Index: llvm/lib/Transforms/IPO/AttributorAttributes.cpp
===================================================================
--- llvm/lib/Transforms/IPO/AttributorAttributes.cpp
+++ llvm/lib/Transforms/IPO/AttributorAttributes.cpp
@@ -1980,6 +1980,40 @@
       return true;
     };
 
+    auto InspectCallSiteForUB = [&](Instruction &I) {
+      // Skip instructions that are already saved.
+      if (AssumedNoUBInsts.count(&I) || KnownUBInsts.count(&I))
+        return true;
+      // Check nonnull argument attribute violation for each callsite.
+      CallBase *CB = cast<CallBase>(&I);
+      const Function *Callee = CB->getCalledFunction();
+      if (!Callee)
+        return true;
+      unsigned ArgNum =
+          std::min(CB->getNumArgOperands(), (unsigned)Callee->arg_size());
+      for (unsigned idx = 0; idx < ArgNum; idx++) {
+        Value *ArgVal = CB->getArgOperand(idx);
+        const auto &ValueSimplifyAA =
+            A.getAAFor<AAValueSimplify>(*this, IRPosition::value(*ArgVal));
+        Optional<Value *> SimplifiedVal =
+            ValueSimplifyAA.getAssumedSimplifiedValue(A);
+
+        // If current argument is known to be simplified to null pointer,
+        // this callsite is considered UB.
+        if (!ValueSimplifyAA.isKnown() || !SimplifiedVal.hasValue())
+          continue;
+        if (!isa<ConstantPointerNull>(*SimplifiedVal.getValue()))
+          continue;
+        auto &NonNullAA = A.getAAFor<AANonNull>(
+            *this, IRPosition::argument(*Callee->getArg(idx)));
+        if (NonNullAA.isKnownNonNull()) {
+          KnownUBInsts.insert(&I);
+          return true;
+        }
+      }
+      return true;
+    };
+
     A.checkForAllInstructions(InspectMemAccessInstForUB, *this,
                               {Instruction::Load, Instruction::Store,
                                Instruction::AtomicCmpXchg,
@@ -1987,6 +2021,7 @@
                               /* CheckBBLivenessOnly */ true);
     A.checkForAllInstructions(InspectBrInstForUB, *this, {Instruction::Br},
                               /* CheckBBLivenessOnly */ true);
+    A.checkForAllCallLikeInstructions(InspectCallSiteForUB, *this);
     if (NoUBPrevSize != AssumedNoUBInsts.size() ||
         UBPrevSize != KnownUBInsts.size())
       return ChangeStatus::CHANGED;
Index: llvm/test/Transforms/Attributor/undefined_behavior.ll
===================================================================
--- llvm/test/Transforms/Attributor/undefined_behavior.ll
+++ llvm/test/Transforms/Attributor/undefined_behavior.ll
@@ -579,3 +579,139 @@
   %X = call i32 @callee(i1 false, i32* null)
   ret i32 %X
 }
+
+; Tests for nonnull attribute violation.
+
+define void @arg_nonnull(i32* nonnull %a) {
+; IS__TUNIT____: Function Attrs: argmemonly nofree nosync nounwind willreturn writeonly
+; IS__TUNIT____-LABEL: define {{[^@]+}}@arg_nonnull
+; IS__TUNIT____-SAME: (i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[A:%.*]])
+; IS__TUNIT____-NEXT:    store i32 0, i32* [[A]], align 4
+; IS__TUNIT____-NEXT:    ret void
+;
+; IS__CGSCC____: Function Attrs: argmemonly nofree norecurse nosync nounwind willreturn writeonly
+; IS__CGSCC____-LABEL: define {{[^@]+}}@arg_nonnull
+; IS__CGSCC____-SAME: (i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[A:%.*]])
+; IS__CGSCC____-NEXT:    store i32 0, i32* [[A]], align 4
+; IS__CGSCC____-NEXT:    ret void
+;
+  store i32 0, i32* %a
+  ret void
+}
+
+define void @arg_nonnull2(i32* nonnull %a, i32* nonnull %b, i32* %c) {
+; IS__TUNIT____: Function Attrs: argmemonly nofree nosync nounwind willreturn writeonly
+; IS__TUNIT____-LABEL: define {{[^@]+}}@arg_nonnull2
