Local variables are free

This is part II of my irregularly scheduled series on compiler optimization. In part I, I explained how the compiler can optimize away return statements, resulting in missed breakpoints. My given workaround to that problem, though effective, was very ugly and architecture-dependent, much like Cowboys Stadium.

(gdb) break *0x00001fc5 if $eax != 0

Although there’s not much we can do to prevent the compiler optimization, we can greatly simplify our conditional breakpoint. I had suggested rewriting the source code, which was awe-inspiringly prescient, because that’s what I’m going to do now. Here’s the original code:

8	if (ShouldReturn())
9		return;

And here’s the revised code:

8	int localVar = ShouldReturn();
9	if (localVar)
10		return;

The return at line 10 will still be optimized away. However, the revised code allows us to set a simple breakpoint at line 9 that will stop when we want:

(gdb) break 9 if localVar != 0

No knowledge of the architecture, machine registers, or assembly language is required.

From the beginning of time (January 1970, of course), programmers have struggled over coding style. Objective-C programmers, for example, expend undue effort arranging their brackets. (I have [NSMutableArray array] going to the Final Four.) For some, bracket-making becomes a kind of game or contest.

[[[[[[[[[[[[[See how] many] method] calls] we] can] fit] on] one] line] of] source] code];

I’ve changed my coding style over the years, but I’ve settled on one fundamental principle: write your code so that it’s easy to debug. All your fancy margin-aligning isn’t going to help when you need to figure out why your app keeps exploding. If you have nested method calls on one line of code, you can’t easily set a breakpoint in the middle. That’s why I prefer as much as possible to have only one method call per line of code, and create a local variable to store the return value.

There is a misconception that local variables are expensive, in terms of either computation or memory. The truth is that local variables are very cheap, the value meals of the computing world. (Would you like trans fat with your saturated fat?) It only takes one machine instruction to store a pointer address to a local variable. One machine instruction is really quite fast, about as fast as you can get — at least with restrictor plates. With regard to memory, local variables only take up stack space. To create a local variable, you simply move the stack a little. When the method or function returns, the stack is moved back, and thereby the space reserved for local variables is automatically recovered. Of course, you don’t want to create large C arrays on the stack, but a pointer to an Objective-C object only takes 4 bytes on the stack for 32-bit, 8 bytes for 64-bit. The default 32-bit stack size is 8MB, so you’re not going to run out of space unless you have deeply recursive calls.

Even these small costs are only relevant in the context of your app’s unoptimized, debug configuration. For your customers, on the other hand, local variables are free. As in Mumia, or Bird. When you compile your app using the release configuration, the local variables disappear, the compiler optimizes them away. (By the way, this is one of the reasons that debugging the release build of your app can be a frustrating and/or wacky experience.) To see the optimization in action, let’s consider some sample code:

1  #import <Foundation/Foundation.h>
2
3  @interface MyObject : NSObject {}
4  @end
5
6  @implementation MyObject
7
8  -(NSString *)myDirectProcessName {
9  	return [[[NSProcessInfo processInfo] processName] lowercaseString];
10 }
11
12 -(NSString *)myRoundaboutProcessName {
13 	NSString *myRoundaboutProcessName = nil;
14 	NSProcessInfo *processInfo = [NSProcessInfo processInfo];
15 	NSString *processName = [processInfo processName];
16 	NSString *lowercaseString = [processName lowercaseString];
17 	myRoundaboutProcessName = lowercaseString;
18 	return myRoundaboutProcessName;
19 }
20
21 @end
22
23 int main(int argc, const char *argv[]) {
24 	NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
25 	MyObject *myObject = [[[MyObject alloc] init] autorelease];
26 	NSLog(@"My direct process name: %@", [myObject myDirectProcessName]);
27 	NSLog(@"My roundabout process name: %@", [myObject myRoundaboutProcessName]);
28 	[pool release];
29 	return 0;
30 }

The above code is obviously contrived and useless. It only has value for explanatory purposes, and perhaps in the app store for $0.99. The methods -myRoundaboutProcessName and -myDirectProcessName do the same thing, the former with and the latter without local variables. Here’s the i386 disassembly for the methods when compiled using the debug configuration:

