diff --git a/llvm/include/llvm/CodeGen/StackProtector.h b/llvm/include/llvm/CodeGen/StackProtector.h
--- a/llvm/include/llvm/CodeGen/StackProtector.h
+++ b/llvm/include/llvm/CodeGen/StackProtector.h
@@ -95,7 +95,7 @@
                                 bool InStruct = false) const;
 
   /// Check whether a stack allocation has its address taken.
-  bool HasAddressTaken(const Instruction *AI);
+  bool HasAddressTaken(const Instruction *AI, uint64_t AllocSize);
 
   /// RequiresStackProtector - Check whether or not this function needs a
   /// stack protector based upon the stack protector level.
diff --git a/llvm/lib/CodeGen/StackProtector.cpp b/llvm/lib/CodeGen/StackProtector.cpp
--- a/llvm/lib/CodeGen/StackProtector.cpp
+++ b/llvm/lib/CodeGen/StackProtector.cpp
@@ -18,6 +18,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/Analysis/EHPersonalities.h"
+#include "llvm/Analysis/MemoryLocation.h"
 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/CodeGen/TargetLowering.h"
@@ -161,9 +162,16 @@
   return NeedsProtector;
 }
 
-bool StackProtector::HasAddressTaken(const Instruction *AI) {
+bool StackProtector::HasAddressTaken(const Instruction *AI,
+                                     uint64_t AllocSize) {
+  const DataLayout &DL = M->getDataLayout();
   for (const User *U : AI->users()) {
     const auto *I = cast<Instruction>(U);
+    // If this instruction accesses memory make sure it doesn't access beyond
+    // the bounds of the allocated object.
+    Optional<MemoryLocation> MemLoc = MemoryLocation::getOrNone(I);
+    if (MemLoc.hasValue() && MemLoc->Size.getValue() > AllocSize)
+      return true;
     switch (I->getOpcode()) {
     case Instruction::Store:
       if (AI == cast<StoreInst>(I)->getValueOperand())
@@ -189,11 +197,26 @@
     }
     case Instruction::Invoke:
       return true;
+    case Instruction::GetElementPtr: {
+      // If the GEP offset is out-of-bounds, or is non-constant and so has to be
+      // assumed to be potentially out-of-bounds, then any memory access that
+      // would use it could also be out-of-bounds meaning stack protection is
+      // required.
+      const GetElementPtrInst *GEP = cast<GetElementPtrInst>(I);
+      unsigned TypeSize = DL.getIndexTypeSizeInBits(I->getType());
+      APInt Offset(TypeSize, 0);
+      APInt MaxOffset(TypeSize, AllocSize);
+      if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.ugt(MaxOffset))
+        return true;
+      // Adjust AllocSize to be the space remaining after this offset.
+      if (HasAddressTaken(I, AllocSize - Offset.getLimitedValue()))
+        return true;
+      break;
+    }
     case Instruction::BitCast:
-    case Instruction::GetElementPtr:
     case Instruction::Select:
     case Instruction::AddrSpaceCast:
-      if (HasAddressTaken(I))
+      if (HasAddressTaken(I, AllocSize))
         return true;
       break;
     case Instruction::PHI: {
@@ -201,7 +224,7 @@
       // they are only visited once.
       const auto *PN = cast<PHINode>(I);
       if (VisitedPHIs.insert(PN).second)
-        if (HasAddressTaken(PN))
+        if (HasAddressTaken(PN, AllocSize))
           return true;
       break;
     }
@@ -330,7 +353,8 @@
           continue;
         }
 
-        if (Strong && HasAddressTaken(AI)) {
+        if (Strong && HasAddressTaken(AI, M->getDataLayout().getTypeAllocSize(
+                                              AI->getAllocatedType()))) {
           ++NumAddrTaken;
           Layout.insert(std::make_pair(AI, MachineFrameInfo::SSPLK_AddrOf));
           ORE.emit([&]() {
@@ -342,6 +366,9 @@
           });
           NeedsProtector = true;
         }
+        // Clear any PHIs that we visited, to make sure we examine all uses of
+        // any subsequent allocas that we look at.
