[SILGen] Enable alternative entry point name.

include/swift/AST/ASTContext.h

@@ -1151,6 +1151,10 @@ class ASTContext final {
   /// standard library's String implementation.
   bool isASCIIString(StringRef s) const;
 
+  /// Retrieve the name of to be used for the entry point, either main or an
+  /// alternative specified via the -entry-point-function-name frontend flag.
+  std::string getEntryPointFunctionName() const;
+
 private:
   friend Decl;
   Optional<RawComment> getRawComment(const Decl *D);

include/swift/Basic/LangOptions.h

@@ -82,6 +82,9 @@ namespace swift {
     /// The target variant SDK version, if known.
     Optional<llvm::VersionTuple> VariantSDKVersion;
 
+    /// The alternate name to use for the entry point instead of main.
+    std::string entryPointFunctionName = "main";
+
     ///
     /// Language features
     ///

include/swift/Option/FrontendOptions.td

@@ -733,6 +733,9 @@ def disable_interface_lockfile : Flag<["-"], "disable-interface-lock">,
 def bridging_header_directory_for_print: Separate<["-"], "bridging-header-directory-for-print">, MetaVarName<"<path>">,
     HelpText<"Directory for bridging header to be printed in compatibility header">;
 
+def entry_point_function_name: Separate<["-"], "entry-point-function-name">, MetaVarName<"<string>">, 
+    HelpText<"Name of the entry point function">;
+
 def target_sdk_version : Separate<["-"], "target-sdk-version">,
   HelpText<"The version of target SDK used for compilation">;
 

include/swift/SIL/SILFunction.h

@@ -27,9 +27,6 @@
 #include "swift/SIL/SILLinkage.h"
 #include "swift/SIL/SILPrintContext.h"
 
-/// The symbol name used for the program entry point function.
-#define SWIFT_ENTRY_POINT_FUNCTION "main"
-
 namespace swift {
 
 class ASTContext;

lib/AST/ASTContext.cpp

@@ -4811,6 +4811,10 @@ ASTContext::SILTransformCtors ASTContext::getIRGenSILTransforms() const {
   return passes;
 }
 
+std::string ASTContext::getEntryPointFunctionName() const {
+  return LangOpts.entryPointFunctionName;
+}
+
 SILLayout *SILLayout::get(ASTContext &C,
                           CanGenericSignature Generics,
                           ArrayRef<SILField> Fields) {

lib/Frontend/CompilerInvocation.cpp

@@ -661,6 +661,10 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_entry_point_function_name)) {
+    Opts.entryPointFunctionName = A->getValue();
+  }
+
   if (FrontendOpts.RequestedAction == FrontendOptions::ActionType::EmitSyntax) {
     Opts.BuildSyntaxTree = true;
     Opts.VerifySyntaxTree = true;

lib/IRGen/GenDecl.cpp

@@ -590,9 +590,9 @@ void IRGenModule::emitRuntimeRegistration() {
     return;
 
   // Find the entry point.
-  SILFunction *EntryPoint =
-    getSILModule().lookUpFunction(SWIFT_ENTRY_POINT_FUNCTION);
-  
+  SILFunction *EntryPoint = getSILModule().lookUpFunction(
+      getSILModule().getASTContext().getEntryPointFunctionName());
+
   // If we're debugging (and not in the REPL), we don't have a
   // main. Find a function marked with the LLDBDebuggerFunction
   // attribute instead.
@@ -1894,7 +1894,8 @@ static std::string getEntryPointSection(IRGenModule &IGM) {
 
 void IRGenerator::emitEntryPointInfo() {
   SILFunction *entrypoint = nullptr;
-  if (!(entrypoint = SIL.lookUpFunction(SWIFT_ENTRY_POINT_FUNCTION))) {
+  if (!(entrypoint = SIL.lookUpFunction(
+            SIL.getASTContext().getEntryPointFunctionName()))) {
     return;
   }
   auto &IGM = *getGenModule(entrypoint);

