With ARC, what's better: alloc or autorelease initializers?

Is it better (faster & more efficient) to use alloc or autorelease initializers. E.g.:

- (NSString *)hello:(NSString *)name {
    return [[NSString alloc] initWithFormat:@"Hello, %@", name];
}

OR

- (NSString *)hello:(NSString *)name {
    return [NSString stringWithFormat:@"Hello, %@", name];
//    return [@"Hello, " stringByAppendingString:name]; // even simpler
}

I know that in most cases, performance here shouldn't matter. But, I'd still like to get in the habit of doing it the better way.

If they do exactly the same thing, then I prefer the latter option because it's shorter to type and more readable.

In Xcode 4.2, is there a way to see what ARC compiles to, i.e., where it puts retain, release, autorelease, etc? This feature would be very useful while switching over to ARC. I know you shouldn't have to think about this stuff, but it'd help me figure out the answer to questions like these.

The difference is subtle, but you should opt for the autorelease versions. Firstly, your code is much more readable. Secondly, on inspection of the optimized assembly output, the autorelease version is slightly more optimal.

The autorelease version,

- (NSString *)hello:(NSString *)name {
    return [NSString stringWithFormat:@"Hello, %@", name];
}

translates to

"-[SGCAppDelegate hello:]":
    push    {r7, lr}
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_-(LPC0_0+4))
    mov r3, r2
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_-(LPC0_0+4))
    movw    r0, :lower16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC0_1+4))
    movt    r0, :upper16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC0_1+4))
    add r1, pc
    add r0, pc
    mov r7, sp
    ldr r1, [r1]
    ldr r0, [r0]
    movw    r2, :lower16:(L__unnamed_cfstring_-(LPC0_2+4))
    movt    r2, :upper16:(L__unnamed_cfstring_-(LPC0_2+4))
    add r2, pc
    blx _objc_msgSend    ; stringWithFormat:
    pop {r7, pc}

Whereas the [[alloc] init] version looks like the following:

"-[SGCAppDelegate hello:]":
    push    {r4, r5, r6, r7, lr}
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_2-(LPC1_0+4))
    add r7, sp, #12
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_2-(LPC1_0+4))
    movw    r0, :lower16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC1_1+4))
    movt    r0, :upper16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC1_1+4))
    add r1, pc
    add r0, pc
    ldr r5, [r1]
    ldr r6, [r0]
    mov r0, r2
    blx _objc_retain    ; ARC retains the name string temporarily
    mov r1, r5
    mov r4, r0
    mov r0, r6
    blx _objc_msgSend   ; call to alloc
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_4-(LPC1_2+4))
    mov r3, r4
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_4-(LPC1_2+4))
    add r1, pc
    ldr r1, [r1]
    movw    r2, :lower16:(L__unnamed_cfstring_-(LPC1_3+4))
    movt    r2, :upper16:(L__unnamed_cfstring_-(LPC1_3+4))
    add r2, pc
    blx _objc_msgSend   ; call to initWithFormat:
    mov r5, r0
    mov r0, r4
    blx _objc_release   ; ARC releases the name string
    mov r0, r5
    pop.w   {r4, r5, r6, r7, lr}
    b.w _objc_autorelease

As expected, it is a little longer, because it is calling the alloc and initWithFormat: methods. What is particularly interesting is ARC is generating sub-optimal code here, as it retains the name string (noted by call to _objc_retain) and later released after the call to initWithFormat:.

If we add the __unsafe_unretained ownership qualifier, as in the following example, the code is rendered optimally. __unsafe_unretained indicates to the compiler to use primitive (copy pointer) assignment semantics.

- (NSString *)hello:(__unsafe_unretained NSString *)name {
    return [[NSString alloc] initWithFormat:@"Hello, %@", name];
}

as follows:

"-[SGCAppDelegate hello:]":
    push    {r4, r7, lr}
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_2-(LPC1_0+4))
    add r7, sp, #4
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_2-(LPC1_0+4))
    movw    r0, :lower16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC1_1+4))
    movt    r0, :upper16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC1_1+4))
    add r1, pc
    add r0, pc
    mov r4, r2
    ldr r1, [r1]
    ldr r0, [r0]
    blx _objc_msgSend
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_4-(LPC1_2+4))
    mov r3, r4
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_4-(LPC1_2+4))
    add r1, pc
    ldr r1, [r1]
    movw    r2, :lower16:(L__unnamed_cfstring_-(LPC1_3+4))
    movt    r2, :upper16:(L__unnamed_cfstring_-(LPC1_3+4))
    add r2, pc
    blx _objc_msgSend
    .loc    1 31 1
    pop.w   {r4, r7, lr}
    b.w _objc_autorelease

[NSString stringWithFormat:] is less code. But be aware that the object may end up in the autorelease pool. And that currently happens even with ARC and -Os compiler optimization.

Currently the performance of [[NSString alloc] initWithFormat:] is better on both iOS (tested with iOS 5.1.1 and Xcode 4.3.3) and OS X (tested with OS X 10.7.4 and Xcode 4.3.3). I modified @Pascal's sample code to include the autorelease pool drain times and got the following results:

• The ARC optimization does not prevent the objects to end up in the autorelease pool.

• Including time for clearing out the release pool with 1 million objects, [[NSString alloc] initWithFormat:] is around 14% faster on iPhone 4S, and around 8% faster on OS X

• Having an @autoreleasepool around the loop releases all objects at the and of the loop, which eats up a lot of memory.

  

• The memory spikes can be prevented by using an @autoreleasepool inside the loop. The performance stays roughly the same, but the memory consumption then is flat.

I disagree with the other answers, the autorelease version (your 2nd example) is not necessarily better.

The autorelease version behaves just as is it did before ARC. It allocates and inits and then autoreleases, which means the pointer to the object needs to be stored to be autoreleased later the next time the autorelease pool is drained. This uses slightly more memory as the pointer to that object needs to be kept around until it is processed. The object also sticks around longer than if it was immediately released. This can be an issue if you are calling this many times in a loop so the autorelease pool would not have a chance to be drained. This could cause you to run out of memory.

The first example behaves differently than it did before ARC. With ARC, the compiler will now insert a "release" for you (NOT an autorelease like the 2nd example). It does this at the end of the block where the memory is allocated. Usually this is at the end of the function where it is called. In your example, from viewing the assembly, it seems like the object may in fact be autoreleased. This might be due to the fact the compiler doesn't know where the function returns to and thus where the end of the block is. In the majority of the cases where a release is added by the compiler at the end of a block, the alloc/init method will result in better performance, at least in terms of memory usage, just as it did before ARC.