JIT: properly update loop memory dependence when loops are removed (#…

…56436)

If a loop is removed (because of unrolling) then the loop dependence
tracking introduced in #55936 and #56184 may not properly update.

So when a loop is removed, walk up the chain of parent loops looking
for one that is not removed, and record the dependence on that parent.

Addresses last part of #54118.

src/coreclr/jit/optimizer.cpp

@@ -6067,17 +6067,43 @@ void Compiler::optRecordLoopMemoryDependence(GenTree* tree, BasicBlock* block, V
 
     // Find the loop associated with this memory VN.
     //
-    unsigned const updateLoopNum = vnStore->LoopOfVN(memoryVN);
+    unsigned updateLoopNum = vnStore->LoopOfVN(memoryVN);
 
-    if (updateLoopNum == BasicBlock::NOT_IN_LOOP)
+    if (updateLoopNum >= BasicBlock::MAX_LOOP_NUM)
     {
+        // There should be only two special non-loop loop nums.
+        //
+        assert((updateLoopNum == BasicBlock::MAX_LOOP_NUM) || (updateLoopNum == BasicBlock::NOT_IN_LOOP));
+
         // memoryVN defined outside of any loop, we can ignore.
         //
         JITDUMP("      ==> Not updating loop memory dependence of [%06u], memory " FMT_VN " not defined in a loop\n",
                 dspTreeID(tree), memoryVN);
         return;
     }
 
+    // If the loop was removed, then record the dependence in the nearest enclosing loop, if any.
+    //
+    while ((optLoopTable[updateLoopNum].lpFlags & LPFLG_REMOVED) != 0)
+    {
+        unsigned const updateParentLoopNum = optLoopTable[updateLoopNum].lpParent;
+
+        if (updateParentLoopNum == BasicBlock::NOT_IN_LOOP)
+        {
+            // Memory VN was defined in a loop, but no longer.
+            //
+            JITDUMP("      ==> Not updating loop memory dependence of [%06u], memory " FMT_VN
+                    " no longer defined in a loop\n",
+                    dspTreeID(tree), memoryVN);
+            break;
+        }
+
+        JITDUMP("      ==> " FMT_LP " removed, updating dependence to parent " FMT_LP "\n", updateLoopNum,
+                updateParentLoopNum);
+
+        updateLoopNum = updateParentLoopNum;
+    }
+
     // If the update block is not the the header of a loop containing
     // block, we can also ignore the update.
     //

src/tests/JIT/Regression/JitBlue/Runtime_54118/Runtime_54118_2.cs

@@ -0,0 +1,37 @@
+// 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;
+
+// Generated by Fuzzlyn v1.2 on 2021-07-22 13:36:33
+// Seed: 11656046881568048475
+// Reduced from 224.5 KiB to 0.4 KiB in 00:02:42
+// Debug: Outputs 1
+// Release: Outputs 0
+
+public class Program
+{
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static bool Eval(byte b) => b == 100;
+
+    static ulong[] s_34 = new ulong[]{0};
+    public static int Main()
+    {
+        byte[] vr1 = new byte[]{0};
+        bool result = false;
+        for (int vr2 = 0; vr2 < 2; vr2++)
+        {
+            for (int vr3 = 0; vr3 < 1; vr3++)
+            {
+                ulong vr4 = s_34[0]++;
+            }
+
+            vr1[0] = 100;
+            var vr5 = vr1[0];
+            result = Eval(vr5);
+        }
+
+        return result ? 100 : -1;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_54118/Runtime_54118_2.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>