[AArch64] Add hidden option to enable subreg liveness tracking.

Subreg liveness tracking is disabled by default for now until all issues
are ironed out. This option allows the feature to be used in tests.

Author: sdesmalen-arm

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

@@ -86,6 +86,13 @@ static cl::alias AArch64StreamingStackHazardSize(
     cl::desc("alias for -aarch64-streaming-hazard-size"),
     cl::aliasopt(AArch64StreamingHazardSize));
 
+// Subreg liveness tracking is disabled by default for now until all issues
+// are ironed out. This option allows the feature to be used in tests.
+static cl::opt<bool>
+    EnableSubregLivenessTracking("aarch64-enable-subreg-liveness-tracking",
+                                 cl::init(false), cl::Hidden,
+                                 cl::desc("Enable subreg liveness tracking"));
+
 unsigned AArch64Subtarget::getVectorInsertExtractBaseCost() const {
   if (OverrideVectorInsertExtractBaseCost.getNumOccurrences() > 0)
     return OverrideVectorInsertExtractBaseCost;
@@ -380,6 +387,8 @@ AArch64Subtarget::AArch64Subtarget(const Triple &TT, StringRef CPU,
     ReserveXRegisterForRA.set(29);
 
   AddressCheckPSV.reset(new AddressCheckPseudoSourceValue(TM));
+
+  EnableSubregLiveness = EnableSubregLivenessTracking.getValue();
 }
 
 const CallLowering *AArch64Subtarget::getCallLowering() const {

llvm/lib/Target/AArch64/AArch64Subtarget.h

@@ -90,6 +90,8 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
   unsigned VScaleForTuning = 2;
   TailFoldingOpts DefaultSVETFOpts = TailFoldingOpts::Disabled;
 
+  bool EnableSubregLiveness;
+
   /// TargetTriple - What processor and OS we're targeting.
   Triple TargetTriple;
 
@@ -153,6 +155,7 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
   const Triple &getTargetTriple() const { return TargetTriple; }
   bool enableMachineScheduler() const override { return true; }
   bool enablePostRAScheduler() const override { return usePostRAScheduler(); }
+  bool enableSubRegLiveness() const override { return EnableSubregLiveness; }
 
   bool enableMachinePipeliner() const override;
   bool useDFAforSMS() const override { return false; }