Index: llvm/lib/Analysis/GlobalsModRef.cpp
===================================================================
--- llvm/lib/Analysis/GlobalsModRef.cpp
+++ llvm/lib/Analysis/GlobalsModRef.cpp
@@ -354,7 +354,24 @@
           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();
+          if (!F)
+            return true;
+          if (!F->isDeclaration() || !F->hasFnAttribute(Attribute::NoCallback))
+            return true;
+          if (!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)) {
Index: llvm/test/Analysis/GlobalsModRef/noescape-nocapture-nocallback.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/GlobalsModRef/noescape-nocapture-nocallback.ll
@@ -0,0 +1,152 @@
+; RUN: opt < %s -aa-pipeline=basic-aa,globals-aa -S -passes='require<globals-aa>,function(loop-mssa(licm))' | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%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
+
+define dso_local void @test0() {
+; 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: call void @nocapture_nocallback_func(ptr @obj0)
+; CHECK: load ptr, ptr @obj0
+; CHECK: br label %[[exit_latch:.*]]
+; CHECK: [[exit_latch]]:
+; CHECK: br i1 %{{.*}}, label %[[body:.*]], label %[[exit:.*]]
+; CHECK: [[body]]:
+; CHECK-NOT: load ptr, ptr @obj0
+; CHECK: br label %[[inc_latch:.*]]
+; CHECK: [[inc_latch]]:
+; CHECK: br label %[[exit_latch]]
+; CHECK: [[exit]]:
+; CHECK: ret void
+
+entry:
+  %i = alloca i32, align 4
+  call void @nocapture_nocallback_func(ptr @obj0)
+  store i32 0, ptr %i, align 4
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i32, ptr %i, align 4
+  %cmp = icmp slt i32 %0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %1 = load ptr, ptr @obj0, align 8
+  %2 = load i32, ptr %i, align 4
+  %idxprom = sext i32 %2 to i64
+  %arrayidx = getelementptr inbounds i32, ptr %1, i64 %idxprom
+  store i32 -1, ptr %arrayidx, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %3 = load i32, ptr %i, align 4
+  %inc = add nsw i32 %3, 1
+  store i32 %inc, ptr %i, align 4
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test1() {
+; Check that load from @obj1 is not hoisted from the loop,
+; because 'nocallback' is missing:
+; CHECK-LABEL: @test1
+; CHECK: call void @nocapture_func(ptr @obj1)
+; CHECK: br label %[[exit_latch:.*]]
+; CHECK: [[exit_latch]]:
+; CHECK: br i1 %cmp, label %[[body:.*]], label %[[exit:.*]]
+; CHECK: [[body]]:
+; CHECK: load ptr, ptr @obj1
+; CHECK: br label %[[inc_latch:.*]]
+; CHECK: [[inc_latch]]:
+; CHECK: br label %[[exit_latch]]
+; CHECK: [[exit]]:
+; CHECK: ret void
+
+entry:
+  %i = alloca i32, align 4
+  call void @nocapture_func(ptr @obj1)
+  store i32 0, ptr %i, align 4
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i32, ptr %i, align 4
+  %cmp = icmp slt i32 %0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %1 = load ptr, ptr @obj1, align 8
+  %2 = load i32, ptr %i, align 4
+  %idxprom = sext i32 %2 to i64
+  %arrayidx = getelementptr inbounds i32, ptr %1, i64 %idxprom
+  store i32 -1, ptr %arrayidx, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %3 = load i32, ptr %i, align 4
+  %inc = add nsw i32 %3, 1
+  store i32 %inc, ptr %i, align 4
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+define dso_local void @test2() {
+; Check that load from @obj2 is not hoisted from the loop,
+; because 'nocapture' is missing:
+; CHECK-LABEL: @test2
+; CHECK: call void @nocallback_func(ptr @obj2)
+; CHECK: br label %[[exit_latch:.*]]
+; CHECK: [[exit_latch]]:
+; CHECK: br i1 %cmp, label %[[body:.*]], label %[[exit:.*]]
+; CHECK: [[body]]:
+; CHECK: load ptr, ptr @obj2
+; CHECK: br label %[[inc_latch:.*]]
+; CHECK: [[inc_latch]]:
+; CHECK: br label %[[exit_latch]]
+; CHECK: [[exit]]:
+; CHECK: ret void
+
+entry:
+  %i = alloca i32, align 4
+  call void @nocallback_func(ptr @obj2)
+  store i32 0, ptr %i, align 4
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i32, ptr %i, align 4
+  %cmp = icmp slt i32 %0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %1 = load ptr, ptr @obj2, align 8
+  %2 = load i32, ptr %i, align 4
+  %idxprom = sext i32 %2 to i64
+  %arrayidx = getelementptr inbounds i32, ptr %1, i64 %idxprom
+  store i32 -1, ptr %arrayidx, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %3 = load i32, ptr %i, align 4
+  %inc = add nsw i32 %3, 1
+  store i32 %inc, ptr %i, align 4
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+declare void @nocapture_nocallback_func(ptr nocapture) #0
+declare void @nocapture_func(ptr nocapture)
+declare void @nocallback_func(ptr) #0
+
+attributes #0 = { nocallback }