Mark main() as used

This prevents dead code stripping from removing the main() function used in widgets. This special case was added to support the integrated REPL, which was removed after Swift 5.3, and so it is no longer necessary.

Fixes rdar://65862827.

Author: beccadax

lib/IRGen/GenDecl.cpp

@@ -1908,11 +1908,7 @@ void irgen::updateLinkageForDefinition(IRGenModule &IGM,
 
   // Everything externally visible is considered used in Swift.
   // That mostly means we need to be good at not marking things external.
-  //
-  // Exclude "main", because it should naturally be used, and because adding it
-  // to llvm.used leaves a dangling use when the REPL attempts to discard
-  // intermediate mains.
-  if (LinkInfo::isUsed(IRL) && global->getName() != SWIFT_ENTRY_POINT_FUNCTION)
+  if (LinkInfo::isUsed(IRL))
     IGM.addUsedGlobal(global);
 }
 
@@ -2003,11 +1999,7 @@ llvm::Function *irgen::createFunction(IRGenModule &IGM,
 
   // Everything externally visible is considered used in Swift.
   // That mostly means we need to be good at not marking things external.
-  //
-  // Exclude "main", because it should naturally be used, and because adding it
-  // to llvm.used leaves a dangling use when the REPL attempts to discard
-  // intermediate mains.
-  if (linkInfo.isUsed() && name != SWIFT_ENTRY_POINT_FUNCTION) {
+  if (linkInfo.isUsed()) {
     IGM.addUsedGlobal(fn);
   }
 

test/IRGen/unused.sil

@@ -3,6 +3,8 @@
 // REQUIRES: CPU=x86_64
 
 sil_stage canonical
+import Builtin
+import Swift
 
 sil private @foo : $@convention(thin) () -> () {
 bb0:
@@ -43,12 +45,20 @@ bb0:
   return %1 : $()
 }
 
-// CHECK-macho: @llvm.used = appending global [2 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* bitcast (i16* @__swift_reflection_version to i8*)], section "llvm.metadata"
-// CHECK-elf: @llvm.used = appending global [3 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* bitcast (i16* @__swift_reflection_version to i8*), i8* getelementptr inbounds ([0 x i8], [0 x i8]* @_swift1_autolink_entries, i32 0, i32 0)], section "llvm.metadata"
+sil @main : $@convention(c) (Int32, UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>) -> Int32 {
+bb0(%0 : $Int32, %1 : $UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>):
+  %2 = integer_literal $Builtin.Int32, 0          // user: %3
+  %3 = struct $Int32 (%2 : $Builtin.Int32)        // user: %4
+  return %3 : $Int32                              // id: %4
+}
+
+// CHECK-macho: @llvm.used = appending global [3 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* bitcast (i32 (i32, i8**)* @main to i8*), i8* bitcast (i16* @__swift_reflection_version to i8*)], section "llvm.metadata"
+// CHECK-elf: @llvm.used = appending global [4 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* bitcast (i32 (i32, i8**)* @main to i8*), i8* bitcast (i16* @__swift_reflection_version to i8*), i8* getelementptr inbounds ([{{[0-9]+}} x i8], [{{[0-9]+}} x i8]* @_swift1_autolink_entries, i32 0, i32 0)], section "llvm.metadata"
 
 // CHECK: define linkonce_odr hidden swiftcc void @qux()
 // CHECK: define hidden swiftcc void @fred()
 // CHECK: define{{( dllexport)?}}{{( protected)?}} swiftcc void @frieda()
+// CHECK: define{{( dllexport)?}}{{( protected)?}} i32 @main
 
 // NEGATIVE-NOT: @foo
 // NEGATIVE-NOT: @bar