[asan] Set alignment of __asan_global_* globals to sizeof(GlobalStruct)

[kubamracek]

Summary

When using profiling and ASan together (-fprofile-instr-generate -fcoverage-mapping -fsanitize=address), at least on Darwin, the section of globals that ASan emits (__asan_globals) is misaligned and starts at an odd offset. This really doesn't have anything to do with profiling, but it triggers the issue because profiling emits a string section, which can have arbitrary size.

lib/Transforms/Instrumentation/AddressSanitizer.cpp

bool AddressSanitizerModule::InstrumentGlobals(IRBuilder<> &IRB, Module &M) {
   //   size_t has_dynamic_init;
   //   void *source_location;
   //   size_t odr_indicator;
   // We initialize an array of such structures and pass it to a run-time call.
   StructType *GlobalStructTy =
       StructType::get(IntptrTy, IntptrTy, IntptrTy, IntptrTy, IntptrTy,
                       IntptrTy, IntptrTy, IntptrTy, nullptr);
   unsigned SizeOfGlobalStruct = DL.getTypeAllocSize(GlobalStructTy);
-  assert((isPowerOf2_32(SizeOfGlobalStruct) ||
-          !TargetTriple.isOSBinFormatCOFF()) &&
+  assert(isPowerOf2_32(SizeOfGlobalStruct) &&
          "global metadata will not be padded appropriately");
   SmallVector<Constant *, 16> Initializers(UseMetadataArray ? n : 0);
 
   // On recent Mach-O platforms, use a structure which binds the liveness of
   // the global variable to the metadata struct. Keep the list of "Liveness" GV
   // created to be added to llvm.compiler.used
   StructType *LivenessTy  = nullptr;
   if (UseMachOGlobalsSection)
for (size_t i = 0; i < n; i++) {
     // Create a separate metadata global and put it in the appropriate ASan
     // global registration section.
     GlobalVariable *Metadata = new GlobalVariable(
         M, GlobalStructTy, false, GlobalVariable::InternalLinkage,
         Initializer, Twine("__asan_global_") +
                              GlobalValue::getRealLinkageName(G->getName()));
     Metadata->setSection(getGlobalMetadataSection());
 
+    // We don't want any padding, but we also need a reasonable alignment.
     // The MSVC linker always inserts padding when linking incrementally. We
     // cope with that by aligning each struct to its size, which must be a power
     // of two.
-    if (TargetTriple.isOSBinFormatCOFF())
-      Metadata->setAlignment(SizeOfGlobalStruct);
-    else
-      Metadata->setAlignment(1); // Don't leave padding in between.
 
     // On platforms that support comdats, put the metadata and the
     // instrumented global in the same group. This ensures that the metadata
     // is discarded if the instrumented global is discarded.
     if (UseComdatMetadata) {
       assert(G->hasComdat());
       Metadata->setComdat(G->getComdat());
       continue;

test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll

 
 !llvm.asan.globals = !{!0}
 
 !0 = !{[1 x i32]* @global, !1, !"global", i1 false, i1 false}
 !1 = !{!"test-globals.c", i32 1, i32 5}
 
 
 ; Find the metadata for @global:
-; CHECK: [[METADATA:@.+]] = internal global {{.*}} @global {{.*}} section "__DATA,__asan_globals,regular", align 1
+; CHECK: [[METADATA:@.+]] = internal global {{.*}} @global {{.*}} section "__DATA,__asan_globals,regular", align 64
 
 ; Find the liveness binder for @global and its metadata:
 ; CHECK: @__asan_binder_global = internal global {{.*}} @global {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
 
 ; Test that there is the flag global variable:
 ; CHECK: @__asan_globals_registered = common hidden global i64 0
 
 ; The binder has to be inserted to llvm.compiler.used to avoid being stripped