-[MyObject myDirectProcessName]:
00001d2a	nop
00001d2b	nop
00001d2c	nop
00001d2d	nop
00001d2e	nop
00001d2f	nop
00001d30	pushl	%ebp
00001d31	movl	%esp,%ebp
00001d33	pushl	%ebx
00001d34	subl	$0x14,%esp
00001d37	calll	0x00001d3c
00001d3c	popl	%ebx
00001d3d	leal	0x000012e8(%ebx),%eax
00001d43	movl	(%eax),%eax
00001d45	movl	%eax,%edx
00001d47	leal	0x000012e4(%ebx),%eax
00001d4d	movl	(%eax),%eax
00001d4f	movl	%eax,0x04(%esp)
00001d53	movl	%edx,(%esp)
00001d56	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001d5b	movl	%eax,%edx
00001d5d	leal	0x000012e0(%ebx),%eax
00001d63	movl	(%eax),%eax
00001d65	movl	%eax,0x04(%esp)
00001d69	movl	%edx,(%esp)
00001d6c	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001d71	movl	%eax,%edx
00001d73	leal	0x000012dc(%ebx),%eax
00001d79	movl	(%eax),%eax
00001d7b	movl	%eax,0x04(%esp)
00001d7f	movl	%edx,(%esp)
00001d82	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001d87	addl	$0x14,%esp
00001d8a	popl	%ebx
00001d8b	leave
00001d8c	ret
-[MyObject myRoundaboutProcessName]:
00001d8d	nop
00001d8e	nop
00001d8f	nop
00001d90	nop
00001d91	nop
00001d92	nop
00001d93	pushl	%ebp
00001d94	movl	%esp,%ebp
00001d96	pushl	%ebx
00001d97	subl	$0x24,%esp
00001d9a	calll	0x00001d9f
00001d9f	popl	%ebx
00001da0	movl	$0x00000000,0xe8(%ebp)
00001da7	leal	0x00001285(%ebx),%eax
00001dad	movl	(%eax),%eax
00001daf	movl	%eax,%edx
00001db1	leal	0x00001281(%ebx),%eax
00001db7	movl	(%eax),%eax
00001db9	movl	%eax,0x04(%esp)
00001dbd	movl	%edx,(%esp)
00001dc0	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001dc5	movl	%eax,0xec(%ebp)
00001dc8	movl	0xec(%ebp),%edx
00001dcb	leal	0x0000127d(%ebx),%eax
00001dd1	movl	(%eax),%eax
00001dd3	movl	%eax,0x04(%esp)
00001dd7	movl	%edx,(%esp)
00001dda	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001ddf	movl	%eax,0xf0(%ebp)
00001de2	movl	0xf0(%ebp),%edx
00001de5	leal	0x00001279(%ebx),%eax
00001deb	movl	(%eax),%eax
00001ded	movl	%eax,0x04(%esp)
00001df1	movl	%edx,(%esp)
00001df4	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001df9	movl	%eax,0xf4(%ebp)
00001dfc	movl	0xf4(%ebp),%eax
00001dff	movl	%eax,0xe8(%ebp)
00001e02	movl	0xe8(%ebp),%eax
00001e05	addl	$0x24,%esp
00001e08	popl	%ebx
00001e09	leave
00001e0a	ret

As expected, -myRoundaboutProcessName makes more room on the stack than -myDirectProcessName:

00001d34	subl	$0x14,%esp
00001d97	subl	$0x24,%esp

At 00001da0, -myRoundaboutProcessName sets the value of the local variable to nil, as in line 13 of the source code. The interesting differences, though, are immediately after the calls to objc_msgSend(). By the standard ABI, the register eax contains the return value of objc_msgSend(). In -myDirectProcessName, the value in eax is simply moved to the register edx:

00001d5b	movl	%eax,%edx

In contrast, -myRoundaboutProcessName first stores the value on the stack before moving it to edx. The address on the stack is the space reserved for the local variable:

00001dc5	movl	%eax,0xec(%ebp)
00001dc8	movl	0xec(%ebp),%edx

After the final objc_msgSend() call, -myDirectProcessName doesn’t bother to do much, because the return value in eax will become the return value of the whole method. In -myRoundaboutProcessName, it needs to store values in local variables as in lines 16 and 17 of the source code:

00001df9	movl	%eax,0xf4(%ebp)
00001dfc	movl	0xf4(%ebp),%eax
00001dff	movl	%eax,0xe8(%ebp)
00001e02	movl	0xe8(%ebp),%eax

So that’s how the methods differ in the unoptimized build. Now let’s see what happens when we use the release configuration. Here’s the optimized disassembly for -myDirectProcessName:

-[MyObject myDirectProcessName]:
00001dce	pushl	%ebp
00001dcf	movl	%esp,%ebp
00001dd1	subl	$0x18,%esp
00001dd4	movl	0x00003000,%eax
00001dd9	movl	%eax,0x04(%esp)
00001ddd	movl	0x0000302c,%eax
00001de2	movl	%eax,(%esp)
00001de5	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001dea	movl	0x00003004,%edx
00001df0	movl	%edx,0x04(%esp)
00001df4	movl	%eax,(%esp)
00001df7	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001dfc	movl	0x00003008,%edx
00001e02	movl	%edx,0x0c(%ebp)
00001e05	movl	%eax,0x08(%ebp)
00001e08	leave
00001e09	jmpl	0x0000400a	; symbol stub for: _objc_msgSend

The optimized method is significantly shorter, as expected from the compiler option -Os. First, you’ll notice that all those pesky nop instructions have been deleted. Stallman put them in unoptimized builds just to annoy us. (Or they may have been for Fix and Continue, but I always assume the worst.) There are additional optimizations as well that I won’t belabor here, because I’m eager to get to the climax. (Sorry, dear.) For your enlightenment and enjoyment, here’s the optimized disassembly for -myRoundaboutProcessName:

-[MyObject myRoundaboutProcessName]:
00001e0e	pushl	%ebp
00001e0f	movl	%esp,%ebp
00001e11	subl	$0x18,%esp
00001e14	movl	0x00003000,%eax
00001e19	movl	%eax,0x04(%esp)
00001e1d	movl	0x0000302c,%eax
00001e22	movl	%eax,(%esp)
00001e25	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001e2a	movl	0x00003004,%edx
00001e30	movl	%edx,0x04(%esp)
00001e34	movl	%eax,(%esp)
00001e37	calll	0x0000400a	; symbol stub for: _objc_msgSend
00001e3c	movl	0x00003008,%edx
00001e42	movl	%edx,0x0c(%ebp)
00001e45	movl	%eax,0x08(%ebp)
00001e48	leave
00001e49	jmpl	0x0000400a	; symbol stub for: _objc_msgSend