lib/IRGen/IRGenDebugInfo.cpp

@@ -132,7 +132,7 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
   llvm::DIFile *MainFile = nullptr;
   /// The current module.
   llvm::DIModule *MainModule = nullptr;
-  /// Scope of SWIFT_ENTRY_POINT_FUNCTION.
+  /// Scope of entry point function (main by default).
   llvm::DIScope *EntryPointFn = nullptr;
   /// The artificial type decls for named archetypes.
   llvm::StringMap<TypeAliasDecl *> MetadataTypeDeclCache;
@@ -2084,7 +2084,8 @@ llvm::DIScope *IRGenDebugInfoImpl::getEntryPointFn() {
   // Lazily create EntryPointFn.
   if (!EntryPointFn) {
     EntryPointFn = DBuilder.createReplaceableCompositeType(
-        llvm::dwarf::DW_TAG_subroutine_type, SWIFT_ENTRY_POINT_FUNCTION,
+        llvm::dwarf::DW_TAG_subroutine_type,
+        IGM.getSILModule().getASTContext().getEntryPointFunctionName(),
         MainFile, MainFile, 0);
   }
   return EntryPointFn;
@@ -2219,7 +2220,8 @@ IRGenDebugInfoImpl::emitFunction(const SILDebugScope *DS, llvm::Function *Fn,
     Scope = getOrCreateContext(SILFn->getDeclContext()->getParent());
 
   // We know that main always comes from MainFile.
-  if (LinkageName == SWIFT_ENTRY_POINT_FUNCTION) {
+  if (LinkageName ==
+      IGM.getSILModule().getASTContext().getEntryPointFunctionName()) {
     File = MainFile;
     Line = 1;
     Name = LinkageName;
@@ -2242,7 +2244,9 @@ IRGenDebugInfoImpl::emitFunction(const SILDebugScope *DS, llvm::Function *Fn,
   // have a Swift name, does appear prominently in the source code.
   // ObjC thunks should also not show up in the linetable, because we
   // never want to set a breakpoint there.
-  if ((Name.empty() && LinkageName != SWIFT_ENTRY_POINT_FUNCTION &&
+  if ((Name.empty() &&
+       LinkageName !=
+           IGM.getSILModule().getASTContext().getEntryPointFunctionName() &&
        !isExplicitClosure(SILFn)) ||
       (Rep == SILFunctionTypeRepresentation::ObjCMethod) ||
       isAllocatingConstructor(Rep, DeclCtx)) {
@@ -2279,7 +2283,8 @@ IRGenDebugInfoImpl::emitFunction(const SILDebugScope *DS, llvm::Function *Fn,
     Fn->setSubprogram(SP);
 
   // RAUW the entry point function forward declaration with the real thing.
-  if (LinkageName == SWIFT_ENTRY_POINT_FUNCTION) {
+  if (LinkageName ==
+      IGM.getSILModule().getASTContext().getEntryPointFunctionName()) {
     if (EntryPointFn) {
       assert(EntryPointFn->isTemporary() &&
              "more than one entry point function");

lib/IRGen/IRGenDebugInfo.h

@@ -89,10 +89,10 @@ class IRGenDebugInfo {
   /// artificial inline location pointing to the user's instruction.
   void setInlinedTrapLocation(IRBuilder &Builder, const SILDebugScope *Scope);
 
-  /// Set the location for SWIFT_ENTRY_POINT_FUNCTION.
+  /// Set the location for entry point function (main by default).
   void setEntryPointLoc(IRBuilder &Builder);
 
-  /// Return the scope for SWIFT_ENTRY_POINT_FUNCTION.
+  /// Return the scope for the entry point function (main by default).
   llvm::DIScope *getEntryPointFn();
 
   /// Translate a SILDebugScope into an llvm::DIDescriptor.

lib/IRGen/TypeLayoutVerifier.cpp

@@ -291,8 +291,8 @@ void IRGenModule::emitTypeVerifier() {
     return;
 
