Tweak cross-class coalescing to be more aggressive when the target class is small.

The greedy register allocator has live range splitting and register class
inflation, so it can actually fully undo this join, including restoring the
original register classes.

We still don't want to do this for long live ranges, mostly because of the high
register pressure of there are many constrained live ranges overlapping.

llvm-svn: 131466

1 files changed, 9 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
index cebf4c9f4a5..1a2151fecf2 100644
--- a/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
+++ b/llvm/lib/CodeGen/SimpleRegisterCoalescing.cpp
@@ -987,8 +987,14 @@ SimpleRegisterCoalescing::isWinToJoinCrossClass(unsigned SrcReg,
   LiveInterval &DstInt = li_->getInterval(DstReg);
   unsigned SrcSize = li_->getApproximateInstructionCount(SrcInt);
   unsigned DstSize = li_->getApproximateInstructionCount(DstInt);
-  if (SrcSize <= NewRCCount && DstSize <= NewRCCount)
+
+  // Coalesce aggressively if the intervals are small compared to the number of
+  // registers in the new class. The number 4 is fairly arbitrary, chosen to be
+  // less aggressive than the 8 used for the whole function size.
+  const unsigned ThresSize = 4 * NewRCCount;
+  if (SrcSize <= ThresSize && DstSize <= ThresSize)
     return true;
+
   // Estimate *register use density*. If it doubles or more, abort.
   unsigned SrcUses = std::distance(mri_->use_nodbg_begin(SrcReg),
                                    mri_->use_nodbg_end());
@@ -996,12 +1002,12 @@ SimpleRegisterCoalescing::isWinToJoinCrossClass(unsigned SrcReg,
                                    mri_->use_nodbg_end());
   unsigned NewUses = SrcUses + DstUses;
   unsigned NewSize = SrcSize + DstSize;
-  if (SrcRC != NewRC && SrcSize > NewRCCount) {
+  if (SrcRC != NewRC && SrcSize > ThresSize) {
     unsigned SrcRCCount = allocatableRCRegs_[SrcRC].count();
     if (NewUses*SrcSize*SrcRCCount > 2*SrcUses*NewSize*NewRCCount)
       return false;
   }
-  if (DstRC != NewRC && DstSize > NewRCCount) {
+  if (DstRC != NewRC && DstSize > ThresSize) {
     unsigned DstRCCount = allocatableRCRegs_[DstRC].count();
     if (NewUses*DstSize*DstRCCount > 2*DstUses*NewSize*NewRCCount)
       return false;