[clang]Correctly handle negative line changes in PrintPreprocessedOutput::MoveToLine

[SergejSalnikov]

No description provided.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Sergej Salnikov (SergejSalnikov)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/166631.diff

1 Files Affected:

• (modified) clang/lib/Frontend/PrintPreprocessedOutput.cpp (+1-1)

diff --git a/clang/lib/Frontend/PrintPreprocessedOutput.cpp b/clang/lib/Frontend/PrintPreprocessedOutput.cpp
index 9e046633328d7..32e2b8cdcf4c6 100644
--- a/clang/lib/Frontend/PrintPreprocessedOutput.cpp
+++ b/clang/lib/Frontend/PrintPreprocessedOutput.cpp
@@ -306,7 +306,7 @@ bool PrintPPOutputPPCallbacks::MoveToLine(unsigned LineNo,
     *OS << '\n';
     StartedNewLine = true;
   } else if (!DisableLineMarkers) {
-    if (LineNo - CurLine <= 8) {
+    if (LineNo >= CurLine && LineNo - CurLine <= 8) {
       const char *NewLines = "\n\n\n\n\n\n\n\n";
       OS->write(NewLines, LineNo - CurLine);
     } else {

[shafik]

This change needs a summary explaining the problem and how this fixes it. It is not obvious what your are fixing and without more narrative I can't review this properly. It also needs a test.

[shafik]

I am waiting for an update but want to make sure this does not get merged by accident.

[SergejSalnikov]

After #163982 the debug symbols inside macro arguments are now resolved to the line they appear rather than the line of macro statement.
#168402 will make the new macro argument expansion behaviour to be used for more cases.
Eventually we want the clang -E command to produce the pre-processed code which can be then compiled and the line numbers of debug symbols to match the source file.

The example is a #define SWAP(X, Y) Y; X where the line numbers in generated file will not always increment, but could decrement. that's why we need that new behaviour (see the video attached to #163982).