Store (priority,regnum) pairs in the priority queue instead of providing an

abstract priority queue interface in subclasses that want to override the
priority calculations.

Subclasses must provide a getPriority() implementation instead.

This approach requires less code as long as priorities are expressable as simple
floats, and it avoids the dangers of defining potentially expensive priority
comparison functions.

It also should speed up priority_queue operations since they no longer have to
chase pointers when comparing registers. This is not measurable, though.

Preferably, we shouldn't use floats to guide code generation. The use of floats
here is derived from the use of floats for spill weights. Spill weights have a
dynamic range that doesn't lend itself easily to a fixpoint implementation.

When someone invents a stable spill weight representation, it can be reused for
allocation priorities.

llvm-svn: 121294

lib/CodeGen/RegAllocBase.h

@@ -39,6 +39,7 @@
 
 #include "llvm/ADT/OwningPtr.h"
 #include "LiveIntervalUnion.h"
+#include <queue>
 
 namespace llvm {
 
@@ -48,16 +49,6 @@ class VirtRegMap;
 class LiveIntervals;
 class Spiller;
 
-// Heuristic that determines the priority of assigning virtual to physical
-// registers. The main impact of the heuristic is expected to be compile time.
-// The default is to simply compare spill weights.
-struct LessSpillWeightPriority
-  : public std::binary_function<LiveInterval,LiveInterval, bool> {
-  bool operator()(const LiveInterval *Left, const LiveInterval *Right) const {
-    return Left->weight < Right->weight;
-  }
-};
-
 // Forward declare a priority queue of live virtual registers. If an
 // implementation needs to prioritize by anything other than spill weight, then
 // this will become an abstract base class with virtual calls to push/get.
@@ -128,6 +119,10 @@ protected:
   // Get a temporary reference to a Spiller instance.
   virtual Spiller &spiller() = 0;
 
+  // getPriority - Calculate the allocation priority for VirtReg.
+  // Virtual registers with higher priorities are allocated first.
+  virtual float getPriority(LiveInterval *LI) = 0;
+
   // A RegAlloc pass should override this to provide the allocation heuristics.
   // Each call must guarantee forward progess by returning an available PhysReg
   // or new set of split live virtual registers. It is up to the splitter to
@@ -158,7 +153,7 @@ protected:
 #endif
 
 private:
-  void seedLiveVirtRegs(LiveVirtRegQueue &VirtRegQ);
+  void seedLiveVirtRegs(std::priority_queue<std::pair<float, unsigned> >&);
 
   void spillReg(LiveInterval &VirtReg, unsigned PhysReg,
                 SmallVectorImpl<LiveInterval*> &SplitVRegs);

lib/CodeGen/RegAllocBasic.cpp

@@ -43,8 +43,6 @@
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 
-#include <vector>
-#include <queue>
 #include <cstdlib>
 
 using namespace llvm;
@@ -103,6 +101,8 @@ public:
 
   virtual Spiller &spiller() { return *SpillerInstance; }
 
+  virtual float getPriority(LiveInterval *LI) { return LI->weight; }
+
   virtual unsigned selectOrSplit(LiveInterval &VirtReg,
                                  SmallVectorImpl<LiveInterval*> &SplitVRegs);
 
@@ -230,49 +230,18 @@ void RegAllocBase::releaseMemory() {
   PhysReg2LiveUnion.clear();
 }
 