   // Find the entry point.
-  SILFunction *EntryPoint =
-    getSILModule().lookUpFunction(SWIFT_ENTRY_POINT_FUNCTION);
+  SILFunction *EntryPoint = getSILModule().lookUpFunction(
+      getSILModule().getASTContext().getEntryPointFunctionName());
 
   if (!EntryPoint)
     return;

lib/SILGen/SILGen.cpp

@@ -569,9 +569,10 @@ SILFunction *SILGenModule::emitTopLevelFunction(SILLocation Loc) {
                                    SubstitutionMap(), SubstitutionMap(),
                                    C);
 
+  auto name = getASTContext().getEntryPointFunctionName();
   SILGenFunctionBuilder builder(*this);
   return builder.createFunction(
-      SILLinkage::Public, SWIFT_ENTRY_POINT_FUNCTION, topLevelType, nullptr,
+      SILLinkage::Public, name, topLevelType, nullptr,
       Loc, IsBare, IsNotTransparent, IsNotSerialized, IsNotDynamic,
       ProfileCounter(), IsNotThunk, SubclassScope::NotApplicable);
 }
@@ -1817,8 +1818,9 @@ class SourceFileScope {
     // If this is the script-mode file for the module, create a toplevel.
     if (sf->isScriptMode()) {
       assert(!sgm.TopLevelSGF && "already emitted toplevel?!");
-      assert(!sgm.M.lookUpFunction(SWIFT_ENTRY_POINT_FUNCTION)
-             && "already emitted toplevel?!");
+      assert(!sgm.M.lookUpFunction(
+                 sgm.getASTContext().getEntryPointFunctionName()) &&
+             "already emitted toplevel?!");
 
       RegularLocation TopLevelLoc = RegularLocation::getModuleLocation();
       SILFunction *toplevel = sgm.emitTopLevelFunction(TopLevelLoc);
@@ -1940,8 +1942,9 @@ class SourceFileScope {
     // If the source file contains an artificial main, emit the implicit
     // toplevel code.
     if (auto mainDecl = sf->getMainDecl()) {
-      assert(!sgm.M.lookUpFunction(SWIFT_ENTRY_POINT_FUNCTION)
-             && "already emitted toplevel before main class?!");
+      assert(!sgm.M.lookUpFunction(
+                 sgm.getASTContext().getEntryPointFunctionName()) &&
+             "already emitted toplevel before main class?!");
 
       RegularLocation TopLevelLoc = RegularLocation::getModuleLocation();
       SILFunction *toplevel = sgm.emitTopLevelFunction(TopLevelLoc);

lib/TBDGen/TBDGenVisitor.h

@@ -165,7 +165,8 @@ class TBDGenVisitor : public ASTVisitor<TBDGenVisitor> {
     // TBD for, and not for any statically linked libraries.
     // FIXME: We should have a SymbolSource for main.
     if (file->hasEntryPoint() && file->getParentModule() == SwiftModule)
-      addSymbol("main", SymbolSource::forUnknown());
+      addSymbol(SwiftModule->getASTContext().getEntryPointFunctionName(),
+                SymbolSource::forUnknown());
   }
 
   /// Adds the global symbols associated with the first file.

test/SILGen/Inputs/forward-to-foobar.c

@@ -0,0 +1,5 @@
+extern int foobar(int argc, char *argv[]);
+
+int main(int argc, char *argv[]) {
+    return foobar(argc, argv);
+}

test/SILGen/other-entry-point-function-name.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-clang -c --language=c %S/Inputs/forward-to-foobar.c -o %t/forward.o
+// RUN: %target-build-swift %s %t/forward.o -Xfrontend -entry-point-function-name -Xfrontend foobar -o %t/main
+// RUN: %target-run %t/main | %FileCheck %s
+
+// CHECK: howdy from foobar
+print("howdy from foobar")
+
+