Index: lib/Transforms/Scalar/LoopUnrollPass.cpp
===================================================================
--- lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -274,6 +274,42 @@
   return 0;
 }
 
+// Remove existing unroll metadata and add unroll disable metadata to
+// indicate the loop has already been unrolled.  This prevents a loop
+// from being unrolled more than is directed by a pragma if the loop
+// unrolling pass is run more than once (which it generally is).
+static void SetLoopAlreadyUnrolled(Loop *L) {
+  MDNode *LoopID = L->getLoopID();
+  if (!LoopID) return;
+
+  // First remove any existing loop unrolling metadata.
+  SmallVector<Value *, 4> Vals;
+  // Reserve first location for self reference to the LoopID metadata node.
+  Vals.push_back(nullptr);
+  for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
+    bool IsUnrollMetadata = false;
+    MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
+    if (MD) {
+      const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
+      IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
+    }
+    if (!IsUnrollMetadata) Vals.push_back(LoopID->getOperand(i));
+  }
+
+  // Add unroll(disable) metadata to disable future unrolling.
+  LLVMContext &Context = L->getHeader()->getContext();
+  MDNode *DisableNode =
+      MDNode::get(Context, {MDString::get(Context, "llvm.loop.unroll.enable"),
+                            ConstantInt::get(Type::getInt1Ty(Context), 0)});
+  Vals.push_back(DisableNode);
+
+  MDNode *NewLoopID = MDNode::get(Context, Vals);
+  // Set operand 0 to refer to the loop id itself.
+  NewLoopID->replaceOperandWith(0, NewLoopID);
+  L->setLoopID(NewLoopID);
+  LoopID->replaceAllUsesWith(NewLoopID);
+}
+
 unsigned LoopUnroll::selectUnrollCount(
     const Loop *L, unsigned TripCount, bool HasEnablePragma,
     unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
@@ -428,6 +464,10 @@
   }
 
   if (HasPragma) {
+    // Mark loop as unrolled to prevent unrolling beyond that
+    // requested by the pragma.
+    SetLoopAlreadyUnrolled(L);
+
     // Emit optimization remarks if we are unable to unroll the loop
     // as directed by a pragma.
     DebugLoc LoopLoc = L->getStartLoc();
Index: test/Transforms/LoopUnroll/unroll-pragmas-disabled.ll
===================================================================
--- test/Transforms/LoopUnroll/unroll-pragmas-disabled.ll
+++ test/Transforms/LoopUnroll/unroll-pragmas-disabled.ll
@@ -0,0 +1,69 @@
+; RUN: opt < %s -loop-unroll -S | FileCheck %s
+;
+; Verify that the unrolling pass removes existing loop unrolling metadata
+; and adds a disable unrolling node after unrolling is complete.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; #pragma clang loop  vectorize(enable) unroll(enable) unroll_count(4) vectorize_width(8)
+;
+; Unroll metadata should be replaces with unroll(disable).  Vectorize
+; metadata should be untouched.
+;
+; CHECK-LABEL: @loop1(
+; CHECK: br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[LOOP_1:.*]]
+define void @loop1(i32* nocapture %a) {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %inc = add nsw i32 %0, 1
+  store i32 %inc, i32* %arrayidx, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 64
+  br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !1
+
+for.end:                                          ; preds = %for.body
+  ret void
+}
+!1 = metadata !{metadata !1, metadata !2, metadata !3, metadata !4, metadata !5}
+!2 = metadata !{metadata !"llvm.loop.vectorize.enable", i1 true}
+!3 = metadata !{metadata !"llvm.loop.unroll.enable", i1 true}
+!4 = metadata !{metadata !"llvm.loop.unroll.count", i32 4}
+!5 = metadata !{metadata !"llvm.loop.vectorize.width", i32 8}
+
+; #pragma clang loop unroll(disable)
+;
+; Unroll metadata should not change.
+;
+; CHECK-LABEL: @loop2(
+; CHECK: br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[LOOP_2:.*]]
+define void @loop2(i32* nocapture %a) {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %inc = add nsw i32 %0, 1
+  store i32 %inc, i32* %arrayidx, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 64
+  br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !6
+
+for.end:                                          ; preds = %for.body
+  ret void
+}
+!6 = metadata !{metadata !6, metadata !7}
+!7 = metadata !{metadata !"llvm.loop.unroll.enable", i1 false}
+
+; CHECK: ![[LOOP_1]] = metadata !{metadata ![[LOOP_1]], metadata ![[VEC_ENABLE:.*]], metadata ![[WIDTH_8:.*]], metadata ![[UNROLL_DISABLE:.*]]}
+; CHECK: ![[VEC_ENABLE]] = metadata !{metadata !"llvm.loop.vectorize.enable", i1 true}
+; CHECK: ![[WIDTH_8]] = metadata !{metadata !"llvm.loop.vectorize.width", i32 8}
+; CHECK: ![[UNROLL_DISABLE]] = metadata !{metadata !"llvm.loop.unroll.enable", i1 false}
+; CHECK: ![[LOOP_2]] = metadata !{metadata ![[LOOP_2]], metadata ![[UNROLL_DISABLE:.*]]}
Index: test/Transforms/LoopUnroll/unroll-pragmas.ll
===================================================================
--- test/Transforms/LoopUnroll/unroll-pragmas.ll
+++ test/Transforms/LoopUnroll/unroll-pragmas.ll
@@ -1,4 +1,8 @@
 ; RUN: opt < %s -loop-unroll -S | FileCheck %s
+; RUN: opt < %s -loop-unroll -loop-unroll -S | FileCheck %s
+;
+; Run loop unrolling twice to verify that loop unrolling metadata is properly
+; removed and further unrolling is disabled after the pass is run once.
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"