Prepare RAGreedy::growRegion for compact regions.

A split candidate can have a null PhysReg which means that it doesn't
map to a real interference pattern. Instead, pretend that all through
blocks have interference.

This makes it possible to generate compact regions where the live range
doesn't go through blocks that don't use it. The live range will still
be live between directly connected blocks with uses.

Splitting around a compact region tends to produce a live range with a
high spill weight, so it may evict a less dense live range.

llvm-svn: 135845

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index 570a832303a..0267717b843 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -749,7 +749,14 @@ void RAGreedy::growRegion(GlobalSplitCandidate &Cand) {
     // Any new blocks to add?
     if (ActiveBlocks.size() == AddedTo)
       break;
-    addThroughConstraints(Cand.Intf, makeArrayRef(ActiveBlocks).slice(AddedTo));
+
+    // Compute through constraints from the interference, or assume that all
+    // through blocks prefer spilling when forming compact regions.
+    ArrayRef<unsigned> NewBlocks = makeArrayRef(ActiveBlocks).slice(AddedTo);
+    if (Cand.PhysReg)
+      addThroughConstraints(Cand.Intf, NewBlocks);
+    else
+      SpillPlacer->addPrefSpill(NewBlocks);
     AddedTo = ActiveBlocks.size();
 
     // Perhaps iterating can enable more bundles?