+; IS__TUNIT____-SAME: (i32* nocapture nofree nonnull writeonly [[A:%.*]], i32* nocapture nofree nonnull writeonly [[B:%.*]], i32* nofree writeonly [[C:%.*]])
+; IS__TUNIT____-NEXT:    [[D:%.*]] = icmp eq i32* [[C]], null
+; IS__TUNIT____-NEXT:    br i1 [[D]], label [[T:%.*]], label [[F:%.*]]
+; IS__TUNIT____:       t:
+; IS__TUNIT____-NEXT:    store i32 0, i32* [[A]], align 4
+; IS__TUNIT____-NEXT:    br label [[RET:%.*]]
+; IS__TUNIT____:       f:
+; IS__TUNIT____-NEXT:    store i32 1, i32* [[B]], align 4
+; IS__TUNIT____-NEXT:    br label [[RET]]
+; IS__TUNIT____:       ret:
+; IS__TUNIT____-NEXT:    ret void
+;
+; IS__CGSCC____: Function Attrs: argmemonly nofree norecurse nosync nounwind willreturn writeonly
+; IS__CGSCC____-LABEL: define {{[^@]+}}@arg_nonnull2
+; IS__CGSCC____-SAME: (i32* nocapture nofree nonnull writeonly [[A:%.*]], i32* nocapture nofree nonnull writeonly [[B:%.*]], i32* nofree writeonly [[C:%.*]])
+; IS__CGSCC____-NEXT:    [[D:%.*]] = icmp eq i32* [[C]], null
+; IS__CGSCC____-NEXT:    br i1 [[D]], label [[T:%.*]], label [[F:%.*]]
+; IS__CGSCC____:       t:
+; IS__CGSCC____-NEXT:    store i32 0, i32* [[A]], align 4
+; IS__CGSCC____-NEXT:    br label [[RET:%.*]]
+; IS__CGSCC____:       f:
+; IS__CGSCC____-NEXT:    store i32 1, i32* [[B]], align 4
+; IS__CGSCC____-NEXT:    br label [[RET]]
+; IS__CGSCC____:       ret:
+; IS__CGSCC____-NEXT:    ret void
+;
+  %d = icmp eq i32* %c, null
+  br i1 %d, label %t, label %f
+t:
+  store i32 0, i32* %a
+  br label %ret
+f:
+  store i32 1, i32* %b
+  br label %ret
+ret:
+  ret void
+}
+
+; Pass null directly to argument with nonnull attribute
+define void @arg_nonnull_violation1() {
+; IS__TUNIT____: Function Attrs: nofree nosync nounwind readnone willreturn
+; IS__TUNIT____-LABEL: define {{[^@]+}}@arg_nonnull_violation1()
+; IS__TUNIT____-NEXT:    unreachable
+;
+; IS__CGSCC____: Function Attrs: nofree norecurse nosync nounwind readnone willreturn
+; IS__CGSCC____-LABEL: define {{[^@]+}}@arg_nonnull_violation1()
+; IS__CGSCC____-NEXT:    unreachable
+;
+  call void @arg_nonnull(i32* null)
+  ret void
+}
+
+; A case that depends on value simplification
+define void @arg_nonnull_violation2(i1 %c) {
+; IS__TUNIT____: Function Attrs: nofree nosync nounwind readnone willreturn
+; IS__TUNIT____-LABEL: define {{[^@]+}}@arg_nonnull_violation2
+; IS__TUNIT____-SAME: (i1 [[C:%.*]])
+; IS__TUNIT____-NEXT:    unreachable
+;
+; IS__CGSCC____: Function Attrs: nofree norecurse nosync nounwind readnone willreturn
+; IS__CGSCC____-LABEL: define {{[^@]+}}@arg_nonnull_violation2
+; IS__CGSCC____-SAME: (i1 [[C:%.*]])
+; IS__CGSCC____-NEXT:    unreachable
+;
+  %null = getelementptr i32, i32* null, i32 0
+  %mustnull = select i1 %c, i32* null, i32* %null
+  call void @arg_nonnull(i32* %mustnull)
+  ret void
+}
+
+; Cases for single and multiple violation at a callsite
+define void @arg_nonnull_violation3(i1 %c) {
+; IS__TUNIT____: Function Attrs: nofree nosync nounwind readnone willreturn
+; IS__TUNIT____-LABEL: define {{[^@]+}}@arg_nonnull_violation3
+; IS__TUNIT____-SAME: (i1 [[C:%.*]])
+; IS__TUNIT____-NEXT:    [[PTR:%.*]] = alloca i32, align 4
+; IS__TUNIT____-NEXT:    br i1 [[C]], label [[T:%.*]], label [[F:%.*]]
+; IS__TUNIT____:       t:
+; IS__TUNIT____-NEXT:    call void @arg_nonnull2(i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* noalias nocapture nofree writeonly align 536870912 null)
+; IS__TUNIT____-NEXT:    unreachable
+; IS__TUNIT____:       f:
+; IS__TUNIT____-NEXT:    call void @arg_nonnull2(i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]])
+; IS__TUNIT____-NEXT:    unreachable
