm/utils/gn/build/write_vcsrevision.py

@@ -29,40 +29,39 @@ def main():
     parser.add_argument('-d', '--depfile',
                         help='if set, writes a depfile that causes this script '
                              'to re-run each time the current revision changes')
+    parser.add_argument('--write-git-rev', action='store_true',
+                        help='if set, writes git revision, else writes #undef')
     parser.add_argument('--name', action='append',
                         help='if set, writes a depfile that causes this script '
                              'to re-run each time the current revision changes')
     parser.add_argument('vcs_header', help='path to the output file to write')
     args = parser.parse_args()
 
-    if os.path.isdir(os.path.join(LLVM_DIR, '.svn')):
-        print('SVN support not implemented', file=sys.stderr)
-        return 1
-    if os.path.exists(os.path.join(LLVM_DIR, '.git')):
-        print('non-mono-repo git support not implemented', file=sys.stderr)
-        return 1
-
-    git, use_shell = which('git'), False
-    if not git:
-        git = which('git.exe')
-    if not git:
-        git = which('git.bat')
-        use_shell = True
-
-    git_dir = subprocess.check_output([git, 'rev-parse', '--git-dir'],
-                                      cwd=LLVM_DIR, shell=use_shell).decode().strip()
-    if not os.path.isdir(git_dir):
-        print('.git dir not found at "%s"' % git_dir, file=sys.stderr)
-        return 1
-
-    rev = subprocess.check_output([git, 'rev-parse', '--short', 'HEAD'],
-                                  cwd=git_dir, shell=use_shell).decode().strip()
-    url = subprocess.check_output([git, 'remote', 'get-url', 'origin'],
-                                  cwd=git_dir, shell=use_shell).decode().strip()
     vcsrevision_contents = ''
-    for name in args.name:
-        vcsrevision_contents += '#define %s_REVISION "%s"\n' % (name, rev)
-        vcsrevision_contents += '#define %s_REPOSITORY "%s"\n' % (name, url)
+    if args.write_git_rev:
+        git, use_shell = which('git'), False
+        if not git: git = which('git.exe')
+        if not git: git, use_shell = which('git.bat'), True
+        git_dir = subprocess.check_output(
+                [git, 'rev-parse', '--git-dir'],
+                cwd=LLVM_DIR, shell=use_shell).decode().strip()
+        if not os.path.isdir(git_dir):
+            print('.git dir not found at "%s"' % git_dir, file=sys.stderr)
+            return 1
+
+        rev = subprocess.check_output(
+                [git, 'rev-parse', '--short', 'HEAD'],
+                cwd=git_dir, shell=use_shell).decode().strip()
+        url = subprocess.check_output(
+                [git, 'remote', 'get-url', 'origin'],
+                cwd=git_dir, shell=use_shell).decode().strip()
+        for name in args.name:
+            vcsrevision_contents += '#define %s_REVISION "%s"\n' % (name, rev)
+            vcsrevision_contents += '#define %s_REPOSITORY "%s"\n' % (name, url)
+    else:
+        for name in args.name:
+            vcsrevision_contents += '#undef %s_REVISION\n' % name
+            vcsrevision_contents += '#undef %s_REPOSITORY\n' % name
 
     # If the output already exists and is identical to what we'd write,
     # return to not perturb the existing file's timestamp.