[MIScheduler] Slightly better handling of constrainLocalCopy when both source and dest are local

This fixes PR21792.

Differential Revision: http://reviews.llvm.org/D6823

llvm-svn: 226433

1 files changed, 7 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp
index 9fe23c5b227..a554254794a 100644
--- a/llvm/lib/CodeGen/MachineScheduler.cpp
+++ b/llvm/lib/CodeGen/MachineScheduler.cpp
@@ -1434,12 +1434,15 @@ void CopyConstrain::constrainLocalCopy(SUnit *CopySU, ScheduleDAGMILive *DAG) {
   // Check if either the dest or source is local. If it's live across a back
   // edge, it's not local. Note that if both vregs are live across the back
   // edge, we cannot successfully contrain the copy without cyclic scheduling.
-  unsigned LocalReg = DstReg;
-  unsigned GlobalReg = SrcReg;
+  // If both the copy's source and dest are local live intervals, then we
+  // should treat the dest as the global for the purpose of adding
+  // constraints. This adds edges from source's other uses to the copy.
+  unsigned LocalReg = SrcReg;
+  unsigned GlobalReg = DstReg;
   LiveInterval *LocalLI = &LIS->getInterval(LocalReg);
   if (!LocalLI->isLocal(RegionBeginIdx, RegionEndIdx)) {
-    LocalReg = SrcReg;
-    GlobalReg = DstReg;
+    LocalReg = DstReg;
+    GlobalReg = SrcReg;
     LocalLI = &LIS->getInterval(LocalReg);
     if (!LocalLI->isLocal(RegionBeginIdx, RegionEndIdx))
       return;