[NativeAOT] Use SoftFP ABI on linux-bionic-arm (#100392)

Android uses the SoftFP ABI on ARM32 (https://android.googlesource.com/platform/ndk/+/ndk-r12-release/docs/HardFloatAbi.md).

- Copy Crossgen2 logic for "armel" target into ILCompiler
- Use "armel" for linux-bionic-arm ILC target

src/coreclr/nativeaot/BuildIntegration/Microsoft.DotNet.ILCompiler.SingleEntry.targets

@@ -30,6 +30,10 @@
     <_linuxToken>linux-</_linuxToken>
     <_linuxLibcFlavor Condition="$(_targetOS.StartsWith($(_linuxToken)))">$(_targetOS.SubString($(_linuxToken.Length)))</_linuxLibcFlavor>
     <_targetOS Condition="$(_targetOS.StartsWith($(_linuxToken)))">linux</_targetOS>
+
+    <!-- linux-bionic on ARM uses armel (softfp) ABI -->
+    <_targetArchitectureWithAbi>$(_targetArchitecture)</_targetArchitectureWithAbi>
+    <_targetArchitectureWithAbi Condition="'$(_linuxLibcFlavor)' == 'bionic' and '$(_targetArchitecture)' == 'arm'">armel</_targetArchitectureWithAbi>
   </PropertyGroup>
 
   <PropertyGroup>

src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.targets

@@ -227,7 +227,7 @@ The .NET Foundation licenses this file to you under the MIT license.
       <IlcArg Include="@(MibcFile->'--mibc:%(Identity)')" />
       <IlcArg Condition="$(IlcGenerateMetadataLog) == 'true'" Include="--metadatalog:$(NativeIntermediateOutputPath)%(ManagedBinary.Filename).metadata.csv" />
       <IlcArg Condition="$(_targetOS) != ''" Include="--targetos:$(_targetOS)" />
-      <IlcArg Condition="$(_targetArchitecture) != ''" Include="--targetarch:$(_targetArchitecture)" />
+      <IlcArg Condition="$(_targetArchitectureWithAbi) != ''" Include="--targetarch:$(_targetArchitectureWithAbi)" />
       <IlcArg Condition="$(IlcMultiModule) == 'true'" Include="--multifile" />
       <IlcArg Condition="$(IlcMultiModule) != 'true' and '$(IlcDehydrate)' != 'false' and '$(ControlFlowGuard)' != 'Guard'" Include="--dehydrate" />
       <IlcArg Condition="$(Optimize) == 'true'" Include="-O" />

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/ElfObjectWriter.cs

@@ -35,6 +35,7 @@ internal sealed class ElfObjectWriter : UnixObjectWriter
     {
         private readonly bool _useInlineRelocationAddends;
         private readonly ushort _machine;
+        private readonly bool _useSoftFPAbi;
         private readonly List<ElfSectionDefinition> _sections = new();
         private readonly List<ElfSymbol> _symbols = new();
         private uint _localSymbolCount;
@@ -61,6 +62,7 @@ public ElfObjectWriter(NodeFactory factory, ObjectWritingOptions options)
                 _ => throw new NotSupportedException("Unsupported architecture")
             };
             _useInlineRelocationAddends = _machine is EM_386 or EM_ARM;
+            _useSoftFPAbi = _machine is EM_ARM && factory.Target.Abi == TargetAbi.NativeAotArmel;
 
             // By convention the symbol table starts with empty symbol
             _symbols.Add(new ElfSymbol {});
@@ -513,7 +515,7 @@ private protected override void EmitSectionsAndLayout()
                 attributesBuilder.WriteAttribute(Tag_ABI_FP_number_model, 3); // IEEE 754
                 attributesBuilder.WriteAttribute(Tag_ABI_align_needed, 1); // 8-byte
                 attributesBuilder.WriteAttribute(Tag_ABI_align_preserved, 1); // 8-byte
-                attributesBuilder.WriteAttribute(Tag_ABI_VFP_args, 1); // FP parameters passes in VFP registers
+                attributesBuilder.WriteAttribute(Tag_ABI_VFP_args, _useSoftFPAbi ? 0ul : 1ul); // FP parameters passes in VFP registers
                 attributesBuilder.WriteAttribute(Tag_CPU_unaligned_access, 0); // None
                 attributesBuilder.EndSection();
             }

src/coreclr/tools/aot/ILCompiler/ILCompilerRootCommand.cs

@@ -148,7 +148,7 @@ internal sealed class ILCompilerRootCommand : CliRootCommand
         public CliOption<bool> RootDefaultAssemblies { get; } =
             new("--defaultrooting") { Description = "Root assemblies that are not marked [IsTrimmable]" };
         public CliOption<TargetArchitecture> TargetArchitecture { get; } =
-            new("--targetarch") { CustomParser = result => Helpers.GetTargetArchitecture(result.Tokens.Count > 0 ? result.Tokens[0].Value : null), DefaultValueFactory = result => Helpers.GetTargetArchitecture(result.Tokens.Count > 0 ? result.Tokens[0].Value : null), Description = "Target architecture for cross compilation", HelpName = "arg" };
+            new("--targetarch") { CustomParser = MakeTargetArchitecture, DefaultValueFactory = MakeTargetArchitecture, Description = "Target architecture for cross compilation", HelpName = "arg" };
         public CliOption<TargetOS> TargetOS { get; } =
             new("--targetos") { CustomParser = result => Helpers.GetTargetOS(result.Tokens.Count > 0 ? result.Tokens[0].Value : null), DefaultValueFactory = result => Helpers.GetTargetOS(result.Tokens.Count > 0 ? result.Tokens[0].Value : null), Description = "Target OS for cross compilation", HelpName = "arg" };
         public CliOption<string> JitPath { get; } =
@@ -170,6 +170,7 @@ internal sealed class ILCompilerRootCommand : CliRootCommand
 
         public OptimizationMode OptimizationMode { get; private set; }
         public ParseResult Result;
+        public static bool IsArmel { get; private set; }
 
         public ILCompilerRootCommand(string[] args) : base(".NET Native IL Compiler")
         {
@@ -373,6 +374,18 @@ public static IEnumerable<Func<HelpContext, bool>> GetExtendedHelp(HelpContext _
             };
         }
 
+        private static TargetArchitecture MakeTargetArchitecture(ArgumentResult result)
+        {
+            string firstToken = result.Tokens.Count > 0 ? result.Tokens[0].Value : null;
+            if (firstToken != null && firstToken.Equals("armel", StringComparison.OrdinalIgnoreCase))
+            {
+                IsArmel = true;
+                return Internal.TypeSystem.TargetArchitecture.ARM;
+            }
+
+            return Helpers.GetTargetArchitecture(firstToken);
+        }
+
         private static int MakeParallelism(ArgumentResult result)
         {
             if (result.Tokens.Count > 0)

src/coreclr/tools/aot/ILCompiler/Program.cs

@@ -113,7 +113,7 @@ public int Run()
             SharedGenericsMode genericsMode = SharedGenericsMode.CanonicalReferenceTypes;
 
             var simdVectorLength = instructionSetSupport.GetVectorTSimdVector();
-            var targetAbi = TargetAbi.NativeAot;
+            var targetAbi = ILCompilerRootCommand.IsArmel ? TargetAbi.NativeAotArmel : TargetAbi.NativeAot;
             var targetDetails = new TargetDetails(targetArchitecture, targetOS, targetAbi, simdVectorLength);
             CompilerTypeSystemContext typeSystemContext =
                 new CompilerTypeSystemContext(targetDetails, genericsMode, supportsReflection ? DelegateFeature.All : 0,