Index: llvm/trunk/lib/Transforms/Scalar/LICM.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/LICM.cpp
+++ llvm/trunk/lib/Transforms/Scalar/LICM.cpp
@@ -81,6 +81,11 @@
     DisablePromotion("disable-licm-promotion", cl::Hidden,
                      cl::desc("Disable memory promotion in LICM pass"));
 
+static cl::opt<uint32_t> MaxNumUsesTraversed(
+    "licm-max-num-uses-traversed", cl::Hidden, cl::init(8),
+    cl::desc("Max num uses visited for identifying load "
+             "invariance in loop using invariant start (default = 8)"));
+
 static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI);
 static bool isNotUsedInLoop(const Instruction &I, const Loop *CurLoop,
                             const LoopSafetyInfo *SafetyInfo);
@@ -480,6 +485,59 @@
         SafetyInfo->BlockColors = colorEHFunclets(*Fn);
 }
 
+// Return true if LI is invariant within scope of the loop. LI is invariant if
+// CurLoop is dominated by an invariant.start representing the same memory location
+// and size as the memory location LI loads from, and also the invariant.start
+// has no uses.
+static bool isLoadInvariantInLoop(LoadInst *LI, DominatorTree *DT,
+                                  Loop *CurLoop) {
+  Value *Addr = LI->getOperand(0);
+  const DataLayout &DL = LI->getModule()->getDataLayout();
+  const uint32_t LocSizeInBits = DL.getTypeSizeInBits(
+      cast<PointerType>(Addr->getType())->getElementType());
+
+  // if the type is i8 addrspace(x)*, we know this is the type of
+  // llvm.invariant.start operand
+  auto *PtrInt8Ty = PointerType::get(Type::getInt8Ty(LI->getContext()),
+                                     LI->getPointerAddressSpace());
+  unsigned BitcastsVisited = 0;
+  // Look through bitcasts until we reach the i8* type (this is invariant.start
+  // operand type).
+  while (Addr->getType() != PtrInt8Ty) {
+    auto *BC = dyn_cast<BitCastInst>(Addr);
+    // Avoid traversing high number of bitcast uses.
+    if (++BitcastsVisited > MaxNumUsesTraversed || !BC)
+      return false;
+    Addr = BC->getOperand(0);
+  }
+
+  unsigned UsesVisited = 0;
+  // Traverse all uses of the load operand value, to see if invariant.start is
+  // one of the uses, and whether it dominates the load instruction.
+  for (auto *U : Addr->users()) {
+    // Avoid traversing for Load operand with high number of users.
+    if (++UsesVisited > MaxNumUsesTraversed)
+      return false;
+    IntrinsicInst *II = dyn_cast<IntrinsicInst>(U);
+    // If there are escaping uses of invariant.start instruction, the load maybe
+    // non-invariant.
+    if (!II || II->getIntrinsicID() != Intrinsic::invariant_start ||
+        II->hasNUsesOrMore(1))
+      continue;
+    unsigned InvariantSizeInBits =
+        cast<ConstantInt>(II->getArgOperand(0))->getSExtValue() * 8;
+    // Confirm the invariant.start location size contains the load operand size
+    // in bits. Also, the invariant.start should dominate the load, and we
+    // should not hoist the load out of a loop that contains this dominating
+    // invariant.start.
+    if (LocSizeInBits <= InvariantSizeInBits &&
+        DT->properlyDominates(II->getParent(), CurLoop->getHeader()))
+      return true;
+  }
+
+  return false;
+}
+
 bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
                               Loop *CurLoop, AliasSetTracker *CurAST,
                               LoopSafetyInfo *SafetyInfo,
@@ -496,6 +554,10 @@
     if (LI->getMetadata(LLVMContext::MD_invariant_load))
       return true;
 
+    // This checks for an invariant.start dominating the load.
