If the live interval legnth is essentially zero, i.e. in every live range

the use follows def immediately, it doesn't make sense to spill it and
hope it will be easier to allocate for this LI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28217 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/RegAllocLinearScan.cpp b/lib/CodeGen/RegAllocLinearScan.cpp
index 59467c1..fd7d5a6 100644 (file)
--- a/lib/CodeGen/RegAllocLinearScan.cpp
+++ b/lib/CodeGen/RegAllocLinearScan.cpp
@@ -406,6 +406,14 @@ static void RevertVectorIteratorsTo(RA::IntervalPtrs &V, unsigned Point) {
   }
 }
 
+static bool isZeroLengthInterval(LiveInterval *li) {
+  for (LiveInterval::Ranges::const_iterator
+         i = li->ranges.begin(), e = li->ranges.end(); i != e; ++i)
+    if (i->end - i->start > LiveIntervals::InstrSlots::NUM)
+      return false;
+  return true;
+}
+
 
 /// assignRegOrStackSlotAtInterval - assign a register if one is available, or
 /// spill.
@@ -557,10 +565,16 @@ void RA::assignRegOrStackSlotAtInterval(LiveInterval* cur)
   DEBUG(std::cerr << "\t\tregister with min weight: "
         << mri_->getName(minReg) << " (" << minWeight << ")\n");
 
+  // If the live interval legnth is essentially zero, i.e. in every live range
+  // the use follows def immediately, it doesn't make sense to spill it and
+  // hope it will be easier to allocate for this li.
+  if (isZeroLengthInterval(cur))
+    DEBUG(std::cerr << "\t\tavoid spilling zero length live interval: "
+          << *cur << '\n';);
   // if the current has the minimum weight, we need to spill it and
   // add any added intervals back to unhandled, and restart
   // linearscan.
-  if (cur->weight <= minWeight) {
+  else if (cur->weight <= minWeight) {
     DEBUG(std::cerr << "\t\t\tspilling(c): " << *cur << '\n';);
     int slot = vrm_->assignVirt2StackSlot(cur->reg);
     std::vector<LiveInterval*> added =