+        VisitedPHIs.clear();
       }
     }
   }
diff --git a/llvm/test/CodeGen/X86/stack-guard-oob.ll b/llvm/test/CodeGen/X86/stack-guard-oob.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/stack-guard-oob.ll
@@ -0,0 +1,415 @@
+; RUN: llc -mtriple=i686 -O0 < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64 -O0 < %s | FileCheck %s
+
+; CHECK-LABEL: in_bounds:
+; CHECK-NOT: __stack_chk_guard
+define i32 @in_bounds() #0 {
+  %var = alloca i32, align 4
+  store i32 0, i32* %var, align 4
+  %gep = getelementptr inbounds i32, i32* %var, i32 0
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: constant_out_of_bounds:
+; CHECK: __stack_chk_guard
+define i32 @constant_out_of_bounds() #0 {
+  %var = alloca i32, align 4
+  store i32 0, i32* %var, align 4
+  %gep = getelementptr inbounds i32, i32* %var, i32 1
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: nonconstant_out_of_bounds:
+; CHECK: __stack_chk_guard
+define i32 @nonconstant_out_of_bounds(i32 %n) #0 {
+  %var = alloca i32, align 4
+  store i32 0, i32* %var, align 4
+  %gep = getelementptr inbounds i32, i32* %var, i32 %n
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_before_gep_in_bounds:
+; CHECK-NOT: __stack_chk_guard
+define i32 @phi_before_gep_in_bounds(i32 %k) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %then
+
+if:
+  br label %then
+
+then:
+  %ptr = phi i32* [ %var1, %entry ], [ %var2, %if ]
+  %gep = getelementptr inbounds i32, i32* %ptr, i32 0
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_before_gep_constant_out_of_bounds:
+; CHECK: __stack_chk_guard
+define i32 @phi_before_gep_constant_out_of_bounds(i32 %k) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %then
+
+if:
+  br label %then
+
+then:
+  %ptr = phi i32* [ %var1, %entry ], [ %var2, %if ]
+  %gep = getelementptr inbounds i32, i32* %ptr, i32 1
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_before_gep_nonconstant_out_of_bounds:
+; CHECK: __stack_chk_guard
+define i32 @phi_before_gep_nonconstant_out_of_bounds(i32 %k, i32 %n) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %then
+
+if:
+  br label %then
+
+then:
+  %ptr = phi i32* [ %var1, %entry ], [ %var2, %if ]
+  %gep = getelementptr inbounds i32, i32* %ptr, i32 %n
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_after_gep_in_bounds:
+; CHECK-NOT: __stack_chk_guard
+define i32 @phi_after_gep_in_bounds(i32 %k) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %else
+
+if:
+  %gep1 = getelementptr inbounds i32, i32* %var1, i32 0
+  br label %then
+
+else:
+  %gep2 = getelementptr inbounds i32, i32* %var2, i32 0
+  br label %then
+
+then:
+  %ptr = phi i32* [ %gep1, %if ], [ %gep2, %else ]
+  %ret = load i32, i32* %ptr, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_after_gep_constant_out_of_bounds_a:
+; CHECK: __stack_chk_guard
+define i32 @phi_after_gep_constant_out_of_bounds_a(i32 %k) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %else
+
+if:
+  %gep1 = getelementptr inbounds i32, i32* %var1, i32 0
+  br label %then
+
+else:
+  %gep2 = getelementptr inbounds i32, i32* %var2, i32 1
+  br label %then
+
+then:
+  %ptr = phi i32* [ %gep1, %if ], [ %gep2, %else ]
+  %ret = load i32, i32* %ptr, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_after_gep_constant_out_of_bounds_b:
+; CHECK: __stack_chk_guard
+define i32 @phi_after_gep_constant_out_of_bounds_b(i32 %k) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %else
+
+if:
+  %gep1 = getelementptr inbounds i32, i32* %var1, i32 1
+  br label %then
+
+else:
+  %gep2 = getelementptr inbounds i32, i32* %var2, i32 0
+  br label %then
+
+then:
+  %ptr = phi i32* [ %gep1, %if ], [ %gep2, %else ]
