[llvm] [MachineScheduler] Add option to skip large regions with high pressure (PR #178422)

[Nathan Corbyn via llvm-commits]

https://github.com/cofibrant created https://github.com/llvm/llvm-project/pull/178422

None

>From 662f7d4de4d20c43555b7ed6994a31c05ef27482 Mon Sep 17 00:00:00 2001
From: Nathan Corbyn <n_corbyn at apple.com>
Date: Wed, 28 Jan 2026 13:42:34 +0000
Subject: [PATCH] [MachineScheduler] Add option to skip large regions with high
 pressure

---
 llvm/include/llvm/CodeGen/MachineScheduler.h |  5 +++++
 llvm/lib/CodeGen/MachineScheduler.cpp        | 22 ++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/llvm/include/llvm/CodeGen/MachineScheduler.h b/llvm/include/llvm/CodeGen/MachineScheduler.h
index 33036030679e5..2d60c2faf18aa 100644
--- a/llvm/include/llvm/CodeGen/MachineScheduler.h
+++ b/llvm/include/llvm/CodeGen/MachineScheduler.h
@@ -218,6 +218,11 @@ struct MachineSchedPolicy {
   // Compute DFSResult for use in scheduling heuristics.
   bool ComputeDFSResult = false;
 
+  // Skip scheduling for large regions with critical register pressure.
+  // When a region has more instructions than this threshold and register
+  // pressure exceeds limits, scheduling is skipped. 0 disables this feature.
+  unsigned LargeRegionSkipThreshold = 0;
+
   MachineSchedPolicy() = default;
 };
 
diff --git a/llvm/lib/CodeGen/MachineScheduler.cpp b/llvm/lib/CodeGen/MachineScheduler.cpp
index 05df5ea59d7a3..cb363c8a7c0ee 100644
--- a/llvm/lib/CodeGen/MachineScheduler.cpp
+++ b/llvm/lib/CodeGen/MachineScheduler.cpp
@@ -268,6 +268,12 @@ static cl::opt<unsigned>
                          cl::desc("The threshold for fast cluster"),
                          cl::init(1000));
 
+static cl::opt<unsigned> LargeRegionSkipThreshold(
+    "misched-large-region-skip-threshold", cl::Hidden,
+    cl::desc("Skip scheduling regions with instruction count exceeding this "
+             "threshold when register pressure is critical. 0 disables."),
+    cl::init(0));
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 static cl::opt<bool> MISchedDumpScheduleTrace(
     "misched-dump-schedule-trace", cl::Hidden, cl::init(false),
@@ -1694,6 +1700,18 @@ void ScheduleDAGMILive::schedule() {
   LLVM_DEBUG(SchedImpl->dumpPolicy());
   buildDAGWithRegPressure();
 
+  // Skip scheduling for large regions with critical register pressure that
+  // exceed the large region skip threshold.
+  unsigned SkipThreshold = SchedImpl->getPolicy().LargeRegionSkipThreshold;
+  if (SkipThreshold && NumRegionInstrs > SkipThreshold &&
+      !RegionCriticalPSets.empty()) {
+    LLVM_DEBUG(dbgs() << "Skipping scheduling for region with "
+                      << NumRegionInstrs << " instructions and "
+                      << RegionCriticalPSets.size()
+                      << " critical pressure sets\n");
+    return;
+  }
+
   postProcessDAG();
 
   SmallVector<SUnit*, 8> TopRoots, BotRoots;
@@ -3716,6 +3734,10 @@ void GenericScheduler::initPolicy(MachineBasicBlock::iterator Begin,
     RegionPolicy.OnlyTopDown = false;
   }
 
+  // Apply command-line override for large region skip threshold.
+  if (LargeRegionSkipThreshold.getNumOccurrences())
+    RegionPolicy.LargeRegionSkipThreshold = LargeRegionSkipThreshold;
+
   BotIdx = NumRegionInstrs - 1;
   this->NumRegionInstrs = NumRegionInstrs;
 }