[IRGen] Expose an -Xfrontend flag to disable the LLVM MergeFunctions pass

Author: kubamracek

include/swift/AST/IRGenOptions.h

@@ -371,6 +371,8 @@ class IRGenOptions {
   /// Emit names of struct stored properties and enum cases.
   unsigned EnableReflectionNames : 1;
 
+  unsigned DisableLLVMMergeFunctions : 1;
+
   /// Emit mangled names of anonymous context descriptors.
   unsigned EnableAnonymousContextMangledNames : 1;
 
@@ -568,7 +570,8 @@ class IRGenOptions {
         SwiftAsyncFramePointer(SwiftAsyncFramePointerKind::Auto),
         HasValueNamesSetting(false), ValueNames(false),
         ReflectionMetadata(ReflectionMetadataMode::Runtime),
-        EnableReflectionNames(true), EnableAnonymousContextMangledNames(false),
+        EnableReflectionNames(true), DisableLLVMMergeFunctions(false),
+        EnableAnonymousContextMangledNames(false),
         ForcePublicLinkage(false), LazyInitializeClassMetadata(false),
         LazyInitializeProtocolConformances(false),
         IndirectAsyncFunctionPointer(false),

include/swift/Option/FrontendOptions.td

@@ -617,6 +617,9 @@ def disable_reflection_names : Flag<["-"], "disable-reflection-names">,
   HelpText<"Disable emission of names of stored properties and enum cases in"
            "reflection metadata">;
 
+def disable_llvm_merge_functions_pass : Flag<["-"], "disable-llvm-merge-functions-pass">,
+  HelpText<"Disable the MergeFunctionPass LLVM IR pass">;
+
 def function_sections: Flag<["-"], "function-sections">,
   Flags<[FrontendOption, NoInteractiveOption]>,
   HelpText<"Emit functions to separate sections.">;

lib/Frontend/CompilerInvocation.cpp

@@ -3344,6 +3344,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.EnableReflectionNames = false;
   }
 
+  if (Args.hasArg(OPT_disable_llvm_merge_functions_pass)) {
+    Opts.DisableLLVMMergeFunctions = true;
+  }
+
   if (Args.hasArg(OPT_force_public_linkage)) {
     Opts.ForcePublicLinkage = true;
   }

lib/IRGen/IRGen.cpp

@@ -260,7 +260,7 @@ void swift::performLLVMOptimizations(const IRGenOptions &Opts,
     PTO.LoopInterleaving = true;
     PTO.LoopVectorization = true;
     PTO.SLPVectorization = true;
-    PTO.MergeFunctions = true;
+    PTO.MergeFunctions = !Opts.DisableLLVMMergeFunctions;
     // Splitting trades code size to enhance memory locality, avoid in -Osize.
     DoHotColdSplit = Opts.EnableHotColdSplit && !Opts.optimizeForSize();
     level = llvm::OptimizationLevel::Os;
@@ -361,7 +361,8 @@ void swift::performLLVMOptimizations(const IRGenOptions &Opts,
                                         allowlistFiles, ignorelistFiles));
     });
   }
-  if (RunSwiftSpecificLLVMOptzns) {
+
+  if (RunSwiftSpecificLLVMOptzns && !Opts.DisableLLVMMergeFunctions) {
     PB.registerOptimizerLastEPCallback(
         [&](ModulePassManager &MPM, OptimizationLevel Level) {
           if (Level != OptimizationLevel::O0) {