[JIT] LclMorph GT_IND(GT_LCL_VAR_ADDR) => GT_CAST(GT_LCL_VAR) narrow-cast only

src/coreclr/jit/lclmorph.cpp

@@ -464,6 +464,7 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
         None,
         Nop,
         BitCast,
+        NarrowCast,
 #ifdef FEATURE_HW_INTRINSICS
         GetElement,
         WithElement,
@@ -1138,6 +1139,7 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
         unsigned             lclNum      = val.LclNum();
         LclVarDsc*           varDsc      = m_compiler->lvaGetDesc(lclNum);
         GenTreeLclVarCommon* lclNode     = nullptr;
+        bool                 isDef = (user != nullptr) && user->OperIs(GT_ASG) && (user->AsOp()->gtGetOp1() == indir);
 
         switch (transform)
         {
@@ -1244,6 +1246,16 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
                 lclNode = indir->AsLclVarCommon();
                 break;
 
+            case IndirTransform::NarrowCast:
+                assert(varTypeIsIntegral(indir));
+                assert(varTypeIsIntegral(varDsc));
+                assert(genTypeSize(varDsc) >= genTypeSize(indir));
+                assert(!isDef);
+
+                lclNode    = BashToLclVar(indir->gtGetOp1(), lclNum);
+                *val.Use() = m_compiler->gtNewCastNode(genActualType(indir), lclNode, false, indir->TypeGet());
+                break;
+
             case IndirTransform::LclFld:
                 indir->ChangeOper(GT_LCL_FLD);
                 indir->AsLclFld()->SetLclNum(lclNum);
@@ -1266,7 +1278,7 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
 
         GenTreeFlags lclNodeFlags = GTF_EMPTY;
 
-        if (user->OperIs(GT_ASG) && (user->AsOp()->gtGetOp1() == indir))
+        if (isDef)
         {
             lclNodeFlags |= (GTF_VAR_DEF | GTF_DONT_CSE);
 
@@ -1365,6 +1377,12 @@ class LocalAddressVisitor final : public GenTreeVisitor<LocalAddressVisitor>
             }
 #endif // FEATURE_HW_INTRINSICS
 
+            // Turn this into a narrow-cast if we can.
+            if (!isDef && varTypeIsIntegral(indir) && varTypeIsIntegral(varDsc))
+            {
+                return IndirTransform::NarrowCast;
+            }
+
             // Turn this into a bitcast if we can.
             if ((genTypeSize(indir) == genTypeSize(varDsc)) && (varTypeIsFloating(indir) || varTypeIsFloating(varDsc)))
             {

src/tests/JIT/opt/Unsafe/Unsafe.cs

@@ -0,0 +1,46 @@
+// 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;
+
+namespace CodeGenTests
+{
+    class UnsafeTests
+    {
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static byte UnsafeAsNarrowCast_Short(short value)
+        {
+            // X64-NOT: dword ptr
+            return Unsafe.As<short, byte>(ref value);
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static byte UnsafeAsNarrowCast_Int(int value)
+        {
+            // X64-NOT: dword ptr
+            return Unsafe.As<int, byte>(ref value);
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static byte UnsafeAsNarrowCast_Long(long value)
+        {
+            // X64-NOT: qword ptr
+            return Unsafe.As<long, byte>(ref value);
+        }
+
+        static int Main()
+        {
+            if (UnsafeAsNarrowCast_Short(255) != 255)
+                return 0;
+
+            if (UnsafeAsNarrowCast_Int(255) != 255)
+                return 0;
+
+            if (UnsafeAsNarrowCast_Long(255) != 255)
+                return 0;
+
+            return 100;
+        }
+    }
+}

src/tests/JIT/opt/Unsafe/Unsafe.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs">
+      <HasDisasmCheck>true</HasDisasmCheck>
+    </Compile>
+
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_JITMinOpts" Value="0" />
+  </ItemGroup>
+</Project>