objective c - Why does ARC retain method arguments?

Question

When compiling with ARC, method arguments often appear to be retained at the beginning of the method and released at the end. This retain/release pair seems superfluous, and contradicts the idea that ARC "produces the code you would have written anyway". Nobody in those dark, pre-ARC days performed an extra retain/release on all method arguments just to be on the safe side, did they?

Consider:

@interface Test : NSObject
@end

@implementation Test

- (void)testARC:(NSString *)s
{
  [s length];  // no extra retain/release here.
}

- (void)testARC2:(NSString *)s
{
  // ARC inserts [s retain]
  [s length];
  [s length];
  // ARC inserts [s release]
}

- (void)testARC3:(__unsafe_unretained NSString *)s
{
  // no retain -- we used __unsafe_unretained
  [s length];
  [s length];
  // no release -- we used __unsafe_unretained
}

@end

When compiled with Xcode 4.3.2 in release mode, the assembly (such that I'm able to understand it) contained calls to objc_retain and objc_release at the start and end of the second method. What's going on?

This is not a huge problem, but this extra retain/release traffic does show up when using Instruments to profile performance-sensitive code. It seems you can decorate method arguments with __unsafe_unretained to avoid this extra retain/release, as I've done in the third example, but doing so feels quite disgusting.

Answer 1

See this reply from the Objc-language mailing list:

When the compiler doesn't know anything about the memory management behavior of a function or method (and this happens a lot), then the compiler must assume:

1) That the function or method might completely rearrange or replace the entire object graph of the application (it probably won't, but it could). 2) That the caller might be manual reference counted code, and therefore the lifetime of passed in parameters is not realistically knowable.

Given #1 and #2; and given that ARC must never allow an object to be prematurely deallocated, then these two assumptions force the compiler to retain passed in objects more often than not.

I think that the main problem is that your method’s body might lead to the arguments being released, so that ARC has to act defensively and retain them:

- (void) processItems
{
    [self setItems:[NSArray arrayWithObject:[NSNumber numberWithInt:0]]];
    [self doSomethingSillyWith:[items lastObject]];
}

- (void) doSomethingSillyWith: (id) foo
{
    [self setItems:nil];
    NSLog(@"%@", foo); // if ARC did not retain foo, you could be in trouble
}

That might also be the reason that you don’t see the extra retain when there’s just a single call in your method.