Fix up some alignments (#5517)

* Fix up some alignments

* Optional fields is a bunch of bytes without relocs. We don't need a pointer alignment.
* Sealed vtables are a bunch of 32bit relative relocs. They are not pointer sized.
* R2R helpers don't have addresses that are visible from managed code. We don't need to align them at 4 byte boundaries.

Saves around 10 kB on the size of a Hello world. Won't make a huge difference, but this was an easy thing.

* Fix ARM64 code alignment

6 files changed, 43 insertions, 7 deletions

diff --git a/src/Common/src/TypeSystem/Common/TargetDetails.cs b/src/Common/src/TypeSystem/Common/TargetDetails.cs
index a17a5157a..b7fc058bf 100644
--- a/src/Common/src/TypeSystem/Common/TargetDetails.cs
+++ b/src/Common/src/TypeSystem/Common/TargetDetails.cs
@@ -130,17 +130,36 @@ namespace Internal.TypeSystem
         }
 
         /// <summary>
-        /// Gets the minimum required method alignment.
+        /// Gets the minimum required alignment for methods whose address is visible
+        /// to managed code.
         /// </summary>
         public int MinimumFunctionAlignment
         {
             get
             {
                 // We use a minimum alignment of 4 irrespective of the platform.
+                // This is to prevent confusing the method address with a fat function pointer.
                 return 4;
             }
         }
 
+        public int MinimumCodeAlignment
+        {
+            get
+            {
+                switch (Architecture)
+                {
+                    case TargetArchitecture.ARM:
+                    case TargetArchitecture.ARMEL:
+                        return 2;
+                    case TargetArchitecture.ARM64:
+                        return 4;
+                    default:
+                        return 1;
+                }
+            }
+        }
+
         public TargetDetails(TargetArchitecture architecture, TargetOS targetOS, TargetAbi abi)
         {
             Architecture = architecture;
diff --git a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/AssemblyStubNode.cs b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/AssemblyStubNode.cs
index db568a3e8..0bc9db8c6 100644
--- a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/AssemblyStubNode.cs
+++ b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/AssemblyStubNode.cs
@@ -16,6 +16,12 @@ namespace ILCompiler.DependencyAnalysis
         {
         }
 
+        /// <summary>
+        /// Gets a value indicating whether the stub's address is visible from managed code
+        /// and could be a target of a managed calli.
+        /// </summary>
+        protected virtual bool IsVisibleFromManagedCode => true;
+
         public override ObjectNodeSection Section => ObjectNodeSection.TextSection;
 
         public override bool StaticDependenciesAreComputed => true;
@@ -26,19 +32,26 @@ namespace ILCompiler.DependencyAnalysis
 
         public override ObjectData GetData(NodeFactory factory, bool relocsOnly)
         {
+            // If the address is expected to be visible from managed code, we need to align
+            // at the managed code boundaries to prevent the stub from being confused with
+            // a fat fuction pointer. Otherwise we can align tighter.
+            int alignment = IsVisibleFromManagedCode ?
+                factory.Target.MinimumFunctionAlignment :
+                factory.Target.MinimumCodeAlignment;
+
             switch (factory.Target.Architecture)
             {
                 case TargetArchitecture.X64:
                     X64.X64Emitter x64Emitter = new X64.X64Emitter(factory, relocsOnly);
                     EmitCode(factory, ref x64Emitter, relocsOnly);
-                    x64Emitter.Builder.RequireInitialAlignment(factory.Target.MinimumFunctionAlignment);
+                    x64Emitter.Builder.RequireInitialAlignment(alignment);
                     x64Emitter.Builder.AddSymbol(this);
                     return x64Emitter.Builder.ToObjectData();
 
                 case TargetArchitecture.X86:
                     X86.X86Emitter x86Emitter = new X86.X86Emitter(factory, relocsOnly);
                     EmitCode(factory, ref x86Emitter, relocsOnly);
-                    x86Emitter.Builder.RequireInitialAlignment(factory.Target.MinimumFunctionAlignment);
+                    x86Emitter.Builder.RequireInitialAlignment(alignment);
                     x86Emitter.Builder.AddSymbol(this);
                     return x86Emitter.Builder.ToObjectData();
 
