[llvm] [asan] Ensure __asan_register_elf_globals is called in COMDAT asan.module_ctor (PR #67745)

[Vitaly Buka via llvm-commits]

================
@@ -2517,27 +2513,35 @@ bool ModuleAddressSanitizer::InstrumentGlobals(IRBuilder<> &IRB, Module &M,
   }
   appendToCompilerUsed(M, ArrayRef<GlobalValue *>(GlobalsToAddToUsedList));
 
-  std::string ELFUniqueModuleId =
-      (UseGlobalsGC && TargetTriple.isOSBinFormatELF()) ? getUniqueModuleId(&M)
-                                                        : "";
-
-  if (!ELFUniqueModuleId.empty()) {
-    InstrumentGlobalsELF(IRB, M, NewGlobals, Initializers, ELFUniqueModuleId);
+  if (UseGlobalsGC && TargetTriple.isOSBinFormatELF()) {
+    // Use COMDAT and register globals even if n == 0 to ensure that (a) the
+    // linkage unit will only have one module constructor, and (b) the register
+    // function will be called. The module destructor is not created when n ==
+    // 0.
     *CtorComdat = true;
-  } else if (UseGlobalsGC && TargetTriple.isOSBinFormatCOFF()) {
-    InstrumentGlobalsCOFF(IRB, M, NewGlobals, Initializers);
-  } else if (UseGlobalsGC && ShouldUseMachOGlobalsSection()) {
-    InstrumentGlobalsMachO(IRB, M, NewGlobals, Initializers);
+    InstrumentGlobalsELF(IRB, M, NewGlobals, Initializers,
+                         getUniqueModuleId(&M));
+  } else if (n == 0) {
+    // When UseGlobalsGC is false, COMDAT can still be used if n == 0, because
+    // all compile units will have identical module constructor/destructor.
+    *CtorComdat = TargetTriple.isOSBinFormatELF();
   } else {
-    InstrumentGlobalsWithMetadataArray(IRB, M, NewGlobals, Initializers);
+    *CtorComdat = false;
+    if (UseGlobalsGC && TargetTriple.isOSBinFormatCOFF()) {
+      InstrumentGlobalsCOFF(IRB, M, NewGlobals, Initializers);
+    } else if (UseGlobalsGC && ShouldUseMachOGlobalsSection()) {
+      InstrumentGlobalsMachO(IRB, M, NewGlobals, Initializers);
+    } else {
+      InstrumentGlobalsWithMetadataArray(IRB, M, NewGlobals, Initializers);
+    }
   }
 
   // Create calls for poisoning before initializers run and unpoisoning after.
   if (HasDynamicallyInitializedGlobals)
     createInitializerPoisonCalls(M, ModuleName);
 
   LLVM_DEBUG(dbgs() << M);
-  return true;
+  return n != 0;
----------------
vitalybuka wrote:

Not important, as we don't use return value. I guess it's true if "changed"
Maybe still return true, or make it void?

https://github.com/llvm/llvm-project/pull/67745