[WebAssembly] Lower ASan constructor priority on Emscripten

Summary:
This change gives Emscripten the ability to use more than one constructor
priorities that runs before ASan. By convention, constructor priorites 0-100
are reserved for use by the system. ASan on Emscripten now uses priority 50,
leaving plenty of room for use by Emscripten before and after ASan.

This change is done in response to:
emscripten-core/emscripten#9076 (comment)

Reviewers: kripken, tlively, aheejin

Reviewed By: tlively

Subscribers: cfe-commits, dschuff, sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D65684

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368101 91177308-0d34-0410-b5e6-96231b3b80d8

Author: quantum5

lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -129,6 +129,8 @@ static const uintptr_t kRetiredStackFrameMagic = 0x45E0360E;
 static const char *const kAsanModuleCtorName = "asan.module_ctor";
 static const char *const kAsanModuleDtorName = "asan.module_dtor";
 static const uint64_t kAsanCtorAndDtorPriority = 1;
+// On Emscripten, the system needs more than one priorities for constructors.
+static const uint64_t kAsanEmscriptenCtorAndDtorPriority = 50;
 static const char *const kAsanReportErrorTemplate = "__asan_report_";
 static const char *const kAsanRegisterGlobalsName = "__asan_register_globals";
 static const char *const kAsanUnregisterGlobalsName =
@@ -530,6 +532,14 @@ static size_t RedzoneSizeForScale(int MappingScale) {
   return std::max(32U, 1U << MappingScale);
 }
 
+static uint64_t GetCtorAndDtorPriority(Triple &TargetTriple) {
+  if (TargetTriple.isOSEmscripten()) {
+    return kAsanEmscriptenCtorAndDtorPriority;
+  } else {
+    return kAsanCtorAndDtorPriority;
+  }
+}
+
 namespace {
 
 /// Module analysis for getting various metadata about the module.
@@ -1777,7 +1787,8 @@ void ModuleAddressSanitizer::createInitializerPoisonCalls(
       if (F->getName() == kAsanModuleCtorName) continue;
       ConstantInt *Priority = dyn_cast<ConstantInt>(CS->getOperand(0));
       // Don't instrument CTORs that will run before asan.module_ctor.
-      if (Priority->getLimitedValue() <= kAsanCtorAndDtorPriority) continue;
+      if (Priority->getLimitedValue() <= GetCtorAndDtorPriority(TargetTriple))
+        continue;
       poisonOneInitializer(*F, ModuleName);
     }
   }
@@ -2429,22 +2440,22 @@ bool ModuleAddressSanitizer::instrumentModule(Module &M) {
     Changed |= InstrumentGlobals(IRB, M, &CtorComdat);
   }
 
+  const uint64_t Priority = GetCtorAndDtorPriority(TargetTriple);
+
   // Put the constructor and destructor in comdat if both
   // (1) global instrumentation is not TU-specific
   // (2) target is ELF.
   if (UseCtorComdat && TargetTriple.isOSBinFormatELF() && CtorComdat) {
     AsanCtorFunction->setComdat(M.getOrInsertComdat(kAsanModuleCtorName));
-    appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority,
-                        AsanCtorFunction);
+    appendToGlobalCtors(M, AsanCtorFunction, Priority, AsanCtorFunction);
     if (AsanDtorFunction) {
       AsanDtorFunction->setComdat(M.getOrInsertComdat(kAsanModuleDtorName));
-      appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority,
-                          AsanDtorFunction);
+      appendToGlobalDtors(M, AsanDtorFunction, Priority, AsanDtorFunction);
     }
   } else {
-    appendToGlobalCtors(M, AsanCtorFunction, kAsanCtorAndDtorPriority);
+    appendToGlobalCtors(M, AsanCtorFunction, Priority);
     if (AsanDtorFunction)
-      appendToGlobalDtors(M, AsanDtorFunction, kAsanCtorAndDtorPriority);
+      appendToGlobalDtors(M, AsanDtorFunction, Priority);
   }
 
   return Changed;