[RegAllocGreedy] Provide a subtarget hook to disable the local reassignment

heuristic.
By default, no functionality change.
This is a follow-up of r212099.

This hook provides a finer grain to control the optimization.

<rdar://problem/17444599>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212204 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Quentin Colombet

include/llvm/Target/TargetSubtargetInfo.h

@@ -100,6 +100,12 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
                                      AntiDepBreakMode& Mode,
                                      RegClassVector& CriticalPathRCs) const;
 
+  /// \brief True if the subtarget should run the local reassignment
+  /// heuristic of the register allocator.
+  /// This heuristic may be compile time intensive, \p OptLevel provides
+  /// a finer grain to tune the register allocator.
+  virtual bool enableRALocalReassignment(CodeGenOpt::Level OptLevel) const;
+
   /// \brief Enable use of alias analysis during code generation (during MI
   /// scheduling, DAGCombine, etc.).
   virtual bool useAA() const;

lib/CodeGen/RegAllocGreedy.cpp

@@ -44,6 +44,7 @@
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Timer.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetSubtargetInfo.h"
 #include <queue>
 
 using namespace llvm;
@@ -83,7 +84,7 @@ static cl::opt<bool> EnableLocalReassignment(
     "enable-local-reassign", cl::Hidden,
     cl::desc("Local reassignment can yield better allocation decisions, but "
              "may be compile time intensive"),
-    cl::init(true));
+    cl::init(false));
 
 // FIXME: Find a good default for this flag and remove the flag.
 static cl::opt<unsigned>
@@ -291,6 +292,10 @@ class RAGreedy : public MachineFunctionPass,
   /// Callee-save register cost, calculated once per machine function.
   BlockFrequency CSRCost;
 
+  /// Run or not the local reassignment heuristic. This information is
+  /// obtained from the TargetSubtargetInfo.
+  bool EnableLocalReassign;
+
 public:
   RAGreedy();
 
@@ -737,7 +742,7 @@ bool RAGreedy::canEvictInterference(LiveInterval &VirtReg, unsigned PhysReg,
       // Evicting another local live range in this case could lead to suboptimal
       // coloring.
       if (!MaxCost.isMax() && IsLocal && LIS->intervalIsInOneMBB(*Intf) &&
-          (!EnableLocalReassignment || !canReassign(*Intf, PhysReg))) {
+          (!EnableLocalReassign || !canReassign(*Intf, PhysReg))) {
         return false;
       }
     }
@@ -2314,9 +2319,14 @@ bool RAGreedy::runOnMachineFunction(MachineFunction &mf) {
                << "********** Function: " << mf.getName() << '\n');
 
   MF = &mf;
-  TRI = MF->getTarget().getRegisterInfo();
-  TII = MF->getTarget().getInstrInfo();
+  const TargetMachine &TM = MF->getTarget();
+  TRI = TM.getRegisterInfo();
+  TII = TM.getInstrInfo();
   RCI.runOnMachineFunction(mf);
+
+  EnableLocalReassign = EnableLocalReassignment ||
+    TM.getSubtargetImpl()->enableRALocalReassignment(TM.getOptLevel());
+
   if (VerifyEnabled)
     MF->verify(this, "Before greedy register allocator");
 

lib/Target/TargetSubtargetInfo.cpp

@@ -47,6 +47,11 @@ bool TargetSubtargetInfo::enableMachineScheduler() const {
   return false;
 }
 
+bool TargetSubtargetInfo::enableRALocalReassignment(
+    CodeGenOpt::Level OptLevel) const {
+  return true;
+}
+
 bool TargetSubtargetInfo::enablePostMachineScheduler() const {
   return false;
 }