Provide a target override for the latest regalloc heuristic.

This is a temporary workaround for native arm linux builds:
PR18996: Changing regalloc order breaks "lencod" on native arm linux builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202433 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetRegisterInfo.h

@@ -683,6 +683,12 @@ public:
   /// (3) Bottom-up allocation is no longer guaranteed to optimally color.
   virtual bool reverseLocalAssignment() const { return false; }
 
+  /// Allow the target to override register assignment heuristics based on the
+  /// live range size. If this returns false, then local live ranges are always
+  /// assigned in order regardless of their size. This is a temporary hook for
+  /// debugging downstream codegen failures exposed by regalloc.
+  virtual bool mayOverrideLocalAssignment() const { return true; }
+
   /// requiresRegisterScavenging - returns true if the target requires (and can
   /// make use of) the register scavenger.
   virtual bool requiresRegisterScavenging(const MachineFunction &MF) const {

lib/CodeGen/RegAllocGreedy.cpp

@@ -457,7 +457,7 @@ void RAGreedy::enqueue(PQueue &CurQueue, LiveInterval *LI) {
     // Giant live ranges fall back to the global assignment heuristic, which
     // prevents excessive spilling in pathological cases.
     bool ReverseLocal = TRI->reverseLocalAssignment();
-    bool ForceGlobal = !ReverseLocal &&
+    bool ForceGlobal = !ReverseLocal && TRI->mayOverrideLocalAssignment() &&
       (Size / SlotIndex::InstrDist) > (2 * MRI->getRegClass(Reg)->getNumRegs());
 
     if (ExtraRegInfo[Reg].Stage == RS_Assign && !ForceGlobal && !LI->empty() &&

lib/Target/ARM/ARMBaseRegisterInfo.cpp

@@ -408,6 +408,11 @@ emitLoadConstPool(MachineBasicBlock &MBB,
     .setMIFlags(MIFlags);
 }
 
+bool ARMBaseRegisterInfo::mayOverrideLocalAssignment() const {
+  // The native linux build hits a downstream codegen bug when this is enabled.
+  return STI.isTargetDarwin();
+}
+
 bool ARMBaseRegisterInfo::
 requiresRegisterScavenging(const MachineFunction &MF) const {
   return true;

lib/Target/ARM/ARMBaseRegisterInfo.h

@@ -172,6 +172,8 @@ public:
                                  unsigned MIFlags = MachineInstr::NoFlags)const;
 
   /// Code Generation virtual methods...
+  virtual bool mayOverrideLocalAssignment() const;
+
   virtual bool requiresRegisterScavenging(const MachineFunction &MF) const;
 
   virtual bool trackLivenessAfterRegAlloc(const MachineFunction &MF) const;