[assembly-preparer] Skip re-serializing assemblies that weren't modified.

tools/assembly-preparer/AssemblyPreparer.cs

@@ -225,7 +225,10 @@ bool RunSteps (IList<ConfigurationAwareStep> steps, out List<ProductException> e
 				// Subscribe once the assemblies have been loaded: accessing the AppBundleRewriter before
 				// that point would create it without finding the corlib and platform assemblies.
 				if (assemblySavedHandler is null && configuration.Assemblies.Count > 0) {
-					assemblySavedHandler = (_) => currentStepModifiedAssemblies = true;
+					assemblySavedHandler = (asm) => {
+						currentStepModifiedAssemblies = true;
+						configuration.ModifiedAssemblies.Add (asm);
+					};
 					configuration.AppBundleRewriter.AssemblySaved += assemblySavedHandler;
 				}
 

tools/assembly-preparer/SaveAssembliesStep.cs

@@ -37,16 +37,17 @@ protected override void TryProcess ()
 				switch (action) {
 				case AssemblyAction.Copy:
 				case AssemblyAction.CopyUsed:
-					if (configuration.Application.IsPostProcessingAssemblies && assembly.InputPath != assembly.OutputPath) {
-						// During post-processing, copy unchanged assemblies to the output directory
-						// so all assemblies are in the same directory (required for AOT compilation).
-						CopyAssemblyToOutput (assembly.InputPath, assembly.OutputPath);
-					} else {
-						assembly.OutputPath = assembly.InputPath;
-					}
+					OutputWithoutRewriting (assembly);
 					continue;
 				case AssemblyAction.Link:
 				case AssemblyAction.Save:
+					if (!configuration.ModifiedAssemblies.Contains (assemblyDefinition)) {
+						// The assembly is marked to be saved (e.g. it's part of the set of assemblies to
+						// trim), but the assembly-preparer didn't actually modify it, so there's no need to
+						// re-serialize it - just output the original assembly.
+						OutputWithoutRewriting (assembly);
+						continue;
+					}
 					log.Log ($"Saving {assembly.InputPath} to {assembly.OutputPath}");
 					break;
 				default:
@@ -80,6 +81,17 @@ protected override void TryProcess ()
 			}
 		}
 
+		void OutputWithoutRewriting (Xamarin.Build.AssemblyPreparerInfo assembly)
+		{
+			if (Configuration.Application.IsPostProcessingAssemblies && assembly.InputPath != assembly.OutputPath) {
+				// During post-processing, copy unchanged assemblies to the output directory
+				// so all assemblies are in the same directory (required for AOT compilation).
+				CopyAssemblyToOutput (assembly.InputPath, assembly.OutputPath);
+			} else {
+				assembly.OutputPath = assembly.InputPath;
+			}
+		}
+
 		void RemoveCrossGen (AssemblyDefinition assemblyDefinition)
 		{
 			// Drop crossgened code from the assembly

tools/dotnet-linker/LinkerConfiguration.cs

@@ -97,6 +97,9 @@ public List<ProductException> Exceptions {
 		public List<AssemblyDefinition> Assemblies => Application.LinkContext.Assemblies;
 		public required List<AssemblyPreparerInfo> AssemblyInfos;
 		public List<(string Path, AssemblyDefinition Assembly, string? OriginatingAssembly)> AddedAssemblies = new ();
+		// The set of assemblies that were modified (i.e. that AppBundleRewriter.SaveAssembly was called for).
+		// Assemblies that aren't modified don't need to be re-serialized when saved.
+		public HashSet<AssemblyDefinition> ModifiedAssemblies = new ();
 #else
 		// The list of assemblies is populated in CollectAssembliesStep.
 		public List<AssemblyDefinition> Assemblies = new List<AssemblyDefinition> ();