Use .alt_entry/.private_extern on Apple platforms

src/coreclr/nativeaot/Runtime/unix/unixasmmacrosamd64.inc

@@ -33,7 +33,12 @@ C_FUNC(\Name):
 .endm
 
 .macro ALTERNATE_ENTRY Name
+#if defined(__APPLE__)
+        .alt_entry C_FUNC(\Name)
+        .private_extern C_FUNC(\Name)
+#else
         .global C_FUNC(\Name)
+#endif
 C_FUNC(\Name):
 .endm
 

src/coreclr/nativeaot/Runtime/unix/unixasmmacrosarm64.inc

@@ -24,8 +24,11 @@ C_FUNC(\Name):
 .endm
 
 .macro ALTERNATE_ENTRY Name
+#if defined(__APPLE__)
+        .alt_entry C_FUNC(\Name)
+        .private_extern C_FUNC(\Name)
+#else
         .global C_FUNC(\Name)
-#if !defined(__APPLE__)
         .hidden C_FUNC(\Name)
 #endif
 C_FUNC(\Name):

src/coreclr/pal/inc/unixasmmacrosarm64.inc

@@ -17,7 +17,12 @@
 .endm
 
 .macro PATCH_LABEL Name
+#if defined(__APPLE__)
+        .alt_entry C_FUNC(\Name)
+        .private_extern C_FUNC(\Name)
+#else
         .global C_FUNC(\Name)
+#endif
 C_FUNC(\Name):
 .endm
 

src/coreclr/tools/Common/Compiler/DependencyAnalysis/ObjectNodeSection.cs

@@ -43,6 +43,17 @@ public bool IsStandardSection
             }
         }
 
+        /// <summary>
+        /// Returns true if the section contains regular data (as opposed to code, debug info or unwinding data)
+        /// </summary>
+        public bool IsDataSection
+        {
+            get
+            {
+                return Type is SectionType.ReadOnly or SectionType.Writeable or SectionType.Uninitialized;
+            }
+        }
+
         public static readonly ObjectNodeSection XDataSection = new ObjectNodeSection("xdata", SectionType.ReadOnly);
         public static readonly ObjectNodeSection DataSection = new ObjectNodeSection("data", SectionType.Writeable);
         public static readonly ObjectNodeSection ReadOnlyDataSection = new ObjectNodeSection("rdata", SectionType.ReadOnly);

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

@@ -96,6 +96,7 @@ internal static class MachNative
         public const byte N_INDR = 0xA;
         public const byte N_SECT = 0xE;
         public const byte N_PBUD = 0xC;
+        public const byte N_PEXT = 0x10;
 
         // Symbol descriptor flags
         public const ushort REFERENCE_FLAG_UNDEFINED_NON_LAZY = 0;
@@ -106,6 +107,7 @@ internal static class MachNative
         public const ushort REFERENCE_FLAG_PRIVATE_UNDEFINED_LAZY = 5;
         public const ushort REFERENCED_DYNAMICALLY = 0x10;
         public const ushort N_NO_DEAD_STRIP = 0x20;
+        public const ushort N_ALT_ENTRY = 0x200;
         public const ushort N_WEAK_REF = 0x40;
         public const ushort N_WEAK_DEF = 0x80;
 

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

@@ -113,7 +113,7 @@ private protected override void EmitSectionsAndLayout()
                     Name = $"lsection{sectionIndex}",
                     Section = section,
                     Value = section.VirtualAddress,
-                    Descriptor = 0,
+                    Descriptor = (ushort)(section.KeepDataLayout ? N_NO_DEAD_STRIP : 0),
                     Type = N_SECT,
                 };
                 _symbolTable.Add(machSymbol);
@@ -355,6 +355,7 @@ private protected override void CreateSection(ObjectNodeSection section, string
             {
                 Log2Alignment = 1,
                 Flags = flags,
+                KeepDataLayout = section.IsDataSection
             };
 
             int sectionIndex = _sections.Count;
@@ -472,6 +473,10 @@ private protected override void EmitSymbolTable(
 
             // Sort and insert all defined symbols
             var sortedDefinedSymbols = new List<MachSymbol>(definedSymbols.Count);
+            // We emit a local N_NO_DEAD_STRIP symbol for the beginning of all sections marked with
+            // KeepDataLayout to ensure the final layout is kept intact by the linker. This works in
+            // tandem with setting the N_ALT_ENTRY flag for all other symbols in the same section in
+            // the loop below.
             foreach ((string name, SymbolDefinition definition) in definedSymbols)
             {
                 MachSection section = _sections[definition.SectionIndex];
@@ -483,8 +488,8 @@ private protected override void EmitSymbolTable(
                     Name = name,
                     Section = section,
                     Value = section.VirtualAddress + (ulong)definition.Value,
-                    Descriptor = N_NO_DEAD_STRIP,
-                    Type = N_SECT | N_EXT,
+                    Descriptor = (ushort)(section.KeepDataLayout || definition.AltEntry ? N_ALT_ENTRY | N_NO_DEAD_STRIP : N_NO_DEAD_STRIP),
+                    Type = (byte)(N_SECT | N_EXT | (definition.Global ? 0 : N_PEXT)),
                 });
             }
             sortedDefinedSymbols.Sort((symA, symB) => string.CompareOrdinal(symA.Name, symB.Name));
@@ -496,7 +501,7 @@ private protected override void EmitSymbolTable(
             }
 