+    if (isLoadInvariantInLoop(LI, DT, CurLoop))
+      return true;
+
     // Don't hoist loads which have may-aliased stores in loop.
     uint64_t Size = 0;
     if (LI->getType()->isSized())
Index: llvm/trunk/test/Transforms/LICM/hoisting.ll
===================================================================
--- llvm/trunk/test/Transforms/LICM/hoisting.ll
+++ llvm/trunk/test/Transforms/LICM/hoisting.ll
@@ -149,3 +149,174 @@
 return:
   ret i32 %sum
 }
+
+declare {}* @llvm.invariant.start.p0i8(i64, i8* nocapture) nounwind readonly
+declare void @llvm.invariant.end.p0i8({}*, i64, i8* nocapture) nounwind
+declare void @escaping.invariant.start({}*) nounwind
+; invariant.start dominates the load, and in this scope, the
+; load is invariant. So, we can hoist the `addrld` load out of the loop.
+define i32 @test_fence(i8* %addr, i32 %n, i8* %volatile) {
+; CHECK-LABEL: @test_fence
+; CHECK-LABEL: entry
+; CHECK: invariant.start
+; CHECK: %addrld = load atomic i32, i32* %addr.i unordered, align 8
+; CHECK: br label %loop
+entry: 
+  %gep = getelementptr inbounds i8, i8* %addr, i64 8
+  %addr.i = bitcast i8* %gep to i32 *
+  store atomic i32 5, i32 * %addr.i unordered, align 8
+  fence release
+  %invst = call {}* @llvm.invariant.start.p0i8(i64 4, i8* %gep)
+  br label %loop
+
+loop:
+  %indvar = phi i32 [ %indvar.next, %loop ], [ 0, %entry ]
+  %sum = phi i32 [ %sum.next, %loop ], [ 0, %entry ]
+  %volload = load atomic i8, i8* %volatile unordered, align 8
+  fence acquire
+  %volchk = icmp eq i8 %volload, 0
+  %addrld = load atomic i32, i32* %addr.i unordered, align 8
+  %sel = select i1 %volchk, i32 0, i32 %addrld
+  %sum.next = add i32 %sel, %sum
+  %indvar.next = add i32 %indvar, 1
+  %cond = icmp slt i32 %indvar.next, %n
+  br i1 %cond, label %loop, label %loopexit
+
+loopexit:
+  ret i32 %sum
+}
+
+
+
+; Same as test above, but the load is no longer invariant (presence of
+; invariant.end). We cannot hoist the addrld out of loop.
+define i32 @test_fence1(i8* %addr, i32 %n, i8* %volatile) {
+; CHECK-LABEL: @test_fence1
+; CHECK-LABEL: entry
+; CHECK: invariant.start
+; CHECK-NEXT: invariant.end
+; CHECK-NEXT: br label %loop
+entry:
+  %gep = getelementptr inbounds i8, i8* %addr, i64 8
+  %addr.i = bitcast i8* %gep to i32 *
+  store atomic i32 5, i32 * %addr.i unordered, align 8
+  fence release
+  %invst = call {}* @llvm.invariant.start.p0i8(i64 4, i8* %gep)
+  call void @llvm.invariant.end.p0i8({}* %invst, i64 4, i8* %gep)
+  br label %loop
+
+loop:
+  %indvar = phi i32 [ %indvar.next, %loop ], [ 0, %entry ]
+  %sum = phi i32 [ %sum.next, %loop ], [ 0, %entry ]
+  %volload = load atomic i8, i8* %volatile unordered, align 8
+  fence acquire
+  %volchk = icmp eq i8 %volload, 0
+  %addrld = load atomic i32, i32* %addr.i unordered, align 8
+  %sel = select i1 %volchk, i32 0, i32 %addrld
+  %sum.next = add i32 %sel, %sum
+  %indvar.next = add i32 %indvar, 1
+  %cond = icmp slt i32 %indvar.next, %n
+  br i1 %cond, label %loop, label %loopexit
+
+loopexit:
+  ret i32 %sum
+}
+
+; same as test above, but instead of invariant.end, we have the result of
+; invariant.start escaping through a call. We cannot hoist the load.
