Index: llvm/trunk/lib/CodeGen/CriticalAntiDepBreaker.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/CriticalAntiDepBreaker.cpp
+++ llvm/trunk/lib/CodeGen/CriticalAntiDepBreaker.cpp
@@ -91,7 +91,14 @@
 
 void CriticalAntiDepBreaker::Observe(MachineInstr *MI, unsigned Count,
                                      unsigned InsertPosIndex) {
-  if (MI->isDebugValue())
+  // Kill instructions can define registers but are really nops, and there might
+  // be a real definition earlier that needs to be paired with uses dominated by
+  // this kill.
+
+  // FIXME: It may be possible to remove the isKill() restriction once PR18663
+  // has been properly fixed. There can be value in processing kills as seen in
+  // the AggressiveAntiDepBreaker class.
+  if (MI->isDebugValue() || MI->isKill())
     return;
   assert(Count < InsertPosIndex && "Instruction index out of expected range!");
 
@@ -234,6 +241,7 @@
   // Update liveness.
   // Proceeding upwards, registers that are defed but not used in this
   // instruction are now dead.
+  assert(!MI->isKill() && "Attempting to scan a kill instruction");
 
   if (!TII->isPredicated(MI)) {
     // Predicated defs are modeled as read + write, i.e. similar to two
@@ -505,7 +513,14 @@
   unsigned Count = InsertPosIndex - 1;
   for (MachineBasicBlock::iterator I = End, E = Begin; I != E; --Count) {
     MachineInstr *MI = --I;
-    if (MI->isDebugValue())
+    // Kill instructions can define registers but are really nops, and there
+    // might be a real definition earlier that needs to be paired with uses
+    // dominated by this kill.
+    
+    // FIXME: It may be possible to remove the isKill() restriction once PR18663
+    // has been properly fixed. There can be value in processing kills as seen
+    // in the AggressiveAntiDepBreaker class.
+    if (MI->isDebugValue() || MI->isKill())
       continue;
 
     // Check if this instruction has a dependence on the critical path that
Index: llvm/trunk/test/CodeGen/X86/critical-anti-dep-breaker.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/critical-anti-dep-breaker.ll
+++ llvm/trunk/test/CodeGen/X86/critical-anti-dep-breaker.ll
@@ -0,0 +1,28 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -relocation-model=pic -post-RA-scheduler=1 -break-anti-dependencies=critical  | FileCheck %s
+
+; PR20308 ( http://llvm.org/bugs/show_bug.cgi?id=20308 )
+; The critical-anti-dependency-breaker must not use register def information from a kill inst.
+; This test case expects such an instruction to appear as a comment with def info for RDI.
+; There is an anti-dependency (WAR) hazard using RAX using default reg allocation and scheduling.
+; The post-RA-scheduler and critical-anti-dependency breaker can eliminate that hazard using R10.
+; That is the first free register that isn't used as a param in the call to "@Image".
+
+@PartClass = external global i32
+@NullToken = external global i64
+
+; CHECK-LABEL: Part_Create:
+; CHECK-DAG: # kill: RDI<def> 
+; CHECK-DAG: movq PartClass@GOTPCREL(%rip), %r10
+define i32 @Part_Create(i64* %Anchor, i32 %TypeNum, i32 %F, i32 %Z, i32* %Status, i64* %PartTkn) {
+  %PartObj = alloca i64*, align 8
+  %Vchunk = alloca i64, align 8
+  %1 = load i64* @NullToken, align 4
+  store i64 %1, i64* %Vchunk, align 8
+  %2 = load i32* @PartClass, align 4
+  call i32 @Image(i64* %Anchor, i32 %2, i32 0, i32 0, i32* %Status, i64* %PartTkn, i64** %PartObj)
+  call i32 @Create(i64* %Anchor)
+  ret i32 %2
+}
+
+declare i32 @Image(i64*, i32, i32, i32, i32*, i64*, i64**)
+declare i32 @Create(i64*)