[LLD][COFF] Survive empty and invalid PCH signature

Solve two issues that showed up when using LLD with Unreal Engine & FASTBuild:
1. It seems the S_OBJNAME record doesn't always record the "precomp signature". We were relying on that to match the PCH.OBJ with their dependent-OBJ.
2. MSVC link.exe is able to link a PCH.OBJ when the "precomp signatureÈ doesn't match, but LLD was failing. This was occuring since the Unreal Engine Build Tool was compiling the PCH.OBJ, but the dependent-OBJ were compiled & cached through FASTBuild. Upon a clean rebuild, the PCH.OBJs were recompiled by the Unreal Build Tool, thus the "precomp signatures" were changing; however the OBJs were already cached by FASTBuild, thus having an old "precomp signatures".

We now ignore "precomp signatures" and properly fallback to cmd-line name lookup, like MSVC link.exe does, and only fail if the PCH.OBJ type stream doesn't match the count expected by the dependent-OBJ.

Differential Revision: https://reviews.llvm.org/D136762

1 files changed, 3 insertions, 0 deletions

diff --git a/lld/docs/ReleaseNotes.rst b/lld/docs/ReleaseNotes.rst
index 0e9029bdcc09..31eb116c383a 100644
--- a/lld/docs/ReleaseNotes.rst
+++ b/lld/docs/ReleaseNotes.rst
@@ -45,6 +45,9 @@ COFF Improvements
   (`D137723 <https://reviews.llvm.org/D137723>`_)
 * Switched from SHA1 to BLAKE3 for PDB type hashing / ``-gcodeview-ghash``
   (`D137101 <https://reviews.llvm.org/D137101>`_)
+* Improvements to the PCH.OBJ files handling. Now LLD behaves the same as MSVC
+  link.exe when merging PCH.OBJ files that don't have the same signature.
+  (`D136762 <https://reviews.llvm.org/D136762>`_)
 
 MinGW Improvements
 ------------------