[RegAllocGreedy]: Allow recoloring of done register if it's non-tied

Summary:
If we have a non-allocated register, we allow us to try recoloring of an
already allocated and "Done" register, even if they are of the same
register class, if the non-allocated register has at least one tied def
and the allocated one has none.

It should be easier to recolor the non-tied register than the tied one, so
it might be an improvement even if they use the same regclasses.

Reviewers: qcolombet

Reviewed By: qcolombet

Subscribers: llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D38309

llvm-svn: 314388

1 files changed, 14 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index 1761a72..5bef247 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -2054,6 +2054,15 @@ unsigned RAGreedy::trySplit(LiveInterval &VirtReg, AllocationOrder &Order,
 //                          Last Chance Recoloring
 //===----------------------------------------------------------------------===//
 
+/// Return true if \p reg has any tied def operand.
+static bool hasTiedDef(MachineRegisterInfo *MRI, unsigned reg) {
+  for (const MachineOperand &MO : MRI->def_operands(reg))
+    if (MO.isTied())
+      return true;
+
+  return false;
+}
+
 /// mayRecolorAllInterferences - Check if the virtual registers that
 /// interfere with \p VirtReg on \p PhysReg (or one of its aliases) may be
 /// recolored to free \p PhysReg.
@@ -2082,8 +2091,11 @@ RAGreedy::mayRecolorAllInterferences(unsigned PhysReg, LiveInterval &VirtReg,
       LiveInterval *Intf = Q.interferingVRegs()[i - 1];
       // If Intf is done and sit on the same register class as VirtReg,
       // it would not be recolorable as it is in the same state as VirtReg.
-      if ((getStage(*Intf) == RS_Done &&
-           MRI->getRegClass(Intf->reg) == CurRC) ||
+      // However, if VirtReg has tied defs and Intf doesn't, then
+      // there is still a point in examining if it can be recolorable.
+      if (((getStage(*Intf) == RS_Done &&
+            MRI->getRegClass(Intf->reg) == CurRC) &&
+           !(hasTiedDef(MRI, VirtReg.reg) && !hasTiedDef(MRI, Intf->reg))) ||
           FixedRegisters.count(Intf->reg)) {
         DEBUG(dbgs() << "Early abort: the interference is not recolorable.\n");
         return false;