Index: llvm/trunk/docs/FaultMaps.rst
===================================================================
--- llvm/trunk/docs/FaultMaps.rst
+++ llvm/trunk/docs/FaultMaps.rst
@@ -64,7 +64,7 @@
 
     %ptr = call i32* @get_ptr()
     %ptr_is_null = icmp i32* %ptr, null
-    br i1 %ptr_is_null, label %is_null, label %not_null
+    br i1 %ptr_is_null, label %is_null, label %not_null, !make.implicit !0
   
   not_null:
     %t = load i32, i32* %ptr
@@ -73,6 +73,8 @@
   is_null:
     call void @HFC()
     unreachable
+  
+  !0 = !{}
 
 to control flow implicit in the instruction loading or storing through
 the pointer being null checked:
@@ -94,3 +96,32 @@
 
 The ``ImplicitNullChecks`` pass adds entries to the
 ``__llvm_faultmaps`` section described above as needed.
+
+``make.implicit`` metadata
+--------------------------
+
+Making null checks implicit is an aggressive optimization, and it can
+be a net performance pessimization if too many memory operations end
+up faulting because of it.  A language runtime typically needs to
+ensure that only a negligible number of implicit null checks actually
+fault once the application has reached a steady state.  A standard way
+of doing this is by healing failed implicit null checks into explicit
+null checks via code patching or recompilation.  It follows that there
+are two requirements an explicit null check needs to satisfy for it to
+be profitable to convert it to an implicit null check:
+
+  1. The case where the pointer is actually null (i.e. the "failing"
+     case) is extremely rare.
+
+  2. The failing path heals the implicit null check into an explicit
+     null check so that the application does not repeatedly page
+     fault.
+
+The frontend is expected to mark branches that satisfy (1) and (2)
+using a ``!make.implicit`` metadata node (the actual content of the
+metadata node is ignored).  Only branches that are marked with
+``!make.implicit`` metadata are considered as candidates for
+conversion into implicit null checks.
+
+(Note that while we could deal with (1) using profiling data, dealing
+with (2) requires some information not present in branch profiles.)
Index: llvm/trunk/lib/CodeGen/ImplicitNullChecks.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/ImplicitNullChecks.cpp
+++ llvm/trunk/lib/CodeGen/ImplicitNullChecks.cpp
@@ -124,6 +124,13 @@
     MachineBasicBlock &MBB, SmallVectorImpl<NullCheck> &NullCheckList) {
   typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
 
+  MDNode *BranchMD =
+      MBB.getBasicBlock()
+          ? MBB.getBasicBlock()->getTerminator()->getMetadata("make.implicit")
+          : nullptr;
+  if (!BranchMD)
+    return false;
+
   MachineBranchPredicate MBP;
 
   if (TII->AnalyzeBranchPredicate(MBB, MBP, true))
Index: llvm/trunk/test/CodeGen/X86/implicit-null-check-negative.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/implicit-null-check-negative.ll
+++ llvm/trunk/test/CodeGen/X86/implicit-null-check-negative.ll
@@ -10,7 +10,7 @@
   %c = icmp eq i32* %x, null
 ; It isn't legal to move the load from %x from "not_null" to here --
 ; the store to %y could be aliasing it.
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -24,7 +24,7 @@
 define i32 @imp_null_check_gep_load(i32* %x) {
  entry:
   %c = icmp eq i32* %x, null
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -36,3 +36,19 @@
   %t = load i32, i32* %x.gep
   ret i32 %t
 }
+
+define i32 @imp_null_check_load_no_md(i32* %x) {
+; This is fine, except it is missing the !make.implicit metadata.
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %t = load i32, i32* %x
+  ret i32 %t
+}
+
+!0 = !{}
Index: llvm/trunk/test/CodeGen/X86/implicit-null-check.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/implicit-null-check.ll
+++ llvm/trunk/test/CodeGen/X86/implicit-null-check.ll
@@ -21,7 +21,7 @@
 
  entry:
   %c = icmp eq i32* %x, null
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -42,7 +42,7 @@
 
  entry:
   %c = icmp eq i32* %x, null
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -65,7 +65,7 @@
 
  entry:
   %c = icmp eq i32* %x, null
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -76,6 +76,8 @@
   ret i32 %p1
 }
 
+!0 = !{}
+
 ; CHECK-LABEL: __LLVM_FaultMaps:
 
 ; Version: