[net] iOS fat (armv7 + arm64) apps are not optimal

[spouliot]

iOS fat (armv7 + arm64) app

Assemblies

Assemblies deduplication is not possible (e.g. System.Runtime.dll and MySimpleView.dll should be shared) due to some minor changes, e.g. the path to the .pdb file is not identical between builds. There's no diff when you compare the disassembly of both assemblies.

System.Private.CoreLib.dll is quite different (and afaik the source assembly is not identical anyway) making it hard to share.

System.Private.CoreLib.dll diff

Xamarin.iOS.dll differs due to 32/64 differences. Those could be solved by preserving additional members and moving the bit-aware types to a different (but much smaller) assembly (with type forwarders for compatibility).

--- a.cs	2021-08-25 20:35:18.000000000 -0400
+++ b.cs	2021-08-25 20:35:21.000000000 -0400
@@ -1137,11 +1137,9 @@
 				NSLog(text);
 				throw ErrorHelper.CreateError(8001, text);
 			}
-			if (8 != sizeof(nint))
+			if (4 != sizeof(nint))
 			{
-				_ = 8;
-				_ = 8;
-				string text2 = string.Format("Native type size mismatch between Xamarin.iOS.dll and the executing architecture. Xamarin.iOS.dll was built for {0}-bit, while the current process is {1}-bit.", "32", "64");
+				string text2 = string.Format("Native type size mismatch between Xamarin.iOS.dll and the executing architecture. Xamarin.iOS.dll was built for {0}-bit, while the current process is {1}-bit.", "64", "32");
 				NSLog(text2);
 				throw ErrorHelper.CreateError(8010, text2);
 			}
@@ -2054,7 +2052,7 @@
 
 		private static bool GetIsARM64CallingConvention()
 		{
-			return true;
+			return false;
 		}
 
 		internal static object GetGCHandleTarget(IntPtr P_0)
@@ -3100,16 +3098,16 @@
 		public static extern bool bool_objc_msgSendSuper_IntPtr(IntPtr P_0, IntPtr P_1, IntPtr P_2);
 
 		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
-		public static extern void void_objc_msgSend_IntPtr_UInt64(IntPtr P_0, IntPtr P_1, IntPtr P_2, ulong P_3);
+		public static extern void void_objc_msgSend_IntPtr_UInt32(IntPtr P_0, IntPtr P_1, IntPtr P_2, uint P_3);
 
 		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
 		public static extern void void_objc_msgSend(IntPtr P_0, IntPtr P_1);
 
-		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
-		public static extern CGRect CGRect_objc_msgSend(IntPtr P_0, IntPtr P_1);
+		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend_stret")]
+		public static extern void CGRect_objc_msgSend_stret(out CGRect P_0, IntPtr P_1, IntPtr P_2);
 
-		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSendSuper")]
-		public static extern CGRect CGRect_objc_msgSendSuper(IntPtr P_0, IntPtr P_1);
+		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSendSuper_stret")]
+		public static extern void CGRect_objc_msgSendSuper_stret(out CGRect P_0, IntPtr P_1, IntPtr P_2);
 
 		[DllImport("/usr/lib/libobjc.dylib", EntryPoint = "objc_msgSend")]
 		public static extern void void_objc_msgSend_IntPtr_IntPtr_bool(IntPtr P_0, IntPtr P_1, IntPtr P_2, IntPtr P_3, [MarshalAs(UnmanagedType.I1)] bool P_4);
@@ -3234,8 +3232,7 @@
 		public void SetTitle(string? P_0, UIControlState P_1)
 		{
 			IntPtr intPtr = CFString.CreateNative(P_0);
-			_ = 8;
-			Messaging.void_objc_msgSend_IntPtr_UInt64(base.Handle, Selector.GetHandle("setTitle:forState:"), intPtr, (ulong)P_1);
+			Messaging.void_objc_msgSend_IntPtr_UInt32(base.Handle, Selector.GetHandle("setTitle:forState:"), intPtr, (uint)P_1);
 			CFString.ReleaseNative(intPtr);
 		}
 	}
@@ -3282,7 +3279,8 @@
 			[Export("bounds")]
 			get
 			{
-				return Messaging.CGRect_objc_msgSend(base.Handle, Selector.GetHandle("bounds"));
+				Messaging.CGRect_objc_msgSend_stret(out var result, base.Handle, Selector.GetHandle("bounds"));
+				return result;
 			}
 		}
 
