Index: llvm/lib/Transforms/IPO/Attributor.cpp
===================================================================
--- llvm/lib/Transforms/IPO/Attributor.cpp
+++ llvm/lib/Transforms/IPO/Attributor.cpp
@@ -2458,15 +2458,53 @@
                       << " is assumed NoAlias in the definition\n");
 
     // (ii) Check whether the value is captured in the scope using AANoCapture.
-    //      FIXME: This is conservative though, it is better to look at CFG and
-    //             check only uses possibly executed before this callsite.
+    //      Look at CFG and check only uses possibly executed before this
+    //      callsite.
+
+    auto UsePred = [&](const Use &U, bool &Follow) -> bool {
+      Instruction *UserI = cast<Instruction>(U.getUser());
+      const Function *ScopeFn = IRP.getAnchorScope();
+      if (ScopeFn) {
+        const auto &ReachabilityAA =
+            A.getAAFor<AAReachability>(*this, IRPosition::function(*ScopeFn),
+                                       /* TrackDependence */ false);
+
+        if (!ReachabilityAA.isAssumedReachable(UserI, getCtxI()))
+          return true;
+
+        if (auto *CB = dyn_cast<CallBase>(UserI)) {
+          if (CB->isArgOperand(&U)) {
+            unsigned ArgNo = CB->getArgOperandNo(&U);
+
+            const auto &NoCaptureAA = A.getAAFor<AANoCapture>(
+                *this, IRPosition::callsite_argument(*CB, ArgNo));
+
+            if (!NoCaptureAA.isAssumedNoCapture())
+              return true;
+          }
+        }
+      }
+
+      if (isa<GetElementPtrInst>(U) || isa<BitCastInst>(U) || isa<PHINode>(U) ||
+          isa<SelectInst>(U)) {
+        Follow = true;
+        return true;
+      }
+
+      LLVM_DEBUG(dbgs() << "[Attributor][AANoAliasCSArg] Unknown user: " << *U
+                        << "\n");
+      return false;
+    };
 
     auto &NoCaptureAA = A.getAAFor<AANoCapture>(*this, IRP);
     if (!NoCaptureAA.isAssumedNoCaptureMaybeReturned()) {
-      LLVM_DEBUG(
-          dbgs() << "[Attributor][AANoAliasCSArg] " << V
-                 << " cannot be noalias as it is potentially captured\n");
-      return indicatePessimisticFixpoint();
+      // Check all of V's uses.
+      if (!A.checkForAllUses(UsePred, *this, V)) {
+        LLVM_DEBUG(
+            dbgs() << "[Attributor][AANoAliasCSArg] " << V
+                   << " cannot be noalias as it is potentially captured\n");
+        return indicatePessimisticFixpoint();
+      }
     }
 
     // (iii) Check there is no other pointer argument which could alias with the
Index: llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll
===================================================================
--- llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll
+++ llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll
@@ -33,10 +33,10 @@
 ; CHECK-NEXT:    [[ALLOC1:%.*]] = alloca i8, align 8
 ; CHECK-NEXT:    [[ALLOC2:%.*]] = alloca i8, align 8
 ; CHECK-NEXT:    [[THREAD:%.*]] = alloca i64, align 8
-; CHECK-NEXT:    [[CALL:%.*]] = call i32 @pthread_create(i64* nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @foo, i8* noalias nofree readnone null)
-; CHECK-NEXT:    [[CALL1:%.*]] = call i32 @pthread_create(i64* nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @bar, i8* nofree nonnull readnone align 8 dereferenceable(8) bitcast (i8** @GlobalVPtr to i8*))
-; CHECK-NEXT:    [[CALL2:%.*]] = call i32 @pthread_create(i64* nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @baz, i8* noalias nocapture nofree nonnull readnone align 8 dereferenceable(1) [[ALLOC1]])
-; CHECK-NEXT:    [[CALL3:%.*]] = call i32 @pthread_create(i64* nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @buz, i8* nofree nonnull readnone align 8 dereferenceable(1) [[ALLOC2]])
+; CHECK-NEXT:    [[CALL:%.*]] = call i32 @pthread_create(i64* noalias nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @foo, i8* noalias nofree readnone null)
+; CHECK-NEXT:    [[CALL1:%.*]] = call i32 @pthread_create(i64* noalias nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @bar, i8* nofree nonnull readnone align 8 dereferenceable(8) bitcast (i8** @GlobalVPtr to i8*))
+; CHECK-NEXT:    [[CALL2:%.*]] = call i32 @pthread_create(i64* noalias nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @baz, i8* noalias nocapture nofree nonnull readnone align 8 dereferenceable(1) [[ALLOC1]])
+; CHECK-NEXT:    [[CALL3:%.*]] = call i32 @pthread_create(i64* noalias nonnull align 8 dereferenceable(8) [[THREAD]], %union.pthread_attr_t* noalias null, i8* (i8*)* nonnull @buz, i8* noalias nofree nonnull readnone align 8 dereferenceable(1) [[ALLOC2]])
 ; CHECK-NEXT:    ret i32 0
 ;
 entry:
@@ -84,7 +84,7 @@
 
