lib/CodeGen/LiveIntervalUnion.cpp

   1 //===-- LiveIntervalUnion.cpp - Live interval union data structure --------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // LiveIntervalUnion represents a coalesced set of live intervals. This may be
  11 // used during coalescing to represent a congruence class, or during register
  12 // allocation to model liveness of a physical register.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #define DEBUG_TYPE "regalloc"
  17 #include "LiveIntervalUnion.h"
  18 #include "llvm/Support/Debug.h"
  19 #include "llvm/Support/raw_ostream.h"
  20 #include <algorithm>
  21 using namespace llvm;
  22
  23 // Merge a LiveInterval's segments. Guarantee no overlaps.
  24 //
  25 // Consider coalescing adjacent segments to save space, even though it makes
  26 // extraction more complicated.
  27 void LiveIntervalUnion::unify(LiveInterval &lvr) {
  28   // Insert each of the virtual register's live segments into the map
  29   SegmentIter segPos = segments_.begin();
  30   for (LiveInterval::iterator lvrI = lvr.begin(), lvrEnd = lvr.end();
  31        lvrI != lvrEnd; ++lvrI ) {
  32     LiveSegment segment(lvrI->start, lvrI->end, lvr);
  33     segPos = segments_.insert(segPos, segment);
  34     assert(*segPos == segment && "need equal val for equal key");
  35 #ifndef NDEBUG
  36     // check for overlap (inductively)
  37     if (segPos != segments_.begin()) {
  38       SegmentIter prevPos = segPos;
  39       --prevPos;
  40       assert(prevPos->end <= segment.start && "overlapping segments" );
  41     }
  42     SegmentIter nextPos = segPos;
  43     ++nextPos;
  44     if (nextPos != segments_.end())
  45       assert(segment.end <= nextPos->start && "overlapping segments" );
  46 #endif // NDEBUG
  47   }
  48 }
  49
  50 // Low-level helper to find the first segment in the range [segI,segEnd) that
  51 // intersects with a live virtual register segment, or segI.start >= lvr.end
  52 //
  53 // This logic is tied to the underlying LiveSegments data structure. For now, we
  54 // use a binary search within the vector to find the nearest starting position,
  55 // then reverse iterate to find the first overlap.
  56 //
  57 // Upon entry we have segI.start < lvrSeg.end
  58 // seg   |--...
  59 //        \   .
  60 // lvr ...-|
  61 //
  62 // After binary search, we have segI.start >= lvrSeg.start:
  63 // seg   |--...
  64 //      /
  65 // lvr |--...
  66 //
  67 // Assuming intervals are disjoint, if an intersection exists, it must be the
  68 // segment found or immediately behind it. We continue reverse iterating to
  69 // return the first overlap.
  70 typedef LiveIntervalUnion::SegmentIter SegmentIter;
  71 static SegmentIter upperBound(SegmentIter segBegin,
  72                        SegmentIter segEnd,
  73                        const LiveRange &lvrSeg) {
  74   assert(lvrSeg.end > segBegin->start && "segment iterator precondition");
  75   // get the next LIU segment such that setg.start is not less than
  76   // lvrSeg.start
  77   SegmentIter segI = std::upper_bound(segBegin, segEnd, lvrSeg.start);
  78   while (segI != segBegin) {
  79     --segI;
  80     if (lvrSeg.start >= segI->end)
  81       return ++segI;
  82   }
  83   return segI;
  84 }
  85
  86 // Private interface accessed by Query.
  87 //
  88 // Find a pair of segments that intersect, one in the live virtual register
  89 // (LiveInterval), and the other in this LiveIntervalUnion. The caller (Query)
  90 // is responsible for advancing the LiveIntervalUnion segments to find a
  91 // "notable" intersection, which requires query-specific logic.
  92 //
  93 // This design assumes only a fast mechanism for intersecting a single live
  94 // virtual register segment with a set of LiveIntervalUnion segments.  This may
  95 // be ok since most LVRs have very few segments.  If we had a data
  96 // structure that optimizd MxN intersection of segments, then we would bypass
  97 // the loop that advances within the LiveInterval.
  98 //
  99 // If no intersection exists, set lvrI = lvrEnd, and set segI to the first
 100 // segment whose start point is greater than LiveInterval's end point.
 101 //
 102 // Assumes that segments are sorted by start position in both
 103 // LiveInterval and LiveSegments.
 104 void LiveIntervalUnion::Query::findIntersection(InterferenceResult &ir) const {
 105   LiveInterval::iterator lvrEnd = lvr_.end();
 106   SegmentIter liuEnd = liu_.end();
 107   while (ir.liuSegI_ != liuEnd) {
 108     // Slowly advance the live virtual reg iterator until we surpass the next
 109     // segment in this union. If this is ever used for coalescing of fixed
 110     // registers and we have a LiveInterval with thousands of segments, then use
 111     // upper bound instead.
 112     while (ir.lvrSegI_ != lvrEnd && ir.lvrSegI_->end <= ir.liuSegI_->start)
 113       ++ir.lvrSegI_;
 114     if (ir.lvrSegI_ == lvrEnd)
 115       break;
 116     // lvrSegI_ may have advanced far beyond liuSegI_,
 117     // do a fast intersection test to "catch up"
 118     ir.liuSegI_ = upperBound(ir.liuSegI_, liuEnd, *ir.lvrSegI_);
 119     // Check if no liuSegI_ exists with lvrSegI_->start < liuSegI_.end
 120     if (ir.liuSegI_ == liuEnd)
 121       break;
 122     if (ir.liuSegI_->start < ir.lvrSegI_->end) {
 123       assert(overlap(*ir.lvrSegI_, *ir.liuSegI_) && "upperBound postcondition");
 124       break;
 125     }
 126   }
 127   if (ir.liuSegI_ == liuEnd)
 128     ir.lvrSegI_ = lvrEnd;
 129 }
 130
 131 // Find the first intersection, and cache interference info
 132 // (retain segment iterators into both lvr_ and liu_).
 133 LiveIntervalUnion::InterferenceResult
 134 LiveIntervalUnion::Query::firstInterference() {
 135   if (firstInterference_ != LiveIntervalUnion::InterferenceResult()) {
 136     return firstInterference_;
 137   }
 138   firstInterference_ = InterferenceResult(lvr_.begin(), liu_.begin());
 139   findIntersection(firstInterference_);
 140   return firstInterference_;
 141 }
 142
 143 // Treat the result as an iterator and advance to the next interfering pair
 144 // of segments. This is a plain iterator with no filter.
 145 bool LiveIntervalUnion::Query::nextInterference(InterferenceResult &ir) const {
 146   assert(isInterference(ir) && "iteration past end of interferences");
 147   // Advance either the lvr or liu segment to ensure that we visit all unique
 148   // overlapping pairs.
 149   if (ir.lvrSegI_->end < ir.liuSegI_->end) {
 150     if (++ir.lvrSegI_ == lvr_.end())
 151       return false;
 152   }
 153   else {
 154     if (++ir.liuSegI_ == liu_.end()) {
 155       ir.lvrSegI_ = lvr_.end();
 156       return false;
 157     }
 158   }
 159   if (overlap(*ir.lvrSegI_, *ir.liuSegI_))
 160     return true;
 161   // find the next intersection
 162   findIntersection(ir);
 163   return isInterference(ir);
 164 }