@@ -3321,12 +3319,17 @@
 			[Export("bounds")]
 			get
 			{
-				//Discarded unreachable code: IL_0028
+				//Discarded unreachable code: IL_0035
+				CGRect result;
 				if (base.IsDirectBinding)
 				{
-					return Messaging.CGRect_objc_msgSend(base.Handle, Selector.GetHandle("bounds"));
+					Messaging.CGRect_objc_msgSend_stret(out result, base.Handle, Selector.GetHandle("bounds"));
+				}
+				else
+				{
+					Messaging.CGRect_objc_msgSendSuper_stret(out result, base.SuperHandle, Selector.GetHandle("bounds"));
 				}
-				return Messaging.CGRect_objc_msgSendSuper(base.SuperHandle, Selector.GetHandle("bounds"));
+				return result;
 			}
 		}
 
@@ -4477,15 +4480,15 @@
 	[Serializable]
 	public struct nint : IFormattable, IConvertible, IComparable, IComparable<nint>, IEquatable<nint>
 	{
-		public static readonly int Size = 8;
+		public static readonly int Size = 4;
 
-		public static readonly nint MaxValue = (nint)9223372036854775807L;
+		public static readonly nint MaxValue = 2147483647;
 
-		public static readonly nint MinValue = (nint)(-9223372036854775808L);
+		public static readonly nint MinValue = -2147483648;
 
-		internal long v;
+		internal int v;
 
-		public nint(long P_0)
+		public nint(int P_0)
 		{
 			v = P_0;
 		}
@@ -4497,12 +4500,7 @@
 
 		public static explicit operator int(nint P_0)
 		{
-			return (int)P_0.v;
-		}
-
-		public static explicit operator nint(long P_0)
-		{
-			return new nint(P_0);
+			return P_0.v;
 		}
 
 		public static implicit operator long(nint P_0)
@@ -4576,15 +4574,15 @@
 	[Serializable]
 	public struct nuint : IFormattable, IConvertible, IComparable, IComparable<nuint>, IEquatable<nuint>
 	{
-		public static readonly int Size = 8;
+		public static readonly int Size = 4;
 
-		public static readonly nuint MaxValue = (nuint)18446744073709551615uL;
+		public static readonly nuint MaxValue = 4294967295u;
 
-		public static readonly nuint MinValue = (nuint)0uL;
+		public static readonly nuint MinValue = 0u;
 
-		internal ulong v;
+		internal uint v;
 
-		public nuint(ulong P_0)
+		public nuint(uint P_0)
 		{
 			v = P_0;
 		}
@@ -4594,7 +4592,7 @@
 			return (int)P_0.v;
 		}
 
-		public static explicit operator nuint(ulong P_0)
+		public static implicit operator nuint(uint P_0)
 		{
 			return new nuint(P_0);
 		}
@@ -4655,28 +4653,28 @@
 	[Serializable]
 	public struct nfloat : IFormattable, IConvertible, IComparable, IComparable<nfloat>, IEquatable<nfloat>
 	{
-		public static readonly int Size = 8;
+		public static readonly int Size = 4;
 
-		public static readonly nfloat MaxValue = (nfloat)1.7976931348623157E+308;
+		public static readonly nfloat MaxValue = 3.4028235E+38f;
 
-		public static readonly nfloat MinValue = (nfloat)(-1.7976931348623157E+308);
+		public static readonly nfloat MinValue = -3.4028235E+38f;
 
-		public static readonly nfloat Epsilon = (nfloat)5E-324;
+		public static readonly nfloat Epsilon = 1E-45f;
 
-		public static readonly nfloat NaN = (nfloat)(0.0 / 0.0);
+		public static readonly nfloat NaN = 0f / 0f;
 
-		public static readonly nfloat NegativeInfinity = (nfloat)(-1.0 / 0.0);
+		public static readonly nfloat NegativeInfinity = -1f / 0f;
 
-		public static readonly nfloat PositiveInfinity = (nfloat)(1.0 / 0.0);
+		public static readonly nfloat PositiveInfinity = 1f / 0f;
 
-		internal double v;
+		internal float v;
 
-		public nfloat(double P_0)
+		public nfloat(float P_0)
 		{
 			v = P_0;
 		}
 
-		public static explicit operator nfloat(double P_0)
+		public static implicit operator nfloat(float P_0)
 		{
 			return new nfloat(P_0);
 		}

App Bundle Comparison

Directories / Files	Legacy	net6 rc.1	diff	%
.//_CodeSignature
.//
embedded.mobileprovision	12,391	12,391	0	0.0%
icudt.dat	0	2,176,304	2,176,304	-
Info.plist	1,027	1,027	0	0.0%
MySingleView	7,288,688	14,379,440	7,090,752	97.3%
MySingleView.aotdata.arm64	1,120	816	-304	-27.1%
MySingleView.aotdata.armv7	1,120	816	-304	-27.1%
MySingleView.exe	4,608	0	-4,608	-100.0%
PkgInfo	8	8	0	0.0%
runtimeconfig.bin	0	733	733	-
System.aotdata.arm64	968	0	-968	-100.0%
System.aotdata.armv7	968	0	-968	-100.0%
System.dll	4,608	0	-4,608	-100.0%
System.Private.CoreLib.aotdata.arm64	0	731,992	731,992	-
System.Private.CoreLib.aotdata.armv7	0	683,728	683,728	-
System.Runtime.aotdata.arm64	0	504	504	-
System.Runtime.aotdata.armv7	0	504	504	-
Xamarin.iOS.aotdata.arm64	0	48,328	48,328	-
Xamarin.iOS.aotdata.armv7	0	48,512	48,512	-
.//.monotouch-32
mscorlib.dll	407,552	0	-407,552	-100.0%
Xamarin.iOS.aotdata.armv7	36,480	0	-36,480	-100.0%
Xamarin.iOS.dll	56,832	0	-56,832	-100.0%
.//.monotouch-64
mscorlib.dll	407,552	0	-407,552	-100.0%
Xamarin.iOS.aotdata.arm64	36,568	0	-36,568	-100.0%
Xamarin.iOS.dll	56,832	0	-56,832	-100.0%
.//.xamarin/ios-arm
System.Private.CoreLib.dll	0	765,440	765,440	-
System.Runtime.dll	0	5,120	5,120	-
Xamarin.iOS.dll	0	73,728	73,728	-
.//.xamarin/ios-arm64
System.Private.CoreLib.dll	0	775,680	775,680	-
System.Runtime.dll	0	5,120	5,120	-
Xamarin.iOS.dll	0	73,728	73,728	-
Statistics
Native subtotal	7,612,208	15,161,080	7,548,872	99.2%
Executable	7,288,688	14,379,440	7,090,752	97.3%
AOT data	323,520	781,640	458,120	141.6%
Managed *.dll/exe	937,984	1,708,032	770,048	82.1%
TOTAL	8,895,347	19,801,784	10,906,437	122.6%

Comparison with legacy is a bit harder since the file's location is not identical.

The situation for macOS should be a little different (and simpler) since both slices would be 64 bits.

[spouliot]

Related links (pdb path) from @rolfbjarne

• PDB format and caller file name arguments include full path names in compiler output dotnet/roslyn#949
• Deterministic builds need a mechanism to control PDB path in PE dotnet/roslyn#9813
• Remove pdb path feature flag dotnet/roslyn#19592
• https://docs.microsoft.com/en-us/visualstudio/msbuild/common-msbuild-project-properties?view=vs-2019

[spouliot]

At least for iOS using App Thinning would be better (for end-users) than doing a better deduplication of the assemblies assemblies -> #12665

[filipnavara]

We get significant differences for all BCL assemblies. I commented on that in dotnet/runtime#62953 and I think that needs to be investigated on the runtime side. Xamarin.Android apparently seems to be doing some deduplication based on ModuleVersionId, presumably the same could be done in the merge task for Xamarin.Mac/iOS.

[filipnavara]

Turns out that the assembly size/content difference we hit on osx-x64/osx-arm64 is from the R2R images used by CoreCLR and Crossgen2. It should not affect the Mono runtime packs and mobile platforms but it's something interesting to keep in mind.

Additionally I discussed it with the MonoVM folks and we found that the deduplication by MVID scenario is broken at the moment. There was an issue with Cecil/Linker in .NET 6 that caused some assemblies to get same MVID despite them not being identical (dotnet/linker#2203). Now that issue was supposedly fixed for .NET 7 but it appears it caused identical assemblies to get different MVIDs too (at least in two runtime assembly packs we compared) so an opposite problem.

[rolfbjarne]

This will not be done in the .NET 7 time frame due to time constraints, so moving to .NET 8.

[rolfbjarne]

This won't be an issue in .NET 8, because we're dropping support for 32-bit iOS (#15216).