+; IS__TUNIT____:       ret:
+; IS__TUNIT____-NEXT:    ret void
+;
+; IS__CGSCC____: Function Attrs: nofree norecurse nosync nounwind readnone willreturn
+; IS__CGSCC____-LABEL: define {{[^@]+}}@arg_nonnull_violation3
+; IS__CGSCC____-SAME: (i1 [[C:%.*]])
+; IS__CGSCC____-NEXT:    [[PTR:%.*]] = alloca i32, align 4
+; IS__CGSCC____-NEXT:    br i1 [[C]], label [[T:%.*]], label [[F:%.*]]
+; IS__CGSCC____:       t:
+; IS__CGSCC____-NEXT:    call void @arg_nonnull2(i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* noalias nocapture nofree writeonly align 536870912 null)
+; IS__CGSCC____-NEXT:    unreachable
+; IS__CGSCC____:       f:
+; IS__CGSCC____-NEXT:    call void @arg_nonnull2(i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* nocapture nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]], i32* nofree nonnull writeonly align 4 dereferenceable(4) [[PTR]])
+; IS__CGSCC____-NEXT:    unreachable
+; IS__CGSCC____:       ret:
+; IS__CGSCC____-NEXT:    ret void
+;
+  %ptr = alloca i32
+  br i1 %c, label %t, label %f
+t:
+  call void @arg_nonnull2(i32* %ptr, i32* %ptr, i32* null)
+  call void @arg_nonnull2(i32* null, i32* %ptr, i32* %ptr)
+  br label %ret
+f:
+  call void @arg_nonnull2(i32* %ptr, i32* %ptr, i32* %ptr)
+  call void @arg_nonnull2(i32* null, i32* null, i32* %ptr)
+  br label %ret
+ret:
+  ret void
+}
Index: llvm/test/Transforms/Attributor/value-simplify.ll
===================================================================
--- llvm/test/Transforms/Attributor/value-simplify.ll
+++ llvm/test/Transforms/Attributor/value-simplify.ll
@@ -391,12 +391,6 @@
 
 define internal void @test_sret(%struct.X* sret %a, %struct.X** %b) {
 ;
-; IS__TUNIT____: Function Attrs: argmemonly nofree nosync nounwind willreturn writeonly
-; IS__TUNIT____-LABEL: define {{[^@]+}}@test_sret
-; IS__TUNIT____-SAME: (%struct.X* noalias nofree nonnull sret writeonly align 536870912 dereferenceable(8) [[A:%.*]], %struct.X** nocapture nofree nonnull writeonly align 8 dereferenceable(8) [[B:%.*]])
-; IS__TUNIT____-NEXT:    store %struct.X* [[A]], %struct.X** [[B]], align 8
-; IS__TUNIT____-NEXT:    ret void
-;
 ; IS__CGSCC____: Function Attrs: argmemonly nofree norecurse nosync nounwind willreturn writeonly
 ; IS__CGSCC____-LABEL: define {{[^@]+}}@test_sret
 ; IS__CGSCC____-SAME: (%struct.X* noalias nofree nonnull sret writeonly align 536870912 dereferenceable(8) [[A:%.*]], %struct.X** nocapture nofree nonnull writeonly align 8 dereferenceable(8) [[B:%.*]])
@@ -412,14 +406,12 @@
 ; IS__TUNIT____: Function Attrs: argmemonly nofree nosync nounwind willreturn writeonly
 ; IS__TUNIT____-LABEL: define {{[^@]+}}@complicated_args_sret
 ; IS__TUNIT____-SAME: (%struct.X** nocapture nofree writeonly [[B:%.*]])
-; IS__TUNIT____-NEXT:    call void @test_sret(%struct.X* noalias nocapture nofree writeonly align 536870912 null, %struct.X** nocapture nofree writeonly align 8 [[B]])
-; IS__TUNIT____-NEXT:    ret void
+; IS__TUNIT____-NEXT:    unreachable
 ;
 ; IS__CGSCC____: Function Attrs: argmemonly nofree norecurse nosync nounwind willreturn writeonly
 ; IS__CGSCC____-LABEL: define {{[^@]+}}@complicated_args_sret
 ; IS__CGSCC____-SAME: (%struct.X** nocapture nofree nonnull writeonly align 8 dereferenceable(8) [[B:%.*]])
-; IS__CGSCC____-NEXT:    call void @test_sret(%struct.X* noalias nocapture nofree nonnull writeonly align 536870912 dereferenceable(8) null, %struct.X** nocapture nofree nonnull writeonly align 8 dereferenceable(8) [[B]])
-; IS__CGSCC____-NEXT:    ret void
+; IS__CGSCC____-NEXT:    unreachable
 ;
   call void @test_sret(%struct.X* null, %struct.X** %b)
   ret void