This is a prototype of an experimental register allocation

framework. It's purpose is not to improve register allocation per se,
but to make it easier to develop powerful live range splitting. I call
it the basic allocator because it is as simple as a global allocator
can be but provides the building blocks for sophisticated register
allocation with live range splitting. 

A minimal implementation is provided that trivially spills whenever it
runs out of registers. I'm checking in now to get high-level design
and style feedback. I've only done minimal testing. The next step is
implementing a "greedy" allocation algorithm that does some register
reassignment and makes better splitting decisions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117174 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 193 insertions, 0 deletions

diff --git a/lib/CodeGen/LiveIntervalUnion.h b/lib/CodeGen/LiveIntervalUnion.h
new file mode 100644
index 0000000000..3953c5930a
--- /dev/null
+++ b/lib/CodeGen/LiveIntervalUnion.h
@@ -0,0 +1,193 @@
+//===-- LiveIntervalUnion.h - Live interval union data struct --*- C++ -*--===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// LiveIntervalUnion is a union of live segments across multiple live virtual
+// registers. This may be used during coalescing to represent a congruence
+// class, or during register allocation to model liveness of a physical
+// register.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_LIVEINTERVALUNION
+#define LLVM_CODEGEN_LIVEINTERVALUNION
+
+#include "llvm/CodeGen/LiveInterval.h"
+#include <vector>
+#include <set>
+
+namespace llvm {
+
+// A LiveSegment is a copy of a LiveRange object used within
+// LiveIntervalUnion. LiveSegment additionally contains a pointer to its
+// original live virtual register (LiveInterval). This allows quick lookup of
+// the live virtual register as we iterate over live segments in a union. Note
+// that LiveRange is misnamed and actually represents only a single contiguous
+// interval within a virtual register's liveness. To limit confusion, in this
+// file we refer it as a live segment.
+struct LiveSegment {
+  SlotIndex start;
+  SlotIndex end;
+  LiveInterval *liveVirtReg;
+
+  LiveSegment(SlotIndex s, SlotIndex e, LiveInterval &lvr)
+    : start(s), end(e), liveVirtReg(&lvr) {}
+
+  bool operator==(const LiveSegment &ls) const {
+    return start == ls.start && end == ls.end && liveVirtReg == ls.liveVirtReg;
+  }
+
+  bool operator!=(const LiveSegment &ls) const {
+    return !operator==(ls);
+  }
+
+  bool operator<(const LiveSegment &ls) const {
+    return start < ls.start || (start == ls.start && end < ls.end);
+  }
+};
+
+/// Compare a live virtual register segment to a LiveIntervalUnion segment.
+inline bool overlap(const LiveRange &lvrSeg, const LiveSegment &liuSeg) {
+  return lvrSeg.start < liuSeg.end && liuSeg.start < lvrSeg.end;
+}
+
+inline bool operator<(SlotIndex V, const LiveSegment &ls) {
+  return V < ls.start;
+}
+
+inline bool operator<(const LiveSegment &ls, SlotIndex V) {
+  return ls.start < V;
+}
+
+/// Union of live intervals that are strong candidates for coalescing into a
+/// single register (either physical or virtual depending on the context).  We
+/// expect the constituent live intervals to be disjoint, although we may
+/// eventually make exceptions to handle value-based interference.
+class LiveIntervalUnion {
+  // A set of live virtual register segments that supports fast insertion,
+  // intersection, and removal. 
+  //
+  // FIXME: std::set is a placeholder until we decide how to
+  // efficiently represent it. Probably need to roll our own B-tree.
+  typedef std::set<LiveSegment> LiveSegments;
+
+  // A set of live virtual registers. Elements have type LiveInterval, where
+  // each element represents the liveness of a single live virtual register.
+  // This is traditionally known as a live range, but we refer is as a live