             _dySymbolTable.ExternalSymbolsIndex = _dySymbolTable.LocalSymbolsCount;
-            _dySymbolTable.ExternalSymbolsCount = (uint)definedSymbols.Count;
+            _dySymbolTable.ExternalSymbolsCount = (uint)sortedDefinedSymbols.Count;
 
             uint savedSymbolIndex = symbolIndex;
             foreach (string externSymbol in undefinedSymbols)
@@ -883,6 +888,7 @@ private sealed class MachSection
             public bool IsInFile => Size > 0 && Type != S_ZEROFILL && Type != S_GB_ZEROFILL && Type != S_THREAD_LOCAL_ZEROFILL;
 
             public bool IsDwarfSection { get; }
+            public bool KeepDataLayout { get; set; }
 
             public IList<MachRelocation> Relocations => relocationCollection ??= new List<MachRelocation>();
             public Stream Stream => dataStream;

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

@@ -18,7 +18,7 @@ namespace ILCompiler.ObjectWriter
 {
     public abstract class ObjectWriter
     {
-        private protected sealed record SymbolDefinition(int SectionIndex, long Value, int Size = 0, bool Global = false);
+        private protected sealed record SymbolDefinition(int SectionIndex, long Value, int Size = 0, bool Global = false, bool AltEntry = false);
         private protected sealed record SymbolicRelocation(long Offset, RelocType Type, string SymbolName, long Addend = 0);
         private protected sealed record BlockToRelocate(int SectionIndex, long Offset, byte[] Data, Relocation[] Relocations);
 
@@ -246,11 +246,12 @@ protected internal void EmitSymbolDefinition(
             string symbolName,
             long offset = 0,
             int size = 0,
-            bool global = false)
+            bool global = false,
+            bool altEntry = false)
         {
             _definedSymbols.Add(
                 symbolName,
-                new SymbolDefinition(sectionIndex, offset, size, global));
+                new SymbolDefinition(sectionIndex, offset, size, global, altEntry));
         }
 
         /// <summary>
@@ -414,22 +415,26 @@ private void EmitObject(string objectFilePath, IReadOnlyCollection<DependencyNod
 
                 sectionWriter.EmitAlignment(nodeContents.Alignment);
 
+                bool hasInitialEntrypoint = false;
                 bool isMethod = node is IMethodBodyNode or AssemblyStubNode;
                 long thumbBit = _nodeFactory.Target.Architecture == TargetArchitecture.ARM && isMethod ? 1 : 0;
                 foreach (ISymbolDefinitionNode n in nodeContents.DefinedSymbols)
                 {
+                    hasInitialEntrypoint |= n.Offset == 0;
                     sectionWriter.EmitSymbolDefinition(
                         n == node ? currentSymbolName : GetMangledName(n),
                         n.Offset + thumbBit,
-                        n.Offset == 0 && isMethod ? nodeContents.Data.Length : 0);
+                        n.Offset == 0 && isMethod ? nodeContents.Data.Length : 0,
+                        altEntry: n.Offset != 0);
                     if (_nodeFactory.GetSymbolAlternateName(n) is string alternateName)
                     {
                         string alternateCName = ExternCName(alternateName);
                         sectionWriter.EmitSymbolDefinition(
                             alternateCName,
                             n.Offset + thumbBit,
                             n.Offset == 0 && isMethod ? nodeContents.Data.Length : 0,
-                            global: true);
+                            global: true,
+                            altEntry: n.Offset != 0);
 
                         if (n is IMethodNode)
                         {
@@ -439,6 +444,11 @@ private void EmitObject(string objectFilePath, IReadOnlyCollection<DependencyNod
                     }
                 }
 
+                // Match the logic for KeepDataLayout in MachSection.CreateSection. If we are in a data section
+                // we don't need to have a symbol for beginning of the node data. For other nodes, particularly
+                // executable code, we enforce it though.
+                Debug.Assert(hasInitialEntrypoint || section.IsDataSection);
+
                 if (nodeContents.Relocs is not null)
                 {
                     blocksToRelocate.Add(new BlockToRelocate(

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

@@ -60,14 +60,16 @@ public readonly void EmitSymbolDefinition(
             string symbolName,
             long relativeOffset = 0,
             int size = 0,
-            bool global = false)
+            bool global = false,
+            bool altEntry = false)
         {
             _objectWriter.EmitSymbolDefinition(
                 SectionIndex,
                 symbolName,
                 Position + relativeOffset,
                 size,
-                global);
+                global,
+                altEntry);
         }
 
         public readonly void EmitSymbolReference(

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

@@ -221,7 +221,7 @@ private protected override void EmitUnwindInfo(
                     string framSymbolName = $"_fram{i}{currentSymbolName}";
                     if (useFrameNames && start != 0)
                     {
-                        sectionWriter.EmitSymbolDefinition(framSymbolName, start);
+                        sectionWriter.EmitSymbolDefinition(framSymbolName, start, altEntry: true);
                     }
 
                     string startSymbolName = useFrameNames && start != 0 ? framSymbolName : currentSymbolName;

src/coreclr/vm/arm64/asmhelpers.S

@@ -176,8 +176,7 @@ NESTED_END ThePreStub, _TEXT
 
 LEAF_ENTRY ThePreStubPatch, _TEXT
     nop
-.globl C_FUNC(ThePreStubPatchLabel)
-C_FUNC(ThePreStubPatchLabel):
+PATCH_LABEL ThePreStubPatchLabel
     ret lr
 LEAF_END ThePreStubPatch, _TEXT