From aa65f93b71dee8cacb22be1957673c8be6a3ec24 Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman@google.com>
Date: Tue, 28 Jan 2025 00:22:25 +0000
Subject: [PATCH] Revert "[MLGO] Count LR Evictions Rather than Relying on
 Cascade (#124440)"

This reverts commit 8cc83b66e20e72cdb3bb5fbd549c941797b0e0c9.

This was causing builbot failures.
https://lab.llvm.org/buildbot/#/builders/90/builds/4198
https://lab.llvm.org/buildbot/#/builders/110/builds/3616
---
 llvm/lib/CodeGen/MLRegAllocEvictAdvisor.cpp | 44 +++------------------
 1 file changed, 6 insertions(+), 38 deletions(-)

diff --git a/llvm/lib/CodeGen/MLRegAllocEvictAdvisor.cpp b/llvm/lib/CodeGen/MLRegAllocEvictAdvisor.cpp
index 51ef625335ea9..9c6487b40d606 100644
--- a/llvm/lib/CodeGen/MLRegAllocEvictAdvisor.cpp
+++ b/llvm/lib/CodeGen/MLRegAllocEvictAdvisor.cpp
@@ -63,11 +63,11 @@ static cl::opt<std::string> InteractiveChannelBaseName(
         "outgoing name should be "
         "<regalloc-evict-interactive-channel-base>.out"));
 
-static cl::opt<unsigned> MaxEvictionCount(
-    "mlregalloc-max-eviction-count", cl::Hidden,
-    cl::desc("The maximum number of times a live range can be "
-             "evicted before preventing it from being evicted"),
-    cl::init(100));
+static cl::opt<unsigned>
+    MaxCascade("mlregalloc-max-cascade", cl::Hidden,
+               cl::desc("The maximum number of times a live range can be "
+                        "evicted before preventing it from being evicted"),
+               cl::init(20));
 
 // Options that only make sense in development mode
 #ifdef LLVM_HAVE_TFLITE
@@ -364,22 +364,6 @@ class MLEvictAdvisor : public RegAllocEvictionAdvisor {
 
   using RegID = unsigned;
   mutable DenseMap<RegID, LIFeatureComponents> CachedFeatures;
-
-  mutable std::unordered_map<unsigned, unsigned> VirtRegEvictionCounts;
-
-  void onEviction(Register RegBeingEvicted) const {
-    // If we cannot find the virtual register in the map, we just assume it has
-    // not been evicted before and thus has a value of zero (which is what the
-    // subscript operator returns by default).
-    ++VirtRegEvictionCounts[RegBeingEvicted.id()];
-  }
-
-  unsigned getEvictionCount(Register Reg) const {
-    auto EvictionCountIt = VirtRegEvictionCounts.find(Reg.id());
-    if (EvictionCountIt != VirtRegEvictionCounts.end())
-      return EvictionCountIt->second;
-    return 0;
-  }
 };
 
 #define _DECL_FEATURES(type, name, shape, _)                                   \
@@ -673,7 +657,7 @@ bool MLEvictAdvisor::loadInterferenceFeatures(
       // threshold, prevent the range from being evicted. We still let the
       // range through if it is urgent as we are required to produce an
       // eviction if the candidate is not spillable.
-      if (getEvictionCount(Intf->reg()) > MaxEvictionCount && !Urgent)
+      if (IntfCascade >= MaxCascade && !Urgent)
         return false;
 
       // Only evict older cascades or live ranges without a cascade.
@@ -819,22 +803,6 @@ MCRegister MLEvictAdvisor::tryFindEvictionCandidate(
   }
   assert(CandidatePos < ValidPosLimit);
   (void)ValidPosLimit;
-
-  // Update information about how many times the virtual registers being
-  // evicted have been evicted so that we can prevent the model from evicting
-  // the same ranges continually and eating compile time.
-  if (CandidatePos == CandidateVirtRegPos) {
-    onEviction(VirtReg.reg());
-  } else {
-    for (MCRegUnit Unit : TRI->regunits(Regs[CandidatePos].first)) {
-      LiveIntervalUnion::Query &Q = Matrix->query(VirtReg, Unit);
-      const auto &IFIntervals = Q.interferingVRegs(EvictInterferenceCutoff);
-      for (const LiveInterval *Intf : reverse(IFIntervals)) {
-        onEviction(Intf->reg());
-      }
-    }
-  }
-
   return Regs[CandidatePos].first;
 }