@@ -46,14 +59,14 @@ namespace ILCompiler.DependencyAnalysis
                 case TargetArchitecture.ARMEL:
                     ARM.ARMEmitter armEmitter = new ARM.ARMEmitter(factory, relocsOnly);
                     EmitCode(factory, ref armEmitter, relocsOnly);
-                    armEmitter.Builder.RequireInitialAlignment(factory.Target.MinimumFunctionAlignment);
+                    armEmitter.Builder.RequireInitialAlignment(alignment);
                     armEmitter.Builder.AddSymbol(this);
                     return armEmitter.Builder.ToObjectData();
 
                 case TargetArchitecture.ARM64:
                     ARM64.ARM64Emitter arm64Emitter = new ARM64.ARM64Emitter(factory, relocsOnly);
                     EmitCode(factory, ref arm64Emitter, relocsOnly);
-                    arm64Emitter.Builder.RequireInitialAlignment(factory.Target.MinimumFunctionAlignment);
+                    arm64Emitter.Builder.RequireInitialAlignment(alignment);
                     arm64Emitter.Builder.AddSymbol(this);
                     return arm64Emitter.Builder.ToObjectData();
 
diff --git a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/EETypeOptionalFieldsNode.cs b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/EETypeOptionalFieldsNode.cs
index 0cc9ef205..ba0b4739e 100644
--- a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/EETypeOptionalFieldsNode.cs
+++ b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/EETypeOptionalFieldsNode.cs
@@ -51,7 +51,7 @@ namespace ILCompiler.DependencyAnalysis
         public override ObjectData GetData(NodeFactory factory, bool relocsOnly = false)
         {
             ObjectDataBuilder objData = new ObjectDataBuilder(factory, relocsOnly);
-            objData.RequireInitialPointerAlignment();
+            objData.RequireInitialAlignment(1);
             objData.AddSymbol(this);
 
             if (!relocsOnly)
diff --git a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunGenericHelperNode.cs b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunGenericHelperNode.cs
index ddd1479b2..e7c36429d 100644
--- a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunGenericHelperNode.cs
+++ b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunGenericHelperNode.cs
@@ -63,6 +63,8 @@ namespace ILCompiler.DependencyAnalysis
             }
         }
 
+        protected override bool IsVisibleFromManagedCode => false;
+
         protected sealed override string GetName(NodeFactory factory) => this.GetMangledName(factory.NameMangler);
         public override bool IsShareable => true;
 
diff --git a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunHelperNode.cs b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunHelperNode.cs
index 7b941ec85..168a14743 100644
--- a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunHelperNode.cs
+++ b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/ReadyToRunHelperNode.cs
@@ -90,6 +90,8 @@ namespace ILCompiler.DependencyAnalysis
             }
         }
 
+        protected override bool IsVisibleFromManagedCode => false;
+
         protected override string GetName(NodeFactory factory) => this.GetMangledName(factory.NameMangler);
 
         public ReadyToRunHelperId Id => _id;
diff --git a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/SealedVTableNode.cs b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/SealedVTableNode.cs
index 4f7e5ebe3..ec1dca987 100644
--- a/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/SealedVTableNode.cs
+++ b/src/ILCompiler.Compiler/src/Compiler/DependencyAnalysis/SealedVTableNode.cs
@@ -128,7 +128,7 @@ namespace ILCompiler.DependencyAnalysis
         public override ObjectData GetData(NodeFactory factory, bool relocsOnly)
         {
             ObjectDataBuilder objData = new ObjectDataBuilder(factory, relocsOnly);
-            objData.RequireInitialPointerAlignment();
+            objData.RequireInitialAlignment(4);
             objData.AddSymbol(this);
 
             if (BuildSealedVTableSlots(factory, relocsOnly))