Support lazy stat'ing of files referenced by module maps.

This patch adds support for a `header` declaration in a module map to specify
certain `stat` information (currently, size and mtime) about that header file.
This has two purposes:

- It removes the need to eagerly `stat` every file referenced by a module map.
  Instead, we track a list of unresolved header files with each size / mtime
  (actually, for simplicity, we track submodules with such headers), and when
  attempting to look up a header file based on a `FileEntry`, we check if there
  are any unresolved header directives with that `FileEntry`'s size / mtime and
  perform deferred `stat`s if so.

- It permits a preprocessed module to be compiled without the original files
  being present on disk. The only reason we used to need those files was to get
  the `stat` information in order to do header -> module lookups when using the
  module. If we're provided with the `stat` information in the preprocessed
  module, we can avoid requiring the files to exist.

Unlike most `header` directives, if a `header` directive with `stat`
information has no corresponding on-disk file the enclosing module is *not*
marked unavailable (so that behavior is consistent regardless of whether we've
resolved a header directive, and so that preprocessed modules don't get marked
unavailable). We could actually do this for all `header` directives: the only
reason we mark the module unavailable if headers are missing is to give a
diagnostic slightly earlier (rather than waiting until we actually try to build
the module / load and validate its .pcm file).

Differential Revision: https://reviews.llvm.org/D33703

llvm-svn: 304515

1 files changed, 7 insertions, 0 deletions

diff --git a/clang/test/Modules/preprocess-missing.modulemap b/clang/test/Modules/preprocess-missing.modulemap
new file mode 100644
index 00000000000..300494672f2
--- /dev/null
+++ b/clang/test/Modules/preprocess-missing.modulemap
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fmodules -fmodule-name=A -x c++-module-map %s -emit-module -o /dev/null -verify
+module A {
+  header "does not exist" { size 12345 } // ok, do not need mtime for explicit module build
+  header "also does not exist" { mtime 12345 }
+}
+#pragma clang module contents
+// expected-error@4 {{cannot emit module A: size must be explicitly specified for missing header file "also does not exist"}}