Index: llvm/trunk/include/llvm/CodeGen/GlobalISel/LegalizerCombiner.h
===================================================================
--- llvm/trunk/include/llvm/CodeGen/GlobalISel/LegalizerCombiner.h
+++ llvm/trunk/include/llvm/CodeGen/GlobalISel/LegalizerCombiner.h
@@ -41,9 +41,7 @@
       Builder.setInstr(MI);
       // We get a copy/trunc/extend depending on the sizes
       Builder.buildAnyExtOrTrunc(DstReg, SrcReg);
-      MI.eraseFromParent();
-      if (MRI.use_empty(DefMI->getOperand(0).getReg()))
-        DeadInsts.push_back(DefMI);
+      markInstAndDefDead(MI, *DefMI, DeadInsts);
       return true;
     }
     return false;
@@ -68,9 +66,7 @@
       // We get a copy/trunc/extend depending on the sizes
       auto SrcCopyOrTrunc = Builder.buildAnyExtOrTrunc(DstTy, TruncSrc);
       Builder.buildAnd(DstReg, SrcCopyOrTrunc, MaskCstMIB);
-      MI.eraseFromParent();
-      if (MRI.use_empty(DefMI->getOperand(0).getReg()))
-        DeadInsts.push_back(DefMI);
+      markInstAndDefDead(MI, *DefMI, DeadInsts);
       return true;
     }
     return false;
@@ -97,9 +93,7 @@
       auto ShlMIB = Builder.buildInstr(TargetOpcode::G_SHL, DstTy,
                                        SrcCopyExtOrTrunc, SizeDiffMIB);
       Builder.buildInstr(TargetOpcode::G_ASHR, DstReg, ShlMIB, SizeDiffMIB);
-      MI.eraseFromParent();
-      if (MRI.use_empty(DefMI->getOperand(0).getReg()))
-        DeadInsts.push_back(DefMI);
+      markInstAndDefDead(MI, *DefMI, DeadInsts);
       return true;
     }
     return false;
@@ -175,17 +169,14 @@
                            MergeI->getOperand(Idx + 1).getReg());
     }
 
-    MI.eraseFromParent();
-    if (MRI.use_empty(MergeI->getOperand(0).getReg()))
-      DeadInsts.push_back(MergeI);
+    markInstAndDefDead(MI, *MergeI, DeadInsts);
     return true;
   }
 
   /// Try to combine away MI.
   /// Returns true if it combined away the MI.
-  /// Caller should not rely in MI existing as it may be deleted.
   /// Adds instructions that are dead as a result of the combine
-  // into DeadInsts
+  /// into DeadInsts, which can include MI.
   bool tryCombineInstruction(MachineInstr &MI,
                              SmallVectorImpl<MachineInstr *> &DeadInsts) {
     switch (MI.getOpcode()) {
@@ -201,6 +192,16 @@
       return tryCombineMerges(MI, DeadInsts);
     }
   }
+
+private:
+  /// Mark MI as dead. If a def of one of MI's operands, DefMI, would also be
+  /// dead due to MI being killed, then mark DefMI as dead too.
+  void markInstAndDefDead(MachineInstr &MI, MachineInstr &DefMI,
+                          SmallVectorImpl<MachineInstr *> &DeadInsts) {
+    DeadInsts.push_back(&MI);
+    if (MRI.hasOneUse(DefMI.getOperand(0).getReg()))
+      DeadInsts.push_back(&DefMI);
+  }
 };
 
 } // namespace llvm
Index: llvm/trunk/test/CodeGen/AArch64/GlobalISel/combine-anyext-crash.mir
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/GlobalISel/combine-anyext-crash.mir
+++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/combine-anyext-crash.mir
@@ -0,0 +1,42 @@
+# RUN: llc -O0 -run-pass=legalizer -global-isel %s -o - | FileCheck %s
+--- |
+  target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64--"
+  
+  define void @test_anyext_crash() {
+  entry:
+    br label %block2
+  
+  block2:
+    %0 = trunc i16 0 to i8
+    %1 = uitofp i8 %0 to double
+    br label %block2
+  }
+  
+
+...
+---
+name:            test_anyext_crash
+alignment:       2
+legalized:       false
+registers:       
+  - { id: 0, class: _, preferred-register: '' }
+  - { id: 1, class: _, preferred-register: '' }
+  - { id: 2, class: _, preferred-register: '' }
+body:             |
+  bb.1:
+   ; Check we don't crash due to trying to legalize a dead instruction.
+   ; CHECK-LABEL: test_anyext_crash
+   ; CHECK-LABEL: bb.1:
+    successors: %bb.2
+  
+    %0(s16) = G_CONSTANT i16 0
+  
+  bb.2:
+    successors: %bb.2
+  
+    %1(s8) = G_TRUNC %0(s16)
+    %2(s64) = G_UITOFP %1(s8)
+    G_BR %bb.2
+
+...