+define i32 @test_fence2(i8* %addr, i32 %n, i8* %volatile) {
+; CHECK-LABEL: @test_fence2
+; CHECK-LABEL: entry
+; CHECK-NOT: load
+; CHECK: br label %loop
+entry:
+  %gep = getelementptr inbounds i8, i8* %addr, i64 8
+  %addr.i = bitcast i8* %gep to i32 *
+  store atomic i32 5, i32 * %addr.i unordered, align 8
+  fence release
+  %invst = call {}* @llvm.invariant.start.p0i8(i64 4, i8* %gep)
+  call void @escaping.invariant.start({}* %invst)
+  br label %loop
+
+loop:
+  %indvar = phi i32 [ %indvar.next, %loop ], [ 0, %entry ]
+  %sum = phi i32 [ %sum.next, %loop ], [ 0, %entry ]
+  %volload = load atomic i8, i8* %volatile unordered, align 8
+  fence acquire
+  %volchk = icmp eq i8 %volload, 0
+  %addrld = load atomic i32, i32* %addr.i unordered, align 8
+  %sel = select i1 %volchk, i32 0, i32 %addrld
+  %sum.next = add i32 %sel, %sum
+  %indvar.next = add i32 %indvar, 1
+  %cond = icmp slt i32 %indvar.next, %n
+  br i1 %cond, label %loop, label %loopexit
+
+loopexit:
+  ret i32 %sum
+}
+
+; FIXME: invariant.start dominates the load, and in this scope, the
+; load is invariant. So, we can hoist the `addrld` load out of the loop.
+; Consider the loadoperand addr.i bitcasted before being passed to
+; invariant.start
+define i32 @test_fence3(i32* %addr, i32 %n, i8* %volatile) {
+; CHECK-LABEL: @test_fence3
+; CHECK-LABEL: entry
+; CHECK: invariant.start
+; CHECK-NOT: %addrld = load atomic i32, i32* %addr.i unordered, align 8
+; CHECK: br label %loop
+entry: 
+  %addr.i = getelementptr inbounds i32, i32* %addr, i64 8
+  %gep = bitcast i32* %addr.i to i8 *
+  store atomic i32 5, i32 * %addr.i unordered, align 8
+  fence release
+  %invst = call {}* @llvm.invariant.start.p0i8(i64 4, i8* %gep)
+  br label %loop
+
+loop:
+  %indvar = phi i32 [ %indvar.next, %loop ], [ 0, %entry ]
+  %sum = phi i32 [ %sum.next, %loop ], [ 0, %entry ]
+  %volload = load atomic i8, i8* %volatile unordered, align 8
+  fence acquire
+  %volchk = icmp eq i8 %volload, 0
+  %addrld = load atomic i32, i32* %addr.i unordered, align 8
+  %sel = select i1 %volchk, i32 0, i32 %addrld
+  %sum.next = add i32 %sel, %sum
+  %indvar.next = add i32 %indvar, 1
+  %cond = icmp slt i32 %indvar.next, %n
+  br i1 %cond, label %loop, label %loopexit
+
+loopexit:
+  ret i32 %sum
+}
+
+; We should not hoist the addrld out of the loop.
+define i32 @test_fence4(i32* %addr, i32 %n, i8* %volatile) {
+; CHECK-LABEL: @test_fence4
+; CHECK-LABEL: entry
+; CHECK-NOT: %addrld = load atomic i32, i32* %addr.i unordered, align 8
+; CHECK: br label %loop
+entry: 
+  %addr.i = getelementptr inbounds i32, i32* %addr, i64 8
+  %gep = bitcast i32* %addr.i to i8 *
+  br label %loop
+
+loop:
+  %indvar = phi i32 [ %indvar.next, %loop ], [ 0, %entry ]
+  %sum = phi i32 [ %sum.next, %loop ], [ 0, %entry ]
+  store atomic i32 5, i32 * %addr.i unordered, align 8
+  fence release
+  %invst = call {}* @llvm.invariant.start.p0i8(i64 4, i8* %gep)
+  %volload = load atomic i8, i8* %volatile unordered, align 8
+  fence acquire
+  %volchk = icmp eq i8 %volload, 0
+  %addrld = load atomic i32, i32* %addr.i unordered, align 8
+  %sel = select i1 %volchk, i32 0, i32 %addrld
+  %sum.next = add i32 %sel, %sum
+  %indvar.next = add i32 %indvar, 1
+  %cond = icmp slt i32 %indvar.next, %n
+  br i1 %cond, label %loop, label %loopexit
+
+loopexit:
+  ret i32 %sum
+}