Index: llvm/trunk/lib/Target/AArch64/AArch64.td
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64.td
+++ llvm/trunk/lib/Target/AArch64/AArch64.td
@@ -32,6 +32,9 @@
 def FeatureCRC : SubtargetFeature<"crc", "HasCRC", "true",
   "Enable ARMv8 CRC-32 checksum instructions">;
 
+def FeaturePerfMon : SubtargetFeature<"perfmon", "HasPerfMon", "true",
+  "Enable ARMv8 PMUv3 Performance Monitors extension">;
+
 /// Cyclone has register move instructions which are "free".
 def FeatureZCRegMove : SubtargetFeature<"zcm", "HasZeroCycleRegMove", "true",
                                         "Has zero-cycle register moves">;
@@ -84,14 +87,16 @@
                                    [FeatureFPARMv8,
                                    FeatureNEON,
                                    FeatureCrypto,
-                                   FeatureCRC]>;
+                                   FeatureCRC,
+                                   FeaturePerfMon]>;
 
 def ProcA57     : SubtargetFeature<"a57", "ARMProcFamily", "CortexA57",
                                    "Cortex-A57 ARM processors",
                                    [FeatureFPARMv8,
                                    FeatureNEON,
                                    FeatureCrypto,
-                                   FeatureCRC]>;
+                                   FeatureCRC,
+                                   FeaturePerfMon]>;
 
 def ProcCyclone : SubtargetFeature<"cyclone", "ARMProcFamily", "Cyclone",
                                    "Cyclone",
@@ -99,11 +104,13 @@
                                    FeatureNEON,
                                    FeatureCrypto,
                                    FeatureCRC,
+                                   FeaturePerfMon,
                                    FeatureZCRegMove, FeatureZCZeroing]>;
 
 def : ProcessorModel<"generic", NoSchedModel, [FeatureFPARMv8,
                                               FeatureNEON,
-                                              FeatureCRC]>;
+                                              FeatureCRC,
+                                              FeaturePerfMon]>;
 
 def : ProcessorModel<"cortex-a53", CortexA53Model, [ProcA53]>;
 def : ProcessorModel<"cortex-a57", CortexA57Model, [ProcA57]>;
Index: llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ llvm/trunk/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -399,6 +399,11 @@
 
   setOperationAction(ISD::PREFETCH, MVT::Other, Custom);
 
+  // Lower READCYCLECOUNTER using an mrs from PMCCNTR_EL0.
+  // This requires the Performance Monitors extension.
+  if (Subtarget->hasPerfMon())
+    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: llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
+++ llvm/trunk/lib/Target/AArch64/AArch64InstrInfo.td
@@ -24,6 +24,7 @@
                                  AssemblerPredicate<"FeatureCrypto", "crypto">;
 def HasCRC           : Predicate<"Subtarget->hasCRC()">,
                                  AssemblerPredicate<"FeatureCRC", "crc">;
+def HasPerfMon       : Predicate<"Subtarget->hasPerfMon()">;
 def IsLE             : Predicate<"Subtarget->isLittleEndian()">;
 def IsBE             : Predicate<"!Subtarget->isLittleEndian()">;
 def IsCyclone        : Predicate<"Subtarget->isCyclone()">;
@@ -404,6 +405,10 @@
 // TPIDR_EL0.
 def : Pat<(AArch64threadpointer), (MRS 0xde82)>;
 
+// The cycle counter PMC register is PMCCNTR_EL0.
+let Predicates = [HasPerfMon] in
+def : Pat<(readcyclecounter), (MRS 0xdce8)>;
+
 // Generic system instructions
 def SYSxt  : SystemXtI<0, "sys">;
 def SYSLxt : SystemLXtI<1, "sysl">;
Index: llvm/trunk/lib/Target/AArch64/AArch64Subtarget.h
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64Subtarget.h
+++ llvm/trunk/lib/Target/AArch64/AArch64Subtarget.h
@@ -44,6 +44,7 @@
   bool HasNEON;
   bool HasCrypto;
   bool HasCRC;
+  bool HasPerfMon;
 
   // HasZeroCycleRegMove - Has zero-cycle register mov instructions.
   bool HasZeroCycleRegMove;
@@ -114,6 +115,7 @@
   bool hasNEON() const { return HasNEON; }
   bool hasCrypto() const { return HasCrypto; }
   bool hasCRC() const { return HasCRC; }
+  bool hasPerfMon() const { return HasPerfMon; }
 
   bool isLittleEndian() const { return IsLittle; }
 
Index: llvm/trunk/lib/Target/AArch64/AArch64Subtarget.cpp
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64Subtarget.cpp
+++ llvm/trunk/lib/Target/AArch64/AArch64Subtarget.cpp
@@ -47,9 +47,9 @@
                                    const TargetMachine &TM, bool LittleEndian)
     : AArch64GenSubtargetInfo(TT, CPU, FS), ARMProcFamily(Others),
       HasV8_1aOps(false), HasFPARMv8(false), HasNEON(false), HasCrypto(false),
-      HasCRC(false), HasZeroCycleRegMove(false), HasZeroCycleZeroing(false),
-      StrictAlign(false), ReserveX18(false), IsLittle(LittleEndian),
-      CPUString(CPU), TargetTriple(TT), FrameLowering(),
+      HasCRC(false), HasPerfMon(false), HasZeroCycleRegMove(false),
+      HasZeroCycleZeroing(false), StrictAlign(false), ReserveX18(false),
+      IsLittle(LittleEndian), CPUString(CPU), TargetTriple(TT), FrameLowering(),
       InstrInfo(initializeSubtargetDependencies(FS)), TSInfo(),
       TLInfo(TM, *this) {}
 
Index: llvm/trunk/test/CodeGen/AArch64/readcyclecounter.ll
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/readcyclecounter.ll
+++ llvm/trunk/test/CodeGen/AArch64/readcyclecounter.ll
@@ -0,0 +1,15 @@
+; RUN: llc -mtriple=aarch64-unknown-unknown -asm-verbose=false < %s |\
+; RUN:   FileCheck %s --check-prefix=CHECK --check-prefix=PERFMON
+; RUN: llc -mtriple=aarch64-unknown-unknown -mattr=-perfmon -asm-verbose=false < %s |\
+; RUN:   FileCheck %s --check-prefix=CHECK --check-prefix=NOPERFMON
+
+define i64 @test_readcyclecounter() nounwind {
+  ; CHECK-LABEL:   test_readcyclecounter:
+  ; PERFMON-NEXT:   mrs x0, PMCCNTR_EL0
+  ; NOPERFMON-NEXT: mov x0, xzr
+  ; CHECK-NEXT:     ret
+  %tmp0 = call i64 @llvm.readcyclecounter()
+  ret i64 %tmp0
+}
+
+declare i64 @llvm.readcyclecounter()