Xcode 8 with "Release: Fastest, Smallest [-Os]" have some weird issue and not functional well in some case

Question

I have 1 target in my project and have 3 build configuration

Debug Production & Release

While I am running my app with debug or production configuration and its working fine with no issues at all, but when I change my build configuration to release it's giving me some weird issues like some string values are not storing in plist file and when I debug my release mode, I didn't get any value in my debugger.

After too much investigation on my code and build setting and some suggestions from SO I have done the below change in my Build setting as mentioned below.

Change Optimization Level under Apple LLVM 8.0 Code Generation header

Getting issue in below Optimization level:

Resolved issue with below change in Optimization level:

Don't know why it's working in None[-O0].

Can anyone help me with the above patch?

Here is one sensitive part where the above Optimisation level affected.

My saved bookmark code where I have value but still it's storing NULL in plist.

- (IBAction)bookmark:(id)sender{
    
    DimensionModel *aDimensionModel = APPDELEGATE.selected_dimensionModel;
    EmirateModel *aEmirateModel = APPDELEGATE.selected_emirateModel;
    DivisionModel *aDivisionModel = APPDELEGATE.selected_divisionModel;
    __weak NSString *aStrVCName = self.bookMarkViewModel.selectedStrVCName;
    
    //Add Title string in Dictionary and then in Plist
    __weak NSString *aStrTitle = [NSString stringWithFormat:@"%@",aDimensionModel.dimension_description];
    aStrTitle = [aStrTitle stringByReplacingOccurrencesOfString:@"(null)" withString:@""];
    
    if (IS_ENGLISH) {
        aStrTitle = [aStrTitle stringByAppendingString:[NSString stringWithFormat:@" - %@ - %@ - %@",aDivisionModel.division_name,aEmirateModel.emirate_name,[self.bookMarkViewModel.strDataType isEqualToString:@"Number"]?@"(0)":@"(%)"]];
    }else{
        aStrTitle = [aStrTitle stringByAppendingString:[NSString stringWithFormat:@" - %@ - %@ - %@",aDivisionModel.division_name,[Utilities getEmirateNameinArabicByCode:aEmirateModel.emirate_code],[self.bookMarkViewModel.strDataType isEqualToString:@"Number"]?@"(0)":@"(%)"]];
    }
    
    if (APPDELEGATE.selectedDimRecordTimeModel.record_year == -1) {
        aStrTitle = [aStrTitle stringByAppendingString:[NSString stringWithFormat:@" - %@",[LanguageUtility getLocalizedStringForKey:@"All Years"]]];
    }else{
        aStrTitle = [aStrTitle stringByAppendingString:[NSString stringWithFormat:@" - %ld",(long)APPDELEGATE.selectedDimRecordTimeModel.record_year]];
    }
    
    
    NSMutableDictionary *aMutDicGraphDetail = [NSMutableDictionary new];
    
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",self.bookMarkViewModel.strSelectedDim] forKey:@"DimCode"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",aStrVCName] forKey:@"VCName"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",PLIST_TITLE_BOOKMARK] forKey:@"PlistBookmark"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",IS_ENGLISH?@"ENGLISH":@"ARABIC"] forKey:@"Language"];
    if (self.bookMarkViewModel.strSelectedDim == nil) {
        [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",[LanguageUtility getLocalizedStringForKey:aStrVCName]] forKey:@"Title"];
    }else{
        [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",aStrTitle] forKey:@"Title"];
    }
    
    
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",APPDELEGATE.selectedDimRecordTimeModel.record_time] forKey:@"RecordTime"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%ld",(long)APPDELEGATE.selectedDimRecordTimeModel.record_year] forKey:@"RecordYear"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",aEmirateModel.emirate_code] forKey:@"EmirateCode"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",self.bookMarkViewModel.strDataType] forKey:@"DataType"];
    
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",aDivisionModel.division_code] forKey:@"DivisionCode"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%d",APPDELEGATE.isDataTypeSwitchHide] forKey:@"switchHide"];
    [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",APPDELEGATE.strYearOrMonthFromRbtnCtrl] forKey:@"YearOrMonthFromRbtnCtrl"];
    [aMutDicGraphDetail setObject:@(self.bookMarkViewModel.selectedIndexPath).stringValue forKey:DimSelectedIndex];
    
    /* Check for RegionVC  and CountryVC*/
    if ([aStrVCName isEqualToString:@"RegionMainVC"] || [aStrVCName isEqualToString:@"CountryMainVC"]){
        [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%@",self.bookMarkViewModel.strComparison] forKey:@"Comparison"];
        [aMutDicGraphDetail setObject:[NSString stringWithFormat:@"%ld",self.bookMarkViewModel.intSelectedRegionId] forKey:@"RegionId"];
    }
    [self.bookMarkViewModel saveBookmark:aMutDicGraphDetail];
    
    aStrVCName = nil;
}

Using the above code all values are successfully stored into plist but aStrTitle has value in it and still, it's storing as null.

Answer 1

Your problem is in this line:

__weak NSString *aStrTitle = [NSString stringWithFormat:@"%@",aDimensionModel.dimension_description];

+[NSString stringWithFormat:] creates a new string with retain count equal to zero. And this is usually okay, since this value is either assigned to a strong variable or passed as a parameter to the method call. In both cases it's retain count gets incremented and the value remains untouched. But you are assigning this new string to the __weak variable! Weak variables don't affect retain count so it remains zero after this line executed. And so the created string gets deleted immediately after creation.

Solution is simple: remove __weak specifier and you'll be good to go.

There is no point in declaring local variables as weak (unless they are captured by the blocks). Also "cleanups" like this one: aStrVCName = nil; have no effect at all.

• All local variables are automatically released when the method finishes its execution.
• aStrVCName is weak anyway, so it never affects retain counts of the objects it's pointing to.

Answer 2

I had a similar issue when migrating to xcode 8 and swift 3. Turns out, swift doesn't like the fact I override functions from the base classes in extensions. see Overriding methods in Swift extensions

For some reason, the compiler doesn't complain about it, and to make things worse, the "strange issues" only started to happen in the Fastest, Smallest [-Os] optimization level.

Seems that in the Fastest, Smallest [-Os] optimization level, the compiler will call the function from the base class, and not the function from the extension. In the None [-O0] optimization level, the compiler will call the function from the extension.

for example:

public class BaseController : UIViewController {
    override func reloadInterface() {
      println("ok")
    }
}

public class NewController : BaseController {
    override func viewDidLoad() {
      super.viewDidLoad()

      reloadInterface()
     }

}

extension NewController {
   override func reloadInterface() {
      println("extended function")
    }
}

will print out "extended function" in none optimization level, and will print out "ok" in fastest optimization level...

to make story short, don't override functions in swift extensions