[C++20][Modules][HU 4/5] Handle pre-processed header units.

We wish to support emitting a pre-processed output for an importable
header unit, that can be consumed to produce the same header units as
the original source.

This means that ee need to find the original filename used to produce
the re-preprocessed output, so that it can be assigned as the module
name.  This is peeked from the first line of the pre-processed source
when the action sets up the files.

Differential Revision: https://reviews.llvm.org/D121098

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
index 68b8232..45245f3 100644
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -843,6 +843,21 @@ bool FrontendAction::BeginSourceFile(CompilerInstance &CI,
   if (!CI.InitializeSourceManager(Input))
     return false;
 
+  if (CI.getLangOpts().CPlusPlusModules && Input.getKind().isHeaderUnit() &&
+      Input.getKind().isPreprocessed() && !usesPreprocessorOnly()) {
+    // We have an input filename like foo.iih, but we want to find the right
+    // module name (and original file, to build the map entry).
+    // Check if the first line specifies the original source file name with a
+    // linemarker.
+    std::string PresumedInputFile = std::string(getCurrentFileOrBufferName());
+    ReadOriginalFileName(CI, PresumedInputFile);
+    // Unless the user overrides this, the module name is the name by which the
+    // original file was known.
+    if (CI.getLangOpts().ModuleName.empty())
+      CI.getLangOpts().ModuleName = std::string(PresumedInputFile);
+    CI.getLangOpts().CurrentModule = CI.getLangOpts().ModuleName;
+  }
+
   // For module map files, we first parse the module map and synthesize a
   // "<module-includes>" buffer before more conventional processing.
   if (Input.getKind().getFormat() == InputKind::ModuleMap) {