Subzero: Optimize live range overlaps() computation through trimming.

src/IceRegAlloc.cpp

Index: src/IceRegAlloc.cpp
diff --git a/src/IceRegAlloc.cpp b/src/IceRegAlloc.cpp
index 69353a1df9bb20ff8dd3ee86cd8ed9b6f5ccc1db..589cd134428565ef469f77171f0eff4fedd6bf42 100644
--- a/src/IceRegAlloc.cpp
+++ b/src/IceRegAlloc.cpp
@@ -98,6 +98,7 @@ void LinearScan::scan(const llvm::SmallBitVector &RegMaskFull) {
       // it was never referenced.
       if (Var->getLiveRange().isEmpty())
         continue;
+      Var->untrimLiveRange();
       LiveRangeWrapper R(Var);
       Unhandled.insert(R);
       if (Var->hasReg()) {
@@ -159,6 +160,7 @@ void LinearScan::scan(const llvm::SmallBitVector &RegMaskFull) {
       Next = I;
       ++Next;
       LiveRangeWrapper Item = *I;
+      Item.Var->trimLiveRange(Cur.range().getStart());
       bool Moved = false;
       if (Item.endsBefore(Cur)) {
         // Move Item from Active to Handled list.
@@ -195,6 +197,16 @@ void LinearScan::scan(const llvm::SmallBitVector &RegMaskFull) {
       Next = I;
       ++Next;
       LiveRangeWrapper Item = *I;
+      Item.Var->trimLiveRange(Cur.range().getStart());
+      // As an optimization, don't bother checking pure point-valued
+      // Inactive ranges, because the overlapsStart() test will never
+      // succeed, and the endsBefore() test will generally only
+      // succeed after the last call instruction, which statistically
+      // happens near the end.  TODO(stichnot): Consider suppressing
+      // this check every N iterations in case calls are only at the
+      // beginning of the function.
+      if (!Item.range().isNonpoints())
+        continue;
       if (Item.endsBefore(Cur)) {
         // Move Item from Inactive to Handled list.
         if (Verbose) {