-namespace llvm {
-/// This class defines a queue of live virtual registers prioritized by spill
-/// weight. The heaviest vreg is popped first.
-///
-/// Currently, this is trivial wrapper that gives us an opaque type in the
-/// header, but we may later give it a virtual interface for register allocators
-/// to override the priority queue comparator.
-class LiveVirtRegQueue {
-  typedef std::priority_queue
-    <LiveInterval*, std::vector<LiveInterval*>, LessSpillWeightPriority>
-    PriorityQ;
-  PriorityQ PQ;
-
-public:
-  // Is the queue empty?
-  bool empty() { return PQ.empty(); }
-
-  // Get the highest priority lvr (top + pop)
-  LiveInterval *get() {
-    LiveInterval *VirtReg = PQ.top();
-    PQ.pop();
-    return VirtReg;
-  }
-  // Add this lvr to the queue
-  void push(LiveInterval *VirtReg) {
-    PQ.push(VirtReg);
-  }
-};
-} // end namespace llvm
-
 // Visit all the live virtual registers. If they are already assigned to a
 // physical register, unify them with the corresponding LiveIntervalUnion,
 // otherwise push them on the priority queue for later assignment.
-void RegAllocBase::seedLiveVirtRegs(LiveVirtRegQueue &VirtRegQ) {
+void RegAllocBase::
+seedLiveVirtRegs(std::priority_queue<std::pair<float, unsigned> > &VirtRegQ) {
   for (LiveIntervals::iterator I = LIS->begin(), E = LIS->end(); I != E; ++I) {
     unsigned RegNum = I->first;
     LiveInterval &VirtReg = *I->second;
-    if (TargetRegisterInfo::isPhysicalRegister(RegNum)) {
+    if (TargetRegisterInfo::isPhysicalRegister(RegNum))
       PhysReg2LiveUnion[RegNum].unify(VirtReg);
-    }
-    else {
-      VirtRegQ.push(&VirtReg);
-    }
+    else
+      VirtRegQ.push(std::make_pair(getPriority(&VirtReg), RegNum));
   }
 }
 
@@ -281,27 +250,28 @@ void RegAllocBase::seedLiveVirtRegs(LiveVirtRegQueue &VirtRegQ) {
 void RegAllocBase::allocatePhysRegs() {
 
   // Push each vreg onto a queue or "precolor" by adding it to a physreg union.
-  LiveVirtRegQueue VirtRegQ;
+  std::priority_queue<std::pair<float, unsigned> > VirtRegQ;
   seedLiveVirtRegs(VirtRegQ);
 
   // Continue assigning vregs one at a time to available physical registers.
   while (!VirtRegQ.empty()) {
     // Pop the highest priority vreg.
-    LiveInterval *VirtReg = VirtRegQ.get();
+    LiveInterval &VirtReg = LIS->getInterval(VirtRegQ.top().second);
+    VirtRegQ.pop();
 
     // selectOrSplit requests the allocator to return an available physical
     // register if possible and populate a list of new live intervals that
     // result from splitting.
     typedef SmallVector<LiveInterval*, 4> VirtRegVec;
     VirtRegVec SplitVRegs;
-    unsigned AvailablePhysReg = selectOrSplit(*VirtReg, SplitVRegs);
+    unsigned AvailablePhysReg = selectOrSplit(VirtReg, SplitVRegs);
 
     if (AvailablePhysReg) {
       DEBUG(dbgs() << "allocating: " << TRI->getName(AvailablePhysReg) <<
-            " " << *VirtReg << '\n');
-      assert(!VRM->hasPhys(VirtReg->reg) && "duplicate vreg in union");
-      VRM->assignVirt2Phys(VirtReg->reg, AvailablePhysReg);
-      PhysReg2LiveUnion[AvailablePhysReg].unify(*VirtReg);
+            " " << VirtReg << '\n');
+      assert(!VRM->hasPhys(VirtReg.reg) && "duplicate vreg in union");
+      VRM->assignVirt2Phys(VirtReg.reg, AvailablePhysReg);
+      PhysReg2LiveUnion[AvailablePhysReg].unify(VirtReg);
     }
     for (VirtRegVec::iterator I = SplitVRegs.begin(), E = SplitVRegs.end();
          I != E; ++I) {
@@ -310,7 +280,8 @@ void RegAllocBase::allocatePhysRegs() {
       DEBUG(dbgs() << "queuing new interval: " << *SplitVirtReg << "\n");
       assert(TargetRegisterInfo::isVirtualRegister(SplitVirtReg->reg) &&
              "expect split value in virtual register");
-      VirtRegQ.push(SplitVirtReg);
+      VirtRegQ.push(std::make_pair(getPriority(SplitVirtReg),
+                                   SplitVirtReg->reg));
     }
   }
 }

lib/CodeGen/RegAllocGreedy.cpp

@@ -70,6 +70,8 @@ public:
 
   virtual Spiller &spiller() { return *SpillerInstance; }
 
+  virtual float getPriority(LiveInterval *LI) { return LI->weight; }
+
   virtual unsigned selectOrSplit(LiveInterval &VirtReg,
                                  SmallVectorImpl<LiveInterval*> &SplitVRegs);