+  %ret = load i32, i32* %ptr, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_different_types_a:
+; CHECK: __stack_chk_guard
+define i64 @phi_different_types_a(i32 %k) #0 {
+entry:
+  %var1 = alloca i64, align 4
+  %var2 = alloca i32, align 4
+  store i64 0, i64* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %then
+
+if:
+  %bitcast = bitcast i32* %var2 to i64*
+  br label %then
+
+then:
+  %ptr = phi i64* [ %var1, %entry ], [ %bitcast, %if ]
+  %ret = load i64, i64* %ptr, align 4
+  ret i64 %ret
+}
+
+; CHECK-LABEL: phi_different_types_b:
+; CHECK: __stack_chk_guard
+define i64 @phi_different_types_b(i32 %k) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i64, align 4
+  store i32 0, i32* %var1, align 4
+  store i64 0, i64* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %then
+
+if:
+  %bitcast = bitcast i32* %var1 to i64*
+  br label %then
+
+then:
+  %ptr = phi i64* [ %var2, %entry ], [ %bitcast, %if ]
+  %ret = load i64, i64* %ptr, align 4
+  ret i64 %ret
+}
+
+; CHECK-LABEL: phi_after_gep_nonconstant_out_of_bounds_a:
+; CHECK: __stack_chk_guard
+define i32 @phi_after_gep_nonconstant_out_of_bounds_a(i32 %k, i32 %n) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %else
+
+if:
+  %gep1 = getelementptr inbounds i32, i32* %var1, i32 0
+  br label %then
+
+else:
+  %gep2 = getelementptr inbounds i32, i32* %var2, i32 %n
+  br label %then
+
+then:
+  %ptr = phi i32* [ %gep1, %if ], [ %gep2, %else ]
+  %ret = load i32, i32* %ptr, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: phi_after_gep_nonconstant_out_of_bounds_b:
+; CHECK: __stack_chk_guard
+define i32 @phi_after_gep_nonconstant_out_of_bounds_b(i32 %k, i32 %n) #0 {
+entry:
+  %var1 = alloca i32, align 4
+  %var2 = alloca i32, align 4
+  store i32 0, i32* %var1, align 4
+  store i32 0, i32* %var2, align 4
+  %cmp = icmp ne i32 %k, 0
+  br i1 %cmp, label %if, label %else
+
+if:
+  %gep1 = getelementptr inbounds i32, i32* %var1, i32 %n
+  br label %then
+
+else:
+  %gep2 = getelementptr inbounds i32, i32* %var2, i32 0
+  br label %then
+
+then:
+  %ptr = phi i32* [ %gep1, %if ], [ %gep2, %else ]
+  %ret = load i32, i32* %ptr, align 4
+  ret i32 %ret
+}
+
+%struct.outer = type { %struct.inner, %struct.inner }
+%struct.inner = type { i32, i32 }
+
+; CHECK-LABEL: struct_in_bounds:
+; CHECK-NOT: __stack_chk_guard
+define void @struct_in_bounds() #0 {
+  %var = alloca %struct.outer, align 4
+  %outergep = getelementptr inbounds %struct.outer, %struct.outer* %var, i32 0, i32 1
+  %innergep = getelementptr inbounds %struct.inner, %struct.inner* %outergep, i32 0, i32 1
+  store i32 0, i32* %innergep, align 4
+  ret void
+}
+
+; CHECK-LABEL: struct_constant_out_of_bounds_a:
+; CHECK: __stack_chk_guard
+define void @struct_constant_out_of_bounds_a() #0 {
+  %var = alloca %struct.outer, align 4
+  %outergep = getelementptr inbounds %struct.outer, %struct.outer* %var, i32 1, i32 0
+  %innergep = getelementptr inbounds %struct.inner, %struct.inner* %outergep, i32 0, i32 0
+  store i32 0, i32* %innergep, align 4
+  ret void
+}
+
+; CHECK-LABEL: struct_constant_out_of_bounds_b:
+; Here the offset is out-of-bounds of the addressed struct.inner member, but
+; still within bounds of the outer struct so no stack guard is needed.
+; CHECK-NOT: __stack_chk_guard
+define void @struct_constant_out_of_bounds_b() #0 {
+  %var = alloca %struct.outer, align 4
+  %outergep = getelementptr inbounds %struct.outer, %struct.outer* %var, i32 0, i32 0
+  %innergep = getelementptr inbounds %struct.inner, %struct.inner* %outergep, i32 1, i32 0
+  store i32 0, i32* %innergep, align 4
+  ret void
+}
+
+; CHECK-LABEL: struct_constant_out_of_bounds_c:
+; Here we are out-of-bounds of both the inner and outer struct.
