[release/8.0-staging] JIT: Home float parameters before integer parameters

src/coreclr/jit/codegencommon.cpp

@@ -6154,8 +6154,14 @@ void CodeGen::genFnProlog()
         };
 
 #if defined(TARGET_AMD64) || defined(TARGET_ARM64) || defined(TARGET_ARM)
-        assignIncomingRegisterArgs(&intRegState);
+        // Handle float parameters first; in the presence of struct promotion
+        // we can have parameters that are homed into float registers but
+        // passed in integer registers. So make sure we get those out of the
+        // integer registers before we potentially override those as part of
+        // handling integer parameters.
+
         assignIncomingRegisterArgs(&floatRegState);
+        assignIncomingRegisterArgs(&intRegState);
 #else
         assignIncomingRegisterArgs(&intRegState);
 #endif

src/coreclr/jit/lsra.cpp

@@ -1663,6 +1663,21 @@ bool LinearScan::isRegCandidate(LclVarDsc* varDsc)
         return false;
     }
 
+    // Avoid allocating parameters that are passed in float regs into integer
+    // registers. We currently home float registers before integer registers,
+    // so that kind of enregistration can trash integer registers containing
+    // other parameters.
+    // We assume that these cases will be homed to float registers if they are
+    // promoted.
+    // TODO-CQ: Combine integer and float register homing to handle these kinds
+    // of conflicts.
+    if ((varDsc->TypeGet() == TYP_STRUCT) && varDsc->lvIsRegArg && !varDsc->lvPromoted &&
+        varTypeUsesIntReg(varDsc->GetRegisterType()) && genIsValidFloatReg(varDsc->GetArgReg()))
+    {
+        compiler->lvaSetVarDoNotEnregister(lclNum DEBUGARG(DoNotEnregisterReason::IsStructArg));
+        return false;
+    }
+
     //  Are we not optimizing and we have exception handlers?
     //   if so mark all args and locals as volatile, so that they
     //   won't ever get enregistered.

src/tests/JIT/Regression/JitBlue/Runtime_96306/Runtime_96306.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.Numerics;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_96306
+{
+    [Fact]
+    public static int TestEntryPoint()
+    {
+        return Foo(new Point2D { V = new Vector2(101, -1) }, 100);
+    }
+
+    // 'a' is passed in rcx but homed into xmm1 after promotion.
+    // 'scale' is passed in xmm1 but spilled because of the call to Bar.
+    // We must take care that we spill 'scale' before we home 'a'.
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static int Foo(Point2D a, float scale)
+    {
+        Bar();
+        return ReturnValue(scale);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static int ReturnValue(float value) => (int)value;
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Bar() { }
+
+    private struct Point2D
+    {
+        public Vector2 V;
+    }
+}
+

src/tests/JIT/Regression/JitBlue/Runtime_96306/Runtime_96306.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+    <DebugType>None</DebugType>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>