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
diff --git a/lib/CodeGen/LiveIntervalUnion.cpp b/lib/CodeGen/LiveIntervalUnion.cpp
new file mode 100644
index 0000000..b22f466
--- /dev/null
+++ b/lib/CodeGen/LiveIntervalUnion.cpp
@@ -0,0 +1,167 @@
+//===-- LiveIntervalUnion.cpp - Live interval union data structure --------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// LiveIntervalUnion represents a coalesced set of live intervals. This may be
+// used during coalescing to represent a congruence class, or during register
+// allocation to model liveness of a physical register.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "regalloc"
+#include "LiveIntervalUnion.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+using namespace llvm;
+
+// Merge a LiveInterval's segments. Guarantee no overlaps.
+void LiveIntervalUnion::unify(LiveInterval &lvr) {
+ // Add this live virtual register to the union
+ LiveVirtRegs::iterator pos = std::upper_bound(lvrs_.begin(), lvrs_.end(),
+ &lvr, less_ptr<LiveInterval>());
+ assert(pos == lvrs_.end() || *pos != &lvr && "duplicate LVR insertion");
+ lvrs_.insert(pos, &lvr);
+ // Insert each of the virtual register's live segments into the map
+ SegmentIter segPos = segments_.begin();
+ for (LiveInterval::iterator lvrI = lvr.begin(), lvrEnd = lvr.end();
+ lvrI != lvrEnd; ++lvrI ) {
+ LiveSegment segment(lvrI->start, lvrI->end, lvr);
+ segPos = segments_.insert(segPos, segment);
+ assert(*segPos == segment && "need equal val for equal key");
+ }
+}
+
+namespace {
+
+// Keep LVRs sorted for fast membership test and extraction.
+struct LessReg
+ : public std::binary_function<LiveInterval*, LiveInterval*, bool> {
+ bool operator()(const LiveInterval *left, const LiveInterval *right) const {
+ return left->reg < right->reg;
+ }
+};
+
+// Low-level helper to find the first segment in the range [segI,segEnd) that
+// intersects with a live virtual register segment, or segI.start >= lvr.end
+//
+// This logic is tied to the underlying LiveSegments data structure. For now, we
+// use a binary search within the vector to find the nearest starting position,
+// then reverse iterate to find the first overlap.
+//
+// Upon entry we have segI.start < lvrSeg.end
+// seg |--...
+// \ .
+// lvr ...-|
+//
+// After binary search, we have segI.start >= lvrSeg.start:
+// seg |--...
+// /
+// lvr |--...
+//
+// Assuming intervals are disjoint, if an intersection exists, it must be the
+// segment found or immediately behind it. We continue reverse iterating to
+// return the first overlap.
+//
+// FIXME: support extract(), handle tombstones of extracted lvrs.
+typedef LiveIntervalUnion::SegmentIter SegmentIter;
+SegmentIter upperBound(SegmentIter segBegin,
+ SegmentIter segEnd,
+ const LiveRange &lvrSeg) {
+ assert(lvrSeg.end > segBegin->start && "segment iterator precondition");
+ // get the next LIU segment such that setg.start is not less than
+ // lvrSeg.start
+ SegmentIter segI = std::upper_bound(segBegin, segEnd, lvrSeg.start);
+ while (segI != segBegin) {
+ --segI;
+ if (lvrSeg.start >= segI->end)
+ return ++segI;
+ }
+ return segI;
+}
+} // end anonymous namespace
+
+// Private interface accessed by Query.
+//
+// Find a pair of segments that intersect, one in the live virtual register
+// (LiveInterval), and the other in this LiveIntervalUnion. The caller (Query)
+// is responsible for advancing the LiveIntervalUnion segments to find a
+// "notable" intersection, which requires query-specific logic.
+//
+// This design assumes only a fast mechanism for intersecting a single live
+// virtual register segment with a set of LiveIntervalUnion segments. This may
+// be ok since most LVRs have very few segments. If we had a data
+// structure that optimizd MxN intersection of segments, then we would bypass
+// the loop that advances within the LiveInterval.
+//
+// If no intersection exists, set lvrI = lvrEnd, and set segI to the first
+// segment whose start point is greater than LiveInterval's end point.
+//
+// Assumes that segments are sorted by start position in both
+// LiveInterval and LiveSegments.
+void LiveIntervalUnion::Query::findIntersection(InterferenceResult &ir) const {
+ LiveInterval::iterator lvrEnd = lvr_.end();
+ SegmentIter liuEnd = liu_.end();
+ while (ir.liuSegI_ != liuEnd) {
+ // Slowly advance the live virtual reg iterator until we surpass the next
+ // segment in this union. If this is ever used for coalescing of fixed
+ // registers and we have a LiveInterval with thousands of segments, then use
+ // upper bound instead.
+ while (ir.lvrSegI_ != lvrEnd && ir.lvrSegI_->end <= ir.liuSegI_->start)
+ ++ir.lvrSegI_;
+ if (ir.lvrSegI_ == lvrEnd)
+ break;
+ // lvrSegI_ may have advanced far beyond liuSegI_,
+ // do a fast intersection test to "catch up"
+ ir.liuSegI_ = upperBound(ir.liuSegI_, liuEnd, *ir.lvrSegI_);
+ // Check if no liuSegI_ exists with lvrSegI_->start < liuSegI_.end
+ if (ir.liuSegI_ == liuEnd)
+ break;
+ if (ir.liuSegI_->start < ir.lvrSegI_->end) {
+ assert(overlap(*ir.lvrSegI_, *ir.liuSegI_) && "upperBound postcondition");
+ break;
+ }
+ }
+ if (ir.liuSegI_ == liuEnd)
+ ir.lvrSegI_ = lvrEnd;
+}
+
+// Find the first intersection, and cache interference info
+// (retain segment iterators into both lvr_ and liu_).
+LiveIntervalUnion::InterferenceResult
+LiveIntervalUnion::Query::firstInterference() {
+ if (firstInterference_ != LiveIntervalUnion::InterferenceResult()) {
+ return firstInterference_;
+ }
+ firstInterference_ = InterferenceResult(lvr_.begin(), liu_.begin());
+ findIntersection(firstInterference_);
+ return firstInterference_;
+}
+
+// Treat the result as an iterator and advance to the next interfering pair
+// of segments. This is a plain iterator with no filter.
+bool LiveIntervalUnion::Query::nextInterference(InterferenceResult &ir) const {
+ assert(isInterference(ir) && "iteration past end of interferences");
+ // Advance either the lvr or liu segment to ensure that we visit all unique
+ // overlapping pairs.
+ if (ir.lvrSegI_->end < ir.liuSegI_->end) {
+ if (++ir.lvrSegI_ == lvr_.end())
+ return false;
+ }
+ else {
+ if (++ir.liuSegI_ == liu_.end()) {
+ ir.lvrSegI_ = lvr_.end();
+ return false;
+ }
+ }
+ if (overlap(*ir.lvrSegI_, *ir.liuSegI_))
+ return true;
+ // find the next intersection
+ findIntersection(ir);
+ return isInterference(ir);
+}