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
@@ -354,7 +354,31 @@
           if (Writers)
             Writers->insert(Call->getParent()->getParent());
         } else {
-          return true; // Argument of an unknown call.
+          // In general, we return true for unknown calls, but there are
+          // some simple checks that we can do for functions that
+          // will never call back into the module.
+          auto *F = Call->getCalledFunction();
+          // TODO: we should be able to remove isDeclaration() check
+          // and let the function body analysis check for captures,
+          // and collect the mod-ref effects. This information will
+          // be later propagated via the call graph.
+          if (!F || !F->isDeclaration())
+            return true;
+          // Note that the NoCallback check here is a little bit too
+          // conservative. If there are no captures of the global
+          // in the module, then this call may not be a capture even
+          // if it does not have NoCallback.
+          if (!Call->hasFnAttr(Attribute::NoCallback) ||
+              !Call->isArgOperand(&U) ||
+              !Call->doesNotCapture(Call->getArgOperandNo(&U)))
+            return true;
+
+          // Conservatively, assume the call reads and writes the global.
+          // We could use memory attributes to make it more precise.
+          if (Readers)
+            Readers->insert(Call->getParent()->getParent());
+          if (Writers)
+            Writers->insert(Call->getParent()->getParent());
         }
       }
     } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
diff --git a/llvm/test/Analysis/GlobalsModRef/noescape-nocapture-nocallback.ll b/llvm/test/Analysis/GlobalsModRef/noescape-nocapture-nocallback.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Analysis/GlobalsModRef/noescape-nocapture-nocallback.ll
@@ -0,0 +1,328 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -aa-pipeline=basic-aa,globals-aa -S -passes='require<globals-aa>,function(loop-mssa(licm))' | FileCheck %s
+
+;Reference C code:
+;struct str {
+;  void **p;
+;};
+;static struct str obj;
+;extern void nocapture_nocallback_func(struct str *);
+;void test(void *p) {
+;  nocapture_nocallback_func(&obj);
+;  for (int i = 0; i < 1000; ++i) {
+;    unknown_call(); // optional
+;    obj.p[i] = p;
+;  }
+;}
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+%struct.str = type { ptr }
+
+@obj0 = internal global %struct.str zeroinitializer, align 8
+@obj1 = internal global %struct.str zeroinitializer, align 8
+@obj2 = internal global %struct.str zeroinitializer, align 8
+@obj3 = internal global %struct.str zeroinitializer, align 8
+@obj4 = internal global %struct.str zeroinitializer, align 8
+@obj5 = internal global %struct.str zeroinitializer, align 8
+
+define dso_local void @test0(ptr %p) {
+; Check that load from @obj0 is hoisted from the loop, meaning
+; that it does not conflict with the store inside the loop:
+; CHECK-LABEL: @test0(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @nocapture_nocallback_func(ptr @obj0)
+; CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr @obj0, align 8
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[I_0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_0]], 1000
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_0]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds ptr, ptr [[TMP0]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store ptr [[P:%.*]], ptr [[ARRAYIDX]], align 8
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_0]], 1
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  call void @nocapture_nocallback_func(ptr @obj0)
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load ptr, ptr @obj0, align 8
+  %idxprom = sext i32 %i.0 to i64
+  %arrayidx = getelementptr inbounds ptr, ptr %0, i64 %idxprom
+  store ptr %p, ptr %arrayidx, align 8
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test1(ptr %p) {
+; Check that load from @obj1 is not hoisted from the loop,
+; because 'nocallback' is missing:
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @nocapture_func(ptr @obj1)
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[I_0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_0]], 1000
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr @obj1, align 8
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_0]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds ptr, ptr [[TMP0]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store ptr [[P:%.*]], ptr [[ARRAYIDX]], align 8
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_0]], 1
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  call void @nocapture_func(ptr @obj1)
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load ptr, ptr @obj1, align 8
+  %idxprom = sext i32 %i.0 to i64
+  %arrayidx = getelementptr inbounds ptr, ptr %0, i64 %idxprom
+  store ptr %p, ptr %arrayidx, align 8
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test2(ptr %p) {
+; Check that load from @obj2 is not hoisted from the loop,
+; because 'nocapture' is missing:
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @nocallback_func(ptr @obj2)
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[I_0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_0]], 1000
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr @obj2, align 8
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_0]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds ptr, ptr [[TMP0]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store ptr [[P:%.*]], ptr [[ARRAYIDX]], align 8
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_0]], 1
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  call void @nocallback_func(ptr @obj2)
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load ptr, ptr @obj2, align 8
+  %idxprom = sext i32 %i.0 to i64
+  %arrayidx = getelementptr inbounds ptr, ptr %0, i64 %idxprom
+  store ptr %p, ptr %arrayidx, align 8
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test3(ptr %p) {
+; Check that load from @obj3 is hoisted from the loop, even though
+; there is unknown call in the loop.
