diff --git a/compiler-rt/lib/dfsan/dfsan.cpp b/compiler-rt/lib/dfsan/dfsan.cpp
--- a/compiler-rt/lib/dfsan/dfsan.cpp
+++ b/compiler-rt/lib/dfsan/dfsan.cpp
@@ -128,6 +128,17 @@
            fname);
 }
 
+extern "C" SANITIZER_INTERFACE_ATTRIBUTE void __dfsan_wrapper_extern_weak_null(
+    const void *addr, char *fname) {
+  if (!addr)
+    Report(
+        "ERROR: DataFlowSanitizer: dfsan generated wrapper calling null "
+        "extern_weak function %s\nIf this only happens with dfsan, the "
+        "dfsan instrumentation pass may be accidentally optimizing out a "
+        "null check\n",
+        fname);
+}
+
 // Use '-mllvm -dfsan-debug-nonzero-labels' and break on this function
 // to try to figure out where labels are being introduced in a nominally
 // label-free program.
diff --git a/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
--- a/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
@@ -439,6 +439,7 @@
   FunctionType *DFSanUnionLoadFnTy;
   FunctionType *DFSanLoadLabelAndOriginFnTy;
   FunctionType *DFSanUnimplementedFnTy;
+  FunctionType *DFSanWrapperExternWeakNullFnTy;
   FunctionType *DFSanSetLabelFnTy;
   FunctionType *DFSanNonzeroLabelFnTy;
   FunctionType *DFSanVarargWrapperFnTy;
@@ -454,6 +455,7 @@
   FunctionCallee DFSanUnionLoadFn;
   FunctionCallee DFSanLoadLabelAndOriginFn;
   FunctionCallee DFSanUnimplementedFn;
+  FunctionCallee DFSanWrapperExternWeakNullFn;
   FunctionCallee DFSanSetLabelFn;
   FunctionCallee DFSanNonzeroLabelFn;
   FunctionCallee DFSanVarargWrapperFn;
@@ -490,6 +492,7 @@
   TransformedFunction getCustomFunctionType(FunctionType *T);
   WrapperKind getWrapperKind(Function *F);
   void addGlobalNameSuffix(GlobalValue *GV);
+  void buildExternWeakCheckIfNeeded(IRBuilder<> &IRB, Function *F);
   Function *buildWrapperFunction(Function *F, StringRef NewFName,
                                  GlobalValue::LinkageTypes NewFLink,
                                  FunctionType *NewFT);
@@ -1041,6 +1044,10 @@
                         /*isVarArg=*/false);
   DFSanUnimplementedFnTy = FunctionType::get(
       Type::getVoidTy(*Ctx), Type::getInt8PtrTy(*Ctx), /*isVarArg=*/false);
+  Type *DFSanWrapperExternWeakNullArgs[2] = {Int8Ptr, Int8Ptr};
+  DFSanWrapperExternWeakNullFnTy =
+      FunctionType::get(Type::getVoidTy(*Ctx), DFSanWrapperExternWeakNullArgs,
+                        /*isVarArg=*/false);
   Type *DFSanSetLabelArgs[4] = {PrimitiveShadowTy, OriginTy,
                                 Type::getInt8PtrTy(*Ctx), IntptrTy};
   DFSanSetLabelFnTy = FunctionType::get(Type::getVoidTy(*Ctx),
@@ -1132,6 +1139,23 @@
   }
 }
 
