JIT: limit phi-refinement to loop-invariant VNs (dotnet#106229)

PhiArg VNs should be invariant in the loop that contains the Phi, otherwise
the same VN may refer to values from more than one iteration.

Fixes dotnet#105792.

src/coreclr/jit/valuenum.cpp

@@ -11147,6 +11147,7 @@ void Compiler::fgValueNumberPhiDef(GenTreeLclVar* newSsaDef, BasicBlock* blk, bo
 
     GenTreePhi*  phiNode = newSsaDef->AsLclVar()->Data()->AsPhi();
     ValueNumPair sameVNP;
+    VNSet        loopInvariantCache(getAllocator(CMK_ValueNumber));
 
     for (GenTreePhi::Use& use : phiNode->Uses())
     {
@@ -11167,16 +11168,34 @@ void Compiler::fgValueNumberPhiDef(GenTreeLclVar* newSsaDef, BasicBlock* blk, bo
 
         ValueNumPair phiArgVNP = lvaGetDesc(phiArg)->GetPerSsaData(phiArg->GetSsaNum())->m_vnPair;
 
-#ifdef DEBUG
-        if (verbose && isUpdate && (phiArgVNP != phiArg->gtVNPair))
+        if (isUpdate && (phiArgVNP != phiArg->gtVNPair))
         {
-            printf("Updating phi arg [%06u] VN from ", dspTreeID(phiArg));
-            vnpPrint(phiArg->gtVNPair, 0);
-            printf(" to ");
-            vnpPrint(phiArgVNP, 0);
-            printf("\n");
-        }
+            FlowGraphNaturalLoop* const blockLoop = m_loops->GetLoopByHeader(blk);
+            bool const canUseNewVN = optVNIsLoopInvariant(phiArgVNP.GetConservative(), blockLoop, &loopInvariantCache);
+
+            if (canUseNewVN)
+            {
+#ifdef DEBUG
+                if (verbose)
+                {
+                    printf("Updating phi arg [%06u] VN from ", dspTreeID(phiArg));
+                    vnpPrint(phiArg->gtVNPair, 0);
+                    printf(" to ");
+                    vnpPrint(phiArgVNP, 0);
+                    printf("\n");
+                }
 #endif
+            }
+            else
+            {
+                JITDUMP("Can't update phi arg [%06u] with " FMT_VN " -- varies in " FMT_LP "\n", dspTreeID(phiArg),
+                        phiArgVNP.GetConservative(), blockLoop->GetIndex());
+
+                // Code below uses phiArgVNP, reset to the old value
+                //
+                phiArgVNP = phiArg->gtVNPair;
+            }
+        }
 
         phiArg->gtVNPair = phiArgVNP;
 

src/tests/JIT/Regression/JitBlue/Runtime_105792/Runtime_105792.cs

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_105792
+{
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static int Problem1(int x)
+    {
+        int y = 0;
+        while (x != 0)
+        {
+            if (y == x) return -1;
+            y = x;
+            Update(ref x);
+        }
+        return x;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static int Problem2(int x)
+    {
+        int y = 0;
+        while (x != 0)
+        {
+            if (y == x + 1) return -1;
+            y = x + 1;
+            Update(ref x);
+        }
+        return x;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Update(ref int x) 
+    {
+        x = x - 1;
+    }
+
+    [Fact]
+    public static int Test1() => Problem1(10) + 100;
+
+    [Fact]
+    public static int Test2() => Problem2(10) + 100;
+}

src/tests/JIT/Regression/JitBlue/Runtime_105792/Runtime_105792.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>