[clang] GCC directive extension extension: Hash NNN lines

Some time back I extended GCC's '# NNN' line marker semantics.
Specifically popping to a blank filename will restore the filename to
that of the popped-to include.  Restore to line 5 of including file
(escaped BOL #'s to avoid git eliding them):

\# 5 "" 2

Added documentation for this line control extension.

This was useful in developing modules tests, but turned out to also be
useful with machine-generated source code.  Specifically, a generated
include file that itself includes fragments from elsewhere.  The
ability to pop to the generated include file -- with its full path
prefix -- is useful for diagnostic & debug purposes.  For instance
something like:

// Machine generated -- DO NOT EDIT
Type Var = {
\# 7 "encoded.dsl" 1 // push to snippet-container
{snippet, of, code}
\# 6 " 2 // Restore to machined-generated source
,
};

// user-code
...
\#include "dsl.h"
...

That pop to "" will restore the filename to '..includepath../dsl.h',
which is better than restoring to plain "dsl.h".

Differential Revision: https://reviews.llvm.org/D113425

1 files changed, 18 insertions, 16 deletions

diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
index 8cba379..c2e7b68 100644
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -207,28 +207,30 @@ void LineTableInfo::AddLineNote(FileID FID, unsigned Offset, unsigned LineNo,
                                 SrcMgr::CharacteristicKind FileKind) {
   std::vector<LineEntry> &Entries = LineEntries[FID];
 
-  // An unspecified FilenameID means use the last filename if available, or the
-  // main source file otherwise.
-  if (FilenameID == -1 && !Entries.empty())
-    FilenameID = Entries.back().FilenameID;
-
   assert((Entries.empty() || Entries.back().FileOffset < Offset) &&
          "Adding line entries out of order!");
 
   unsigned IncludeOffset = 0;
-  if (EntryExit == 0) {  // No #include stack change.
-    IncludeOffset = Entries.empty() ? 0 : Entries.back().IncludeOffset;
-  } else if (EntryExit == 1) {
+  if (EntryExit == 1) {
+    // Push #include
     IncludeOffset = Offset-1;
-  } else if (EntryExit == 2) {
-    assert(!Entries.empty() && Entries.back().IncludeOffset &&
-       "PPDirectives should have caught case when popping empty include stack");
-
-    // Get the include loc of the last entries' include loc as our include loc.
-    IncludeOffset = 0;
-    if (const LineEntry *PrevEntry =
-          FindNearestLineEntry(FID, Entries.back().IncludeOffset))
+  } else {
+    const auto *PrevEntry = Entries.empty() ? nullptr : &Entries.back();
+    if (EntryExit == 2) {
+      // Pop #include
+      assert(PrevEntry && PrevEntry->IncludeOffset &&
+             "PPDirectives should have caught case when popping empty include "
+             "stack");
+      PrevEntry = FindNearestLineEntry(FID, PrevEntry->IncludeOffset);
+    }
+    if (PrevEntry) {
       IncludeOffset = PrevEntry->IncludeOffset;
+      if (FilenameID == -1) {
+        // An unspecified FilenameID means use the previous (or containing)
+        // filename if available, or the main source file otherwise.
+        FilenameID = PrevEntry->FilenameID;
+      }
+    }
   }
 
   Entries.push_back(LineEntry::get(Offset, LineNo, FilenameID, FileKind,