[arm64] Avoid sign-extending TYP_INT register moves

src/coreclr/jit/codegen.h

@@ -1416,8 +1416,9 @@ class CodeGen final : public CodeGenInterface
                          regNumber dstReg,
                          regNumber srcReg,
                          bool      canSkip,
-                         emitAttr  size  = EA_UNKNOWN,
-                         insFlags  flags = INS_FLAGS_DONT_CARE);
+                         emitAttr  size    = EA_UNKNOWN,
+                         insFlags  flags   = INS_FLAGS_DONT_CARE,
+                         var_types dstType = TYP_UNKNOWN);
 
     void inst_RV_RV(instruction ins,
                     regNumber   reg1,
@@ -1637,7 +1638,7 @@ class CodeGen final : public CodeGenInterface
     bool arm_Valid_Imm_For_Add_SP(target_ssize_t imm);
 #endif
 
-    instruction ins_Move_Extend(var_types srcType, bool srcInReg);
+    instruction ins_Move_Extend(var_types srcType, bool srcInReg, var_types dstType = TYP_UNKNOWN);
 
     instruction ins_Copy(var_types dstType);
     instruction ins_Copy(regNumber srcReg, var_types dstType);

src/coreclr/jit/instr.cpp

@@ -790,9 +790,10 @@ void CodeGen::inst_Mov_Extend(var_types srcType,
                               regNumber srcReg,
                               bool      canSkip,
                               emitAttr  size,
-                              insFlags  flags /* = INS_FLAGS_DONT_CARE */)
+                              insFlags  flags /* = INS_FLAGS_DONT_CARE */,
+                              var_types dstType /* = TYP_UNKNOWN */)
 {
-    instruction ins = ins_Move_Extend(srcType, srcInReg);
+    instruction ins = ins_Move_Extend(srcType, srcInReg, dstType);
 
     if (size == EA_UNKNOWN)
     {
@@ -1920,9 +1921,17 @@ bool CodeGenInterface::validImmForBL(ssize_t addr)
  *  Parameters
  *      srcType   - source type
  *      srcInReg  - whether source is in a register
+ *      dstType   - type the value will be consumed as; defaults to srcType. Must share srcType's
+ *                  actual type, as genuine widening (e.g. int -> long) must use a cast instead.
  */
-instruction CodeGen::ins_Move_Extend(var_types srcType, bool srcInReg)
+instruction CodeGen::ins_Move_Extend(var_types srcType, bool srcInReg, var_types dstType /* = TYP_UNKNOWN */)
 {
+    if (dstType == TYP_UNKNOWN)
+    {
+        dstType = srcType;
+    }
+    assert(genActualType(srcType) == genActualType(dstType));
+
     if (varTypeUsesIntReg(srcType))
     {
         instruction ins = INS_invalid;
@@ -2005,14 +2014,8 @@ instruction CodeGen::ins_Move_Extend(var_types srcType, bool srcInReg)
                 }
                 else
                 {
-                    if (srcType == TYP_INT)
-                    {
-                        ins = INS_sxtw;
-                    }
-                    else
-                    {
-                        ins = INS_mov;
-                    }
+                    // A TYP_INT value only needs its low 32 bits; mov Wd, Wn zero extends the rest.
+                    ins = INS_mov;
                 }
             }
         }

src/tests/JIT/opt/InstructionCombining/IntMoveNoSignExtend.cs

@@ -0,0 +1,78 @@
+// 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;
+
+namespace TestIntMoveNoSignExtend
+{
+    public class Program
+    {
+        static int s_value = 7;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static int GetInt() => s_value;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static void Sink(int x) => s_value ^= x;
+
+        // A reg-to-reg move of a TYP_INT value (here the call result kept in a callee-saved register
+        // across the following call) must not be sign extended to 64 bits: the upper bits are never
+        // observed for an int, so a plain 'mov Wd, Wn' suffices instead of 'sxtw Wd, Wn'.
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static int IntMove()
+        {
+            //ARM64-NOT: sxtw {{w[0-9]+}}, {{w[0-9]+}}
+            int a = GetInt();
+            Sink(0);
+            return a + GetInt();
+        }
+
+        // A genuine int -> long widening must still sign extend with 'sxtw Xd, Wn'.
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static long Widen(int x)
+        {
+            //ARM64-FULL-LINE: sxtw {{x[0-9]+}}, {{w[0-9]+}}
+            return x;
+        }
+
+        [Fact]
+        public static int TestEntryPoint()
+        {
+            int result = 100;
+
+            s_value = 7;
+            if (IntMove() != 14)
+            {
+                result = -1;
+            }
+
+            // Negative values must round-trip correctly through the move.
+            s_value = -123456;
+            if (IntMove() != -246912)
+            {
+                result = -1;
+            }
+
+            s_value = int.MinValue;
+            // int.MinValue + int.MinValue overflows to 0 in unchecked int arithmetic.
+            if (IntMove() != 0)
+            {
+                result = -1;
+            }
+
+            // Genuine widening must preserve the sign.
+            if (Widen(-7) != -7L)
+            {
+                result = -1;
+            }
+            if (Widen(int.MinValue) != int.MinValue)
+            {
+                result = -1;
+            }
+
+            return result;
+        }
+    }
+}

src/tests/JIT/opt/InstructionCombining/IntMoveNoSignExtend.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="IntMoveNoSignExtend.cs">
+      <HasDisasmCheck>true</HasDisasmCheck>
+    </Compile>
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_JITMinOpts" Value="0" />
+  </ItemGroup>
+</Project>