+; CHECK: __stack_chk_guard
+define void @struct_constant_out_of_bounds_c() #0 {
+  %var = alloca %struct.outer, align 4
+  %outergep = getelementptr inbounds %struct.outer, %struct.outer* %var, i32 0, i32 1
+  %innergep = getelementptr inbounds %struct.inner, %struct.inner* %outergep, i32 1, i32 0
+  store i32 0, i32* %innergep, align 4
+  ret void
+}
+
+; CHECK-LABEL: struct_nonconstant_out_of_bounds_a:
+; CHECK: __stack_chk_guard
+define void @struct_nonconstant_out_of_bounds_a(i32 %n) #0 {
+  %var = alloca %struct.outer, align 4
+  %outergep = getelementptr inbounds %struct.outer, %struct.outer* %var, i32 %n, i32 0
+  %innergep = getelementptr inbounds %struct.inner, %struct.inner* %outergep, i32 0, i32 0
+  store i32 0, i32* %innergep, align 4
+  ret void
+}
+
+; CHECK-LABEL: struct_nonconstant_out_of_bounds_b:
+; CHECK: __stack_chk_guard
+define void @struct_nonconstant_out_of_bounds_b(i32 %n) #0 {
+  %var = alloca %struct.outer, align 4
+  %outergep = getelementptr inbounds %struct.outer, %struct.outer* %var, i32 0, i32 0
+  %innergep = getelementptr inbounds %struct.inner, %struct.inner* %outergep, i32 %n, i32 0
+  store i32 0, i32* %innergep, align 4
+  ret void
+}
+
+; CHECK-LABEL: bitcast_smaller_load
+; CHECK-NOT: __stack_chk_guard
+define i32 @bitcast_smaller_load() #0 {
+  %var = alloca i64, align 4
+  store i64 0, i64* %var, align 4
+  %bitcast = bitcast i64* %var to i32*
+  %ret = load i32, i32* %bitcast, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: bitcast_same_size_load
+; CHECK-NOT: __stack_chk_guard
+define i32 @bitcast_same_size_load() #0 {
+  %var = alloca i64, align 4
+  store i64 0, i64* %var, align 4
+  %bitcast = bitcast i64* %var to %struct.inner*
+  %gep = getelementptr inbounds %struct.inner, %struct.inner* %bitcast, i32 0, i32 1
+  %ret = load i32, i32* %gep, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: bitcast_larger_load
+; CHECK: __stack_chk_guard
+define i64 @bitcast_larger_load() #0 {
+  %var = alloca i32, align 4
+  store i32 0, i32* %var, align 4
+  %bitcast = bitcast i32* %var to i64*
+  %ret = load i64, i64* %bitcast, align 4
+  ret i64 %ret
+}
+
+; CHECK-LABEL: bitcast_larger_store
+; CHECK: __stack_chk_guard
+define i32 @bitcast_larger_store() #0 {
+  %var = alloca i32, align 4
+  %bitcast = bitcast i32* %var to i64*
+  store i64 0, i64* %bitcast, align 4
+  %ret = load i32, i32* %var, align 4
+  ret i32 %ret
+}
+
+; CHECK-LABEL: bitcast_larger_cmpxchg
+; CHECK: __stack_chk_guard
+define i64 @bitcast_larger_cmpxchg(i64 %desired, i64 %new) #0 {
+  %var = alloca i32, align 4
+  %bitcast = bitcast i32* %var to i64*
+  %pair = cmpxchg i64* %bitcast, i64 %desired, i64 %new seq_cst monotonic
+  %ret = extractvalue { i64, i1 } %pair, 0
+  ret i64 %ret
+}
+
+; CHECK-LABEL: bitcast_larger_atomic_rmw
+; CHECK: __stack_chk_guard
+define i64 @bitcast_larger_atomic_rmw() #0 {
+  %var = alloca i32, align 4
+  %bitcast = bitcast i32* %var to i64*
+  %ret = atomicrmw add i64* %bitcast, i64 1 monotonic
+  ret i64 %ret
+}
+
+%struct.packed = type <{ i16, i32 }>
+
+; CHECK-LABEL: bitcast_overlap
+; CHECK: __stack_chk_guard
+define i32 @bitcast_overlap() #0 {
+  %var = alloca i32, align 4
+  %bitcast = bitcast i32* %var to %struct.packed*
+  %gep = getelementptr inbounds %struct.packed, %struct.packed* %bitcast, i32 0, i32 1
+  %ret = load i32, i32* %gep, align 2
+  ret i32 %ret
+}
+
+%struct.multi_dimensional = type { [10 x [10 x i32]], i32 }
+
+; CHECK-LABEL: multi_dimensional_array
+; CHECK: __stack_chk_guard
+define i32 @multi_dimensional_array() #0 {
+  %var = alloca %struct.multi_dimensional, align 4
+  %gep1 = getelementptr inbounds %struct.multi_dimensional, %struct.multi_dimensional* %var, i32 0, i32 0
+  %gep2 = getelementptr inbounds [10 x [10 x i32]], [10 x [10 x i32]]* %gep1, i32 0, i32 10
+  %gep3 = getelementptr inbounds [10 x i32], [10 x i32]* %gep2, i32 0, i32 5
+  %ret = load i32, i32* %gep3, align 4
+  ret i32 %ret
+}
+
+attributes #0 = { sspstrong }