+void DataFlowSanitizer::buildExternWeakCheckIfNeeded(IRBuilder<> &IRB,
+                                                     Function *F) {
+  // If the function we are wrapping was ExternWeak, it may be null.
+  // The original code before calling this wrapper may have checked for null,
+  // but replacing with a known-to-not-be-null wrapper can break this check.
+  // When replacing uses of the extern weak function with the wrapper we try
+  // to avoid replacing uses in conditionals, but this is not perfect.
+  // In the case where we fail, and accidentially optimize out a null check
+  // for a extern weak function, add a check here to help identify the issue.
+  if (GlobalValue::isExternalWeakLinkage(F->getLinkage())) {
+    std::vector<Value *> Args;
+    Args.push_back(IRB.CreatePointerCast(F, IRB.getInt8PtrTy()));
+    Args.push_back(IRB.CreateGlobalStringPtr(F->getName()));
+    IRB.CreateCall(DFSanWrapperExternWeakNullFn, Args);
+  }
+}
+
 Function *
 DataFlowSanitizer::buildWrapperFunction(Function *F, StringRef NewFName,
                                         GlobalValue::LinkageTypes NewFLink,
@@ -1184,6 +1208,8 @@
   }
   DFSanUnimplementedFn =
       Mod->getOrInsertFunction("__dfsan_unimplemented", DFSanUnimplementedFnTy);
+  DFSanWrapperExternWeakNullFn = Mod->getOrInsertFunction(
+      "__dfsan_wrapper_extern_weak_null", DFSanWrapperExternWeakNullFnTy);
   {
     AttributeList AL;
     AL = AL.addParamAttribute(M.getContext(), 0, Attribute::ZExt);
@@ -1227,6 +1253,8 @@
       DFSanLoadLabelAndOriginFn.getCallee()->stripPointerCasts());
   DFSanRuntimeFunctions.insert(
       DFSanUnimplementedFn.getCallee()->stripPointerCasts());
+  DFSanRuntimeFunctions.insert(
+      DFSanWrapperExternWeakNullFn.getCallee()->stripPointerCasts());
   DFSanRuntimeFunctions.insert(
       DFSanSetLabelFn.getCallee()->stripPointerCasts());
   DFSanRuntimeFunctions.insert(
@@ -2860,16 +2888,19 @@
     CB.setCalledFunction(&F);
     IRB.CreateCall(DFSF.DFS.DFSanUnimplementedFn,
                    IRB.CreateGlobalStringPtr(F.getName()));
+    DFSF.DFS.buildExternWeakCheckIfNeeded(IRB, &F);
     DFSF.setShadow(&CB, DFSF.DFS.getZeroShadow(&CB));
     DFSF.setOrigin(&CB, DFSF.DFS.ZeroOrigin);
     return true;
   case DataFlowSanitizer::WK_Discard:
     CB.setCalledFunction(&F);
+    DFSF.DFS.buildExternWeakCheckIfNeeded(IRB, &F);
     DFSF.setShadow(&CB, DFSF.DFS.getZeroShadow(&CB));
     DFSF.setOrigin(&CB, DFSF.DFS.ZeroOrigin);
     return true;
   case DataFlowSanitizer::WK_Functional:
     CB.setCalledFunction(&F);
+    DFSF.DFS.buildExternWeakCheckIfNeeded(IRB, &F);
     visitInstOperands(CB);
     return true;
   case DataFlowSanitizer::WK_Custom:
diff --git a/llvm/test/Instrumentation/DataFlowSanitizer/extern_weak.ll b/llvm/test/Instrumentation/DataFlowSanitizer/extern_weak.ll
--- a/llvm/test/Instrumentation/DataFlowSanitizer/extern_weak.ll
+++ b/llvm/test/Instrumentation/DataFlowSanitizer/extern_weak.ll
@@ -11,12 +11,15 @@
 
 define noundef i8 @call_if_exists() local_unnamed_addr {
   ; CHECK-LABEL: @call_if_exists.dfsan
+  ; Ensure comparison is preserved
   ; CHECK: br i1 icmp ne ([[FUNCPTRTY:.*]] @ExternWeak, [[FUNCPTRTY]] null), label %use_func, label %avoid_func
   br i1 icmp ne (i8 (i8)* @ExternWeak, i8 (i8)* null), label %use_func, label %avoid_func
 
 use_func:
   ; CHECK: use_func:
-  ; CHECK: call i8 @ExternWeak(i8 {{.*}})
+  ; Ensure extern weak function is validated before being called.
+  ; CHECK: call void @__dfsan_wrapper_extern_weak_null({{[^,]*}}@ExternWeak{{[^,]*}}, {{.*}})
+  ; CHECK-NEXT: call i8 @ExternWeak(i8 {{.*}})
   %1 = call i8 @ExternWeak(i8 4)
   br label %end