Index: lib/Target/AArch64/AArch64DeadRegisterDefinitionsPass.cpp
===================================================================
--- lib/Target/AArch64/AArch64DeadRegisterDefinitionsPass.cpp
+++ lib/Target/AArch64/AArch64DeadRegisterDefinitionsPass.cpp
@@ -68,6 +68,51 @@
   return false;
 }
 
+// Instructions that lose their 'read' operation for a subesquent fence acquire
+// (DMB LD) once the zero register is used.
+//
+// WARNING: The aquire variants of the instructions are also affected, but they
+// are split out into `atomicBarrierDroppedOnZero()` to support annotations on
+// assembly.
+static bool atomicReadDroppedOnZero(unsigned Opcode) {
+  switch (Opcode) {
+    case AArch64::LDADDB:     case AArch64::LDADDH:
+    case AArch64::LDADDW:     case AArch64::LDADDX:
+    case AArch64::LDADDLB:    case AArch64::LDADDLH:
+    case AArch64::LDADDLW:    case AArch64::LDADDLX:
+    case AArch64::LDCLRB:     case AArch64::LDCLRH:
+    case AArch64::LDCLRW:     case AArch64::LDCLRX:
+    case AArch64::LDCLRLB:    case AArch64::LDCLRLH:
+    case AArch64::LDCLRLW:    case AArch64::LDCLRLX:
+    case AArch64::LDEORB:     case AArch64::LDEORH:
+    case AArch64::LDEORW:     case AArch64::LDEORX:
+    case AArch64::LDEORLB:    case AArch64::LDEORLH:
+    case AArch64::LDEORLW:    case AArch64::LDEORLX:
+    case AArch64::LDSETB:     case AArch64::LDSETH:
+    case AArch64::LDSETW:     case AArch64::LDSETX:
+    case AArch64::LDSETLB:    case AArch64::LDSETLH:
+    case AArch64::LDSETLW:    case AArch64::LDSETLX:
+    case AArch64::LDSMAXB:    case AArch64::LDSMAXH:
+    case AArch64::LDSMAXW:    case AArch64::LDSMAXX:
+    case AArch64::LDSMAXLB:   case AArch64::LDSMAXLH:
+    case AArch64::LDSMAXLW:   case AArch64::LDSMAXLX:
+    case AArch64::LDSMINB:    case AArch64::LDSMINH:
+    case AArch64::LDSMINW:    case AArch64::LDSMINX:
+    case AArch64::LDSMINLB:   case AArch64::LDSMINLH:
+    case AArch64::LDSMINLW:   case AArch64::LDSMINLX:
+    case AArch64::LDUMAXB:    case AArch64::LDUMAXH:
+    case AArch64::LDUMAXW:    case AArch64::LDUMAXX:
+    case AArch64::LDUMAXLB:   case AArch64::LDUMAXLH:
+    case AArch64::LDUMAXLW:   case AArch64::LDUMAXLX:
+    case AArch64::LDUMINB:    case AArch64::LDUMINH:
+    case AArch64::LDUMINW:    case AArch64::LDUMINX:
+    case AArch64::LDUMINLB:   case AArch64::LDUMINLH:
+    case AArch64::LDUMINLW:   case AArch64::LDUMINLX:
+    return true;
+  }
+  return false;
+}
+
 void AArch64DeadRegisterDefinitions::processMachineBasicBlock(
     MachineBasicBlock &MBB) {
   const MachineFunction &MF = *MBB.getParent();
@@ -88,7 +133,7 @@
       continue;
     }
 