+; CHECK-LABEL: @test3(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @nocapture_nocallback_func(ptr @obj3)
+; CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr @obj3, align 8
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[I_0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_0]], 1000
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    call void @unknown_call()
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_0]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds ptr, ptr [[TMP0]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store ptr [[P:%.*]], ptr [[ARRAYIDX]], align 8
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_0]], 1
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  call void @nocapture_nocallback_func(ptr @obj3)
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load ptr, ptr @obj3, align 8
+  call void @unknown_call()
+  %idxprom = sext i32 %i.0 to i64
+  %arrayidx = getelementptr inbounds ptr, ptr %0, i64 %idxprom
+  store ptr %p, ptr %arrayidx, align 8
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test4(ptr %p) {
+; Check that load from @obj4 is not hoisted from the loop,
+; because 'nocallback' is missing:
+; CHECK-LABEL: @test4(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @nocapture_func(ptr @obj4)
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[I_0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_0]], 1000
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr @obj4, align 8
+; CHECK-NEXT:    call void @unknown_call()
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_0]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds ptr, ptr [[TMP0]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store ptr [[P:%.*]], ptr [[ARRAYIDX]], align 8
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_0]], 1
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  call void @nocapture_func(ptr @obj4)
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load ptr, ptr @obj4, align 8
+  call void @unknown_call()
+  %idxprom = sext i32 %i.0 to i64
+  %arrayidx = getelementptr inbounds ptr, ptr %0, i64 %idxprom
+  store ptr %p, ptr %arrayidx, align 8
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test5(ptr %p) {
+; Check that load from @obj5 is not hoisted from the loop,
+; because 'nocapture' is missing:
+; CHECK-LABEL: @test5(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @nocallback_func(ptr @obj5)
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    [[I_0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[INC:%.*]], [[FOR_INC:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[I_0]], 1000
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr @obj5, align 8
+; CHECK-NEXT:    call void @unknown_call()
+; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[I_0]] to i64
+; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds ptr, ptr [[TMP0]], i64 [[IDXPROM]]
+; CHECK-NEXT:    store ptr [[P:%.*]], ptr [[ARRAYIDX]], align 8
+; CHECK-NEXT:    br label [[FOR_INC]]
+; CHECK:       for.inc:
+; CHECK-NEXT:    [[INC]] = add nsw i32 [[I_0]], 1
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+
+entry:
+  call void @nocallback_func(ptr @obj5)
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load ptr, ptr @obj5, align 8
+  call void @unknown_call()
+  %idxprom = sext i32 %i.0 to i64
+  %arrayidx = getelementptr inbounds ptr, ptr %0, i64 %idxprom
+  store ptr %p, ptr %arrayidx, align 8
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+declare void @nocapture_nocallback_func(ptr nocapture) nocallback
+declare void @nocapture_func(ptr nocapture)
+declare void @nocallback_func(ptr) nocallback
+; nosync and nocallback are required, otherwise the call
+; will by ModRef for any global:
+declare void @unknown_call() nosync nocallback