Index: lib/CodeGen/MachineSink.cpp
===================================================================
--- lib/CodeGen/MachineSink.cpp
+++ lib/CodeGen/MachineSink.cpp
@@ -27,6 +27,7 @@
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachinePostDominators.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/IR/LLVMContext.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -673,6 +674,49 @@
   return SuccToSinkTo;
 }
 
+/// \brief Return true if MI is likely to be usable as a memory operation by the
+/// implicit null check optimization.
+///
+/// This is a "best effort" heuristic, and should not be relied upon for
+/// correctness.  This returning true does not guarantee that the implicit null
+/// check optimization is legal over MI, and this returning false does not
+/// guarantee MI cannot possibly be used to do a null check.
+static bool SinkingPreventsImplicitNullCheck(MachineInstr *MI,
+                                             const TargetInstrInfo *TII,
+                                             const TargetRegisterInfo *TRI) {
+  typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
+
+  auto *MBB = MI->getParent();
+  if (MBB->pred_size() != 1)
+    return false;
+
+  auto *PredMBB = *MBB->pred_begin();
+  auto *PredBB = PredMBB->getBasicBlock();
+
+  // Frontends that don't use implicit null checks have no reason to emit
+  // branches with make.implicit metadata, and this function should always
+  // return false for them.
+  if (!PredBB ||
+      !PredBB->getTerminator()->getMetadata(LLVMContext::MD_make_implicit))
+    return false;
+
+  unsigned BaseReg, Offset;
+  if (!TII->getMemOpBaseRegImmOfs(MI, BaseReg, Offset, TRI))
+    return false;
+
+  if (!(MI->mayLoad() && !MI->isPredicable()))
+    return false;
+
+  MachineBranchPredicate MBP;
+  if (TII->AnalyzeBranchPredicate(*PredMBB, MBP, false))
+    return false;
+
+  return MBP.LHS.isReg() && MBP.RHS.isImm() && MBP.RHS.getImm() == 0 &&
+         (MBP.Predicate == MachineBranchPredicate::PRED_NE ||
+          MBP.Predicate == MachineBranchPredicate::PRED_EQ) &&
+         MBP.LHS.getReg() == BaseReg;
+}
+
 /// SinkInstruction - Determine whether it is safe to sink the specified machine
 /// instruction out of its current block into a successor.
 bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore,
@@ -691,6 +735,11 @@
   if (MI->isConvergent())
     return false;
 
+  // Don't break implicit null checks.  This is a performance heuristic, and not
+  // required for correctness.
+  if (SinkingPreventsImplicitNullCheck(MI, TII, TRI))
+    return false;
+
   // FIXME: This should include support for sinking instructions within the
   // block they are currently in to shorten the live ranges.  We often get
   // instructions sunk into the top of a large block, but it would be better to
Index: test/CodeGen/X86/machine-sink-and-implicit-null-checks.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/machine-sink-and-implicit-null-checks.ll
@@ -0,0 +1,49 @@
+; RUN: llc -mtriple=x86_64-apple-macosx -O3 -enable-implicit-null-checks -o - < %s 2>&1 | FileCheck %s
+
+declare void @throw0()
+declare void @throw1()
+
+define i1 @f(i8* %p0, i8* %p1) {
+ entry:
+  %c0 = icmp eq i8* %p0, null
+  br i1 %c0, label %throw0, label %continue0, !make.implicit !0
+
+ continue0:
+  %v0 = load i8, i8* %p0
+  %c1 = icmp eq i8* %p1, null
+  br i1 %c1, label %throw1, label %continue1, !make.implicit !0
+
+ continue1:
+  %v1 = load i8, i8* %p1
+  %v = icmp eq i8 %v0, %v1
+  ret i1 %v
+
+ throw0:
+  call void @throw0()
+  unreachable
+
+ throw1:
+  call void @throw1()
+  unreachable
+}
+
+; Check that we have two implicit null checks in @f
+
+; CHECK: __LLVM_FaultMaps:
+; CHECK-NEXT:        .byte   1
+; CHECK-NEXT:        .byte   0
+; CHECK-NEXT:        .short  0
+; CHECK-NEXT:        .long   1
+
+; FunctionInfo[0] =
+
+; FunctionAddress =
+; CHECK-NEXT:        .quad   _f
+
+; NumFaultingPCs =
+; CHECK-NEXT:        .long   2
+
+; Reserved =
+; CHECK-NEXT:        .long   0
+
+!0 = !{}