JIT: Properly handle param reg targets with multiple register assignments (#113499)

LSRA can assign different registers to the same local over its lifetime.
When this happens for a parameter, there is special logic to ensure that
the parameter gets its right initial register assignment. This handling
was missing for mapped parameter register targets.

Author: jakobbotsch

src/coreclr/jit/lclvars.cpp

@@ -4771,7 +4771,7 @@ bool Compiler::lvaIsPreSpilled(unsigned lclNum, regMaskTP preSpillMask)
 //
 void Compiler::lvaUpdateArgWithInitialReg(LclVarDsc* varDsc)
 {
-    noway_assert(varDsc->lvIsParam);
+    assert(varDsc->lvIsParam || varDsc->lvIsParamRegTarget);
 
     if (varDsc->lvIsRegCandidate())
     {
@@ -4790,20 +4790,11 @@ void Compiler::lvaUpdateArgsWithInitialReg()
         return;
     }
 
-    for (unsigned lclNum = 0; lclNum < info.compArgsCount; lclNum++)
+    for (unsigned lclNum = 0; lclNum < lvaCount; lclNum++)
     {
         LclVarDsc* varDsc = lvaGetDesc(lclNum);
 
-        if (varDsc->lvPromoted)
-        {
-            for (unsigned fieldVarNum = varDsc->lvFieldLclStart;
-                 fieldVarNum < varDsc->lvFieldLclStart + varDsc->lvFieldCnt; ++fieldVarNum)
-            {
-                LclVarDsc* fieldVarDsc = lvaGetDesc(fieldVarNum);
-                lvaUpdateArgWithInitialReg(fieldVarDsc);
-            }
-        }
-        else
+        if (varDsc->lvIsParam || varDsc->lvIsParamRegTarget)
         {
             lvaUpdateArgWithInitialReg(varDsc);
         }

src/coreclr/jit/lsra.cpp

@@ -8218,7 +8218,7 @@ void LinearScan::resolveRegisters()
 
                 // Determine initial position for parameters
 
-                if (varDsc->lvIsParam)
+                if (varDsc->lvIsParam || varDsc->lvIsParamRegTarget)
                 {
                     SingleTypeRegSet initialRegMask = interval->firstRefPosition->registerAssignment;
                     regNumber        initialReg = (initialRegMask == RBM_NONE || interval->firstRefPosition->spillAfter)

src/tests/JIT/Regression/JitBlue/Runtime_113486/Runtime_113486.cs

@@ -0,0 +1,79 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// Generated by Fuzzlyn v2.5 on 2025-03-13 04:40:20
+// Run on Arm Linux
+// Seed: 13401882525903110030
+// Reduced from 421.9 KiB to 1.1 KiB in 00:03:35
+// Hits JIT assert in Release:
+// Assertion failed '!"Detected conflicting incoming edges when homing parameter registers"' in 'Program:M42(S0,byte,byte):S0' during 'Generate code' (IL size 83; hash 0x8f48ed9b; FullOpts)
+// 
+//     File: /__w/1/s/src/coreclr/jit/codegencommon.cpp Line: 3225
+// 
+using System;
+using Xunit;
+
+public class Runtime_113486
+{
+    static IRuntime s_rt;
+    static S0[,, ] s_14;
+    static sbyte[][] s_44 = new sbyte[][]
+    {
+        new sbyte[]
+        {
+            0
+        }
+    };
+    
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        try
+        {
+            var vr5 = new S0();
+            var vr6 = s_44[0][0];
+            M42(vr5, vr6, 0);
+        }
+        catch
+        {
+        }
+    }
+
+    static S0 M42(S0 arg0, sbyte arg2, sbyte arg3)
+    {
+        for (int var0 = 0; var0 < 1; var0++)
+        {
+            ushort var1 = (ushort)(-M44());
+            arg0.F0 = var1++;
+            s_rt.WriteLine("c_223", var1);
+        }
+
+        arg3 = arg2;
+        arg0.F1 = arg0.F0;
+        s_rt.WriteLine("c_231", arg3);
+        return arg0;
+    }
+
+    static ushort M44()
+    {
+        return s_14[0, 0, 0].F1--;
+    }
+    
+    struct S0
+    {
+        public ushort F0;
+        public ushort F1;
+    }
+
+    interface IRuntime
+    {
+        void WriteLine<T>(string site, T value);
+    }
+
+    class Runtime : IRuntime
+    {
+        public void WriteLine<T>(string site, T value) => System.Console.WriteLine(value);
+    }
+}
+
+

src/tests/JIT/Regression/JitBlue/Runtime_113486/Runtime_113486.csproj

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