diff --git a/llvm/lib/Analysis/GlobalsModRef.cpp b/llvm/lib/Analysis/GlobalsModRef.cpp
--- a/llvm/lib/Analysis/GlobalsModRef.cpp
+++ b/llvm/lib/Analysis/GlobalsModRef.cpp
@@ -533,7 +533,15 @@
           FI.addModRefInfo(ModRefInfo::ModRef);
           if (!F->onlyAccessesArgMemory())
             FI.setMayReadAnyGlobal();
-          if (!F->isIntrinsic()) {
+          // Intrinsics, like any other synchronizing function, can make effects
+          // of other threads visible. Without nosync we know nothing really.
+          // Similarly, if `nocallback` is missing the function, or intrinsic,
+          // can call into the module arbitrarily. If both are set the function
+          // has an effect but will not interact with accesses of internal
+          // globals inside the module. We are conservative here for optnone
+          // functions, might not be necessary.
+          if (!F->isDeclaration() || !F->hasNoSync() ||
+              !F->hasFnAttribute(Attribute::NoCallback)) {
             KnowNothing = true;
             break;
           }