+  // virtual register to avoid confusing it with the misnamed LiveRange
+  // class.
+  typedef std::vector<LiveInterval*> LiveVirtRegs;
+
+public:
+  // SegmentIter can advance to the next segment ordered by starting position
+  // which may belong to a different live virtual register. We also must be able
+  // to reach the current segment's containing virtual register.
+  typedef LiveSegments::iterator SegmentIter;
+
+  class InterferenceResult;
+  class Query;
+
+private:
+  unsigned repReg_;        // representative register number
+  LiveSegments segments_;  // union of virtual reg segements
+  LiveVirtRegs lvrs_;      // set of live virtual regs in the union
+
+public:
+  // default ctor avoids placement new
+  LiveIntervalUnion() : repReg_(0) {}
+  
+  void init(unsigned repReg) { repReg_ = repReg; }
+
+  SegmentIter begin() { return segments_.begin(); }
+  SegmentIter end() { return segments_.end(); }
+
+  /// FIXME: !!!!!!!!!!! Keeps a non-const ref
+  void unify(LiveInterval &lvr);
+
+  // FIXME: needed by RegAllocGreedy
+  //void extract(const LiveInterval &li);
+
+  /// Cache a single interference test result in the form of two intersecting
+  /// segments. This allows efficiently iterating over the interferences. The
+  /// iteration logic is handled by LiveIntervalUnion::Query which may
+  /// filter interferences depending on the type of query.
+  class InterferenceResult {
+    friend class Query;
+
+    LiveInterval::iterator lvrSegI_; // current position in _lvr
+    SegmentIter liuSegI_;            // current position in _liu
+    
+    // Internal ctor.
+    InterferenceResult(LiveInterval::iterator lvrSegI, SegmentIter liuSegI)
+      : lvrSegI_(lvrSegI), liuSegI_(liuSegI) {}
+
+  public:
+    // Public default ctor.
+    InterferenceResult(): lvrSegI_(), liuSegI_() {}
+
+    // Note: this interface provides raw access to the iterators because the
+    // result has no way to tell if it's valid to dereference them.
+
+    // Access the lvr segment. 
+    const LiveInterval::iterator &lvrSegPos() const { return lvrSegI_; }
+
+    // Access the liu segment.
+    const SegmentIter &liuSeg() const { return liuSegI_; }
+
+    bool operator==(const InterferenceResult &ir) const {
+      return lvrSegI_ == ir.lvrSegI_ && liuSegI_ == ir.liuSegI_;
+    }
+    bool operator!=(const InterferenceResult &ir) const {
+      return !operator==(ir);
+    }
+  };
+
+  /// Query interferences between a single live virtual register and a live
+  /// interval union.
+  class Query {
+    LiveIntervalUnion &liu_;
+    LiveInterval &lvr_;
+    InterferenceResult firstInterference_;
+    // TBD: interfering vregs
+
+  public:
+    Query(LiveInterval &lvr, LiveIntervalUnion &liu): liu_(liu), lvr_(lvr) {}
+
+    LiveInterval &lvr() const { return lvr_; }
+
+    bool isInterference(const InterferenceResult &ir) const {
+      if (ir.lvrSegI_ != lvr_.end()) {
+        assert(overlap(*ir.lvrSegI_, *ir.liuSegI_) &&
+               "invalid segment iterators");
+        return true;
+      }
+      return false;
+    }
+
+    // Does this live virtual register interfere with the union.
+    bool checkInterference() { return isInterference(firstInterference()); }
+
+    // First pair of interfering segments, or a noninterfering result.
+    InterferenceResult firstInterference();
+
+    // Treat the result as an iterator and advance to the next interfering pair
+    // of segments. Visiting each unique interfering pairs means that the same
+    // lvr or liu segment may be visited multiple times.
+    bool nextInterference(InterferenceResult &ir) const;
+        
+    // TBD: bool collectInterferingVirtRegs(unsigned maxInterference)
+
+  private:
+    // Private interface for queries
+    void findIntersection(InterferenceResult &ir) const;
+  };
+};
+
+} // end namespace llvm
+
+#endif // !defined(LLVM_CODEGEN_LIVEINTERVALUNION)