From 3b3da4d94dbdb69ac1cee62cb367b4618f3d110d Mon Sep 17 00:00:00 2001
From: Jakob Botsch Nielsen <jakob.botsch.nielsen@gmail.com>
Date: Thu, 5 Feb 2026 22:09:34 +0100
Subject: [PATCH] JIT: Run liveness + DCE in tier0

---
 src/coreclr/jit/compiler.cpp |  4 +++
 src/coreclr/jit/compiler.h   |  1 +
 src/coreclr/jit/liveness.cpp | 49 ++++++++++++++++++++++++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/src/coreclr/jit/compiler.cpp b/src/coreclr/jit/compiler.cpp
index 379c33ab5a5042..499fadf633b627 100644
--- a/src/coreclr/jit/compiler.cpp
+++ b/src/coreclr/jit/compiler.cpp
@@ -4857,6 +4857,10 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
             opts.optRepeatActive = false;
         }
     }
+    else if (opts.Tier0OptimizationEnabled())
+    {
+        fgTier0Liveness();
+    }
 
     optLoopsCanonical = false;
 
diff --git a/src/coreclr/jit/compiler.h b/src/coreclr/jit/compiler.h
index 7932debc22a854..cac43825f24649 100644
--- a/src/coreclr/jit/compiler.h
+++ b/src/coreclr/jit/compiler.h
@@ -6698,6 +6698,7 @@ class Compiler
     }
 
     void fgSsaLiveness();
+    void fgTier0Liveness();
     void fgAsyncLiveness();
     void fgPostLowerLiveness();
     PhaseStatus fgEarlyLiveness();
diff --git a/src/coreclr/jit/liveness.cpp b/src/coreclr/jit/liveness.cpp
index 971a65cbb9c29e..a8ab06df45d1ae 100644
--- a/src/coreclr/jit/liveness.cpp
+++ b/src/coreclr/jit/liveness.cpp
@@ -2828,6 +2828,55 @@ void Compiler::fgSsaLiveness()
     liveness.Run();
 }
 
+//------------------------------------------------------------------------
+// fgSsaLiveness: Run SSA liveness.
+//
+void Compiler::fgTier0Liveness()
+{
+    struct Tier0Liveness : public Liveness<Tier0Liveness>
+    {
+        enum
+        {
+            SsaLiveness           = false,
+            ComputeMemoryLiveness = false,
+            IsLIR                 = false,
+            IsEarly               = false,
+        };
+
+        Tier0Liveness(Compiler* comp)
+            : Liveness(comp)
+        {
+        }
+    };
+
+    if (m_dfsTree == nullptr)
+    {
+        m_dfsTree = fgComputeDfs();
+    }
+
+    Tier0Liveness liveness(this);
+    liveness.Run();
+
+    // Rest of the compiler does not expect that we started tracking locals, so reset that state.
+    for (unsigned i = 0; i < lvaTrackedCount; i++)
+    {
+        lvaGetDesc(lvaTrackedToVarNum[i])->lvTracked = false;
+    }
+
+    lvaCurEpoch++;
+    lvaTrackedCount             = 0;
+    lvaTrackedCountInSizeTUnits = 0;
+
+    for (BasicBlock* block : Blocks())
+    {
+        block->bbLiveIn  = VarSetOps::UninitVal();
+        block->bbLiveOut = VarSetOps::UninitVal();
+        block->bbVarUse  = VarSetOps::UninitVal();
+        block->bbVarDef  = VarSetOps::UninitVal();
+    }
+    fgBBVarSetsInited = false;
+}
+
 //------------------------------------------------------------------------
 // fgAsyncLiveness: Run async liveness.
 //