 define internal i8* @buz(i8* %arg) {
 ; CHECK-LABEL: define {{[^@]+}}@buz
-; CHECK-SAME: (i8* nofree nonnull readnone returned align 8 dereferenceable(1) [[ARG:%.*]])
+; CHECK-SAME: (i8* noalias nofree nonnull readnone returned align 8 dereferenceable(1) [[ARG:%.*]])
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    ret i8* [[ARG]]
 ;
Index: llvm/test/Transforms/Attributor/callbacks.ll
===================================================================
--- llvm/test/Transforms/Attributor/callbacks.ll
+++ llvm/test/Transforms/Attributor/callbacks.ll
@@ -23,7 +23,7 @@
 ; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i32* [[B]] to i8*
 ; CHECK-NEXT:    store i32 42, i32* [[B]], align 32
 ; CHECK-NEXT:    store i32* [[B]], i32** [[C]], align 64
-; CHECK-NEXT:    call void (i32*, i32*, void (i32*, i32*, ...)*, ...) @t0_callback_broker(i32* noalias null, i32* nonnull align 128 dereferenceable(4) [[PTR]], void (i32*, i32*, ...)* nonnull bitcast (void (i32*, i32*, i32*, i64, i32**)* @t0_callback_callee to void (i32*, i32*, ...)*), i32* align 256 [[A:%.*]], i64 99, i32** noalias nocapture nonnull readonly align 64 dereferenceable(8) [[C]])
+; CHECK-NEXT:    call void (i32*, i32*, void (i32*, i32*, ...)*, ...) @t0_callback_broker(i32* noalias null, i32* noalias nonnull align 128 dereferenceable(4) [[PTR]], void (i32*, i32*, ...)* nonnull bitcast (void (i32*, i32*, i32*, i64, i32**)* @t0_callback_callee to void (i32*, i32*, ...)*), i32* align 256 [[A:%.*]], i64 99, i32** noalias nocapture nonnull readonly align 64 dereferenceable(8) [[C]])
 
 ; CHECK-NEXT:    ret void
 ;
Index: llvm/test/Transforms/Attributor/noalias.ll
===================================================================
--- llvm/test/Transforms/Attributor/noalias.ll
+++ llvm/test/Transforms/Attributor/noalias.ll
@@ -182,14 +182,14 @@
 
 declare void @test10_helper_1(i8* %a)
 define void @test10_helper_2(i8* noalias %a) {
-; CHECK:   tail call void @test10_helper_1(i8* %a)
+; CHECK:   tail call void @test10_helper_1(i8* noalias %a)
   tail call void @test10_helper_1(i8* %a)
   ret void
 }
 define void @test10(i8* noalias %a) {
 ; CHECK: define void @test10(i8* noalias %a)
 ; FIXME: missing noalias
-; CHECK-NEXT:   tail call void @test10_helper_1(i8* %a)
+; CHECK-NEXT:   tail call void @test10_helper_1(i8* noalias %a)
   tail call void @test10_helper_1(i8* %a)
 
 ; CHECK-NEXT:   tail call void @test10_helper_2(i8* noalias %a)