[MachineCSE] Add a threshold to avoid spending too much time in isProfitableToCSE

Currently, it can become extremely costly to compute MayIncreasePressure if the
size of CSUses turns out to be very large. In that case, it's no longer cost
effective to keep computing MayIncreasePressure. Therefore, to limit the amount
of time spent in isProfitableToCSE, we simply conservatively assume
MayIncreasePressure if the size of CSUses is too large. This can reduce overall
compile time by 30% for some benchmarks.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D134003

1 files changed, 16 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineCSE.cpp b/llvm/lib/CodeGen/MachineCSE.cpp
index da2fd66..2de5879e 100644
--- a/llvm/lib/CodeGen/MachineCSE.cpp
+++ b/llvm/lib/CodeGen/MachineCSE.cpp
@@ -60,6 +60,11 @@ STATISTIC(NumCrossBBCSEs,
           "Number of cross-MBB physreg referencing CS eliminated");
 STATISTIC(NumCommutes,  "Number of copies coalesced after commuting");
 
+// Threshold to avoid excessive cost to compute isProfitableToCSE.
+static cl::opt<int>
+    CSUsesThreshold("csuses-threshold", cl::Hidden, cl::init(1024),
+                    cl::desc("Threshold for the size of CSUses"));
+
 namespace {
 
   class MachineCSE : public MachineFunctionPass {
@@ -443,15 +448,23 @@ bool MachineCSE::isProfitableToCSE(Register CSReg, Register Reg,
   if (Register::isVirtualRegister(CSReg) && Register::isVirtualRegister(Reg)) {
     MayIncreasePressure = false;
     SmallPtrSet<MachineInstr*, 8> CSUses;
+    int NumOfUses = 0;
     for (MachineInstr &MI : MRI->use_nodbg_instructions(CSReg)) {
       CSUses.insert(&MI);
-    }
-    for (MachineInstr &MI : MRI->use_nodbg_instructions(Reg)) {
-      if (!CSUses.count(&MI)) {
+      // Too costly to compute if NumOfUses is very large. Conservatively assume
+      // MayIncreasePressure to avoid spending too much time here.
+      if (++NumOfUses > CSUsesThreshold) {
         MayIncreasePressure = true;
         break;
       }
     }
+    if (!MayIncreasePressure)
+      for (MachineInstr &MI : MRI->use_nodbg_instructions(Reg)) {
+        if (!CSUses.count(&MI)) {
+          MayIncreasePressure = true;
+          break;
+        }
+      }
   }
   if (!MayIncreasePressure) return true;