Index: lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.cpp
+++ lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -399,6 +399,11 @@
 
   setOperationAction(ISD::PREFETCH, MVT::Other, Custom);
 
+  // Lower READCYCLECOUNTER using an mrs from PMCCNTR_EL0.
+  // Technically, this requires the Performance Monitors (optional) extension,
+  // but all of the A-class CPUs we support have it.
+  setOperationAction(ISD::READCYCLECOUNTER, MVT::i64, Legal);
+
   if (Subtarget->isTargetMachO()) {
     // For iOS, we don't want to the normal expansion of a libcall to
     // sincos. We want to issue a libcall to __sincos_stret to avoid memory
Index: lib/Target/AArch64/AArch64InstrInfo.td
===================================================================
--- lib/Target/AArch64/AArch64InstrInfo.td
+++ lib/Target/AArch64/AArch64InstrInfo.td
@@ -404,6 +404,9 @@
 // TPIDR_EL0.
 def : Pat<(AArch64threadpointer), (MRS 0xde82)>;
 
+// The cycle counter PMC register is PMCCNTR_EL0.
+def : Pat<(readcyclecounter), (MRS 0xdce8)>;
+
 // Generic system instructions
 def SYSxt  : SystemXtI<0, "sys">;
 def SYSLxt : SystemLXtI<1, "sysl">;
Index: test/CodeGen/AArch64/readcyclecounter.ll
===================================================================
--- test/CodeGen/AArch64/readcyclecounter.ll
+++ test/CodeGen/AArch64/readcyclecounter.ll
@@ -0,0 +1,11 @@
+; RUN: llc -mtriple=arm64-unknown-unknown -asm-verbose=false < %s | FileCheck %s
+
+define i64 @test_readcyclecounter() nounwind {
+  ; CHECK-LABEL: test_readcyclecounter:
+  ; CHECK-NEXT: mrs x0, PMCCNTR_EL0
+  ; CHECK-NEXT: ret
+  %tmp0 = call i64 @llvm.readcyclecounter()
+  ret i64 %tmp0
+}
+
+declare i64 @llvm.readcyclecounter()