[PHIElimination] Compile time optimization for huge functions.

This is a compile-time optimization for PHIElimination (splitting of critical
edges), which was reported at https://bugs.llvm.org/show_bug.cgi?id=44249. As
discussed there, the way to remedy the slowdowns with huge functions is to
pre-compute the live-in registers for each MBB in an efficient way in
PHIElimination.cpp and then pass that information along to
LiveVariabless::addNewBlock().

In all the huge test programs where this slowdown has been noticable, it has
dissapeared entirely with this patch.

Review: Björn Pettersson, Quentin Colombet.

Differential Revision: https://reviews.llvm.org/D73152

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index 1ef7867..ffa68aa 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -871,8 +871,9 @@ bool MachineBasicBlock::canFallThrough() {
   return getFallThrough() != nullptr;
 }
 
-MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ,
-                                                        Pass &P) {
+MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(
+    MachineBasicBlock *Succ, Pass &P,
+    std::vector<SparseBitVector<>> *LiveInSets) {
   if (!canSplitCriticalEdge(Succ))
     return nullptr;
 
@@ -1003,7 +1004,10 @@ MachineBasicBlock *MachineBasicBlock::SplitCriticalEdge(MachineBasicBlock *Succ,
       }
     }
     // Update relevant live-through information.
-    LV->addNewBlock(NMBB, this, Succ);
+    if (LiveInSets != nullptr)
+      LV->addNewBlock(NMBB, this, Succ, *LiveInSets);
+    else
+      LV->addNewBlock(NMBB, this, Succ);
   }
 
   if (LIS) {