[gcov] Set current_working_directory to . for GCC>=9

[MaskRay]

GCC 9 changed the gcov format to set current_working_directory to be
used by gcov --json-format
(linux-test-project/lcov#38).

This change breaks local determinism, which we value more than strict
GCC compatibility. Just write "." to appease GCC's gcov.

Fix #121368

[MaskRay]

@PikachuHyA

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Fangrui Song (MaskRay)

Changes

GCC 9 changed the gcov format to set current_working_directory to be
used by gcov --json-format
(linux-test-project/lcov#38).

This change breaks local determinism, which we value more than strict
GCC compatibility. Just write "." to appease GCC's gcov.

Fix #121368

---

Full diff: https://github.com/llvm/llvm-project/pull/121420.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp (+3-1)

diff --git a/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp b/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp
index f9be7f933d31e4..dc3307a212817a 100644
--- a/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp
+++ b/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp
@@ -973,8 +973,10 @@ bool GCOVProfiler::emitProfileNotes(
         out.write(Tmp, 4);
       }
       write(Stamp);
+      // getcwd() breaks local determinism and is not easy to adjust with
+      // -ffile-prefix-map. Just write "." to appease GCC's gcov.
       if (Version >= 90)
-        writeString(""); // unuseful current_working_directory
+        writeString(".");
       if (Version >= 80)
         write(0); // unuseful has_unexecuted_blocks
 

[MaskRay]

Abandoned. It has been almost 5 years that llvm-cov supports the gcov format used b GCC 9~11. Just bumped Clang's producer format from 4.8 to 11 (9) and folded this change.