[PATCH] D134003: [MachineCSE] Add a threshold to avoid spending too much time in isProfitableToCSE

[Pengxuan Zheng via Phabricator via llvm-commits]

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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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D134003

Files:
  llvm/lib/CodeGen/MachineCSE.cpp


Index: llvm/lib/CodeGen/MachineCSE.cpp
===================================================================
--- llvm/lib/CodeGen/MachineCSE.cpp
+++ llvm/lib/CodeGen/MachineCSE.cpp
@@ -60,6 +60,11 @@
           "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 @@
   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;
 


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