Do not consider single-reg bitcasts multi-reg nodes (#72020)

* Fix an ARM codegen bug

* Do not consider single-reg bitcasts multi-reg nodes

* Add a test

Author: SingleAccretion

src/coreclr/jit/codegenarm.cpp

@@ -1014,7 +1014,7 @@ void CodeGen::genCodeForStoreLclFld(GenTreeLclFld* tree)
 
     GenTree*  data    = tree->gtOp1;
     regNumber dataReg = REG_NA;
-    genConsumeReg(data);
+    genConsumeRegs(data);
 
     if (data->isContained())
     {
@@ -1073,18 +1073,13 @@ void CodeGen::genCodeForStoreLclVar(GenTreeLclVar* tree)
 {
     GenTree* data       = tree->gtOp1;
     GenTree* actualData = data->gtSkipReloadOrCopy();
-    unsigned regCount   = 1;
-    // var = call, where call returns a multi-reg return value
-    // case is handled separately.
+
+    // Stores from a multi-reg source are handled separately.
     if (actualData->IsMultiRegNode())
     {
-        regCount = actualData->GetMultiRegCount(compiler);
-        if (regCount > 1)
-        {
-            genMultiRegStoreToLocal(tree);
-        }
+        genMultiRegStoreToLocal(tree);
     }
-    if (regCount == 1)
+    else
     {
         unsigned   varNum     = tree->GetLclNum();
         LclVarDsc* varDsc     = compiler->lvaGetDesc(varNum);

src/coreclr/jit/gentree.cpp

@@ -812,7 +812,7 @@ bool GenTree::IsMultiRegNode() const
 #if !defined(TARGET_64BIT)
     if (OperIsMultiRegOp())
     {
-        return true;
+        return AsMultiRegOp()->GetRegCount() > 1;
     }
 #endif
 

src/coreclr/jit/lsrabuild.cpp

@@ -3492,7 +3492,7 @@ int LinearScan::BuildStoreLoc(GenTreeLclVarCommon* storeLoc)
 
     // Second, use source registers.
 
-    if (op1->IsMultiRegNode() && (op1->GetMultiRegCount(compiler) > 1))
+    if (op1->IsMultiRegNode())
     {
         // This is the case where the source produces multiple registers.
         // This must be a store lclvar.

src/tests/JIT/Regression/JitBlue/Runtime_71831/Runtime_71831.cs

@@ -0,0 +1,29 @@
+// 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;
+
+unsafe class Runtime_71831
+{
+    private static int Main()
+    {
+        return Problem(100) ? 101 : 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static bool Problem(int i)
+    {
+        StructWithFloats s = default;
+
+        s.FloatOne = BitConverter.Int32BitsToSingle(i);
+
+        return s.FloatOne != *(float*)&i;
+    }
+
+    struct StructWithFloats
+    {
+        public float FloatOne;
+        public float FloatTwo;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_71831/Runtime_71831.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <CLRTestBatchPreCommands><![CDATA[
+$(CLRTestBatchPreCommands)
+set DOTNET_JitNoStructPromotion=1
+]]></CLRTestBatchPreCommands>
+    <BashCLRTestPreCommands><![CDATA[
+$(BashCLRTestPreCommands)
+export DOTNET_JitNoStructPromotion=1
+]]></BashCLRTestPreCommands>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>