-    if (atomicBarrierDroppedOnZero(MI.getOpcode())) {
+    if (atomicBarrierDroppedOnZero(MI.getOpcode()) || atomicReadDroppedOnZero(MI.getOpcode())) {
       LLVM_DEBUG(dbgs() << "    Ignoring, semantics change with xzr/wzr.\n");
       continue;
     }
Index: test/CodeGen/AArch64/atomic-ops-lse.ll
===================================================================
--- test/CodeGen/AArch64/atomic-ops-lse.ll
+++ test/CodeGen/AArch64/atomic-ops-lse.ll
@@ -1311,7 +1311,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stadd w0, [x[[ADDR]]]
+; CHECK: ldadd w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1323,7 +1323,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stadd x0, [x[[ADDR]]]
+; CHECK: ldadd x{{[0-9]}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1387,7 +1387,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: staddl w0, [x[[ADDR]]]
+; CHECK: ldaddl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1399,7 +1399,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: staddl x0, [x[[ADDR]]]
+; CHECK: ldaddl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1696,7 +1696,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stclr w[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldclr w{{[0-9]+}}, w[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1709,7 +1709,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stclr x[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldclr x{{[0-9]+}}, x[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1774,7 +1774,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stclrl w[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldclrl w{{[0-9]*}}, w[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -1787,7 +1787,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stclrl x[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldclrl x{{[0-9]*}}, x[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2306,7 +2306,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stsmax w0, [x[[ADDR]]]
+; CHECK: ldsmax w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2318,7 +2318,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stsmax x0, [x[[ADDR]]]
+; CHECK: ldsmax x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2382,7 +2382,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stsmaxl w0, [x[[ADDR]]]
+; CHECK: ldsmaxl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2394,7 +2394,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stsmaxl x0, [x[[ADDR]]]
+; CHECK: ldsmaxl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2686,7 +2686,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stsmin w0, [x[[ADDR]]]
+; CHECK: ldsmin w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2698,7 +2698,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stsmin x0, [x[[ADDR]]]
+; CHECK: ldsmin x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2762,7 +2762,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stsminl w0, [x[[ADDR]]]
+; CHECK: ldsminl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -2774,7 +2774,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stsminl x0, [x[[ADDR]]]
+; CHECK: ldsminl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -3066,7 +3066,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stset w0, [x[[ADDR]]]
+; CHECK: ldset w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -3078,7 +3078,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stset x0, [x[[ADDR]]]
+; CHECK: ldset x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -3142,7 +3142,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stsetl w0, [x[[ADDR]]]
+; CHECK: ldsetl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -3154,7 +3154,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stsetl x0, [x[[ADDR]]]
+; CHECK: ldsetl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -3467,7 +3467,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stadd w[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldadd w{{[0-9]+}}, w[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
 
   ret void
@@ -3481,7 +3481,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stadd x[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldadd x{{[0-9]+}}, x[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
 
   ret void
@@ -3551,7 +3551,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: staddl w[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldaddl w{{[0-9]*}}, w[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
 
   ret void
@@ -3565,7 +3565,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: staddl x[[NEW:[0-9]+]], [x[[ADDR]]]
+; CHECK: ldaddl x{{[0-9]*}}, x[[NEW:[1-9][0-9]*]], [x[[ADDR]]]
 ; CHECK-NOT: dmb
 
   ret void
@@ -4256,7 +4256,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stumax w0, [x[[ADDR]]]
+; CHECK: ldumax w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4268,7 +4268,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stumax x0, [x[[ADDR]]]
+; CHECK: ldumax x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4332,7 +4332,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stumaxl w0, [x[[ADDR]]]
+; CHECK: ldumaxl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4344,7 +4344,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stumaxl x0, [x[[ADDR]]]
+; CHECK: ldumaxl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4636,7 +4636,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stumin w0, [x[[ADDR]]]
+; CHECK: ldumin w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4648,7 +4648,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stumin x0, [x[[ADDR]]]
+; CHECK: ldumin x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4712,7 +4712,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: stuminl w0, [x[[ADDR]]]
+; CHECK: lduminl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -4724,7 +4724,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: stuminl x0, [x[[ADDR]]]
+; CHECK: lduminl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -5016,7 +5016,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: steor w0, [x[[ADDR]]]
+; CHECK: ldeor w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -5028,7 +5028,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: steor x0, [x[[ADDR]]]
+; CHECK: ldeor x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -5092,7 +5092,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var32
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var32
 
-; CHECK: steorl w0, [x[[ADDR]]]
+; CHECK: ldeorl w{{[0-9]+}}, w{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }
@@ -5104,7 +5104,7 @@
 ; CHECK: adrp [[TMPADDR:x[0-9]+]], var64
 ; CHECK: add x[[ADDR:[0-9]+]], [[TMPADDR]], {{#?}}:lo12:var64
 
-; CHECK: steorl x0, [x[[ADDR]]]
+; CHECK: ldeorl x{{[0-9]+}}, x{{[1-9][0-9]*}}, [x[[ADDR]]]
 ; CHECK-NOT: dmb
   ret void
 }