Module Debugging: Use a nonzero DWO id for precompiled headers.

PCH files don't have a module signature and LLVM uses a nonzero DWO id as
an indicator for skeleton / module CUs. This change pins the DWO id for PCH
files to a known constant value.
The correct long-term solution here is to implement a module signature
that is an actual dterministic hash (at the moment module signatures are
just random nonzero numbers) and then enable this for PCH files as well.

<rdar://problem/24290667>

llvm-svn: 258507

1 files changed, 4 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 28e28489681..4e0cc3f8574 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -1740,11 +1740,14 @@ CGDebugInfo::getOrCreateModuleRef(ExternalASTSource::ASTSourceDescriptor Mod,
 
   bool IsRootModule = M ? !M->Parent : true;
   if (CreateSkeletonCU && IsRootModule) {
+    // PCH files don't have a signature field in the control block,
+    // but LLVM detects skeleton CUs by looking for a non-zero DWO id.
+    uint64_t Signature = Mod.getSignature() ? Mod.getSignature() : ~1U;
     llvm::DIBuilder DIB(CGM.getModule());
     DIB.createCompileUnit(TheCU->getSourceLanguage(), Mod.getModuleName(),
                           Mod.getPath(), TheCU->getProducer(), true,
                           StringRef(), 0, Mod.getASTFile(),
-                          llvm::DIBuilder::FullDebug, Mod.getSignature());
+                          llvm::DIBuilder::FullDebug, Signature);
     DIB.finalize();
   }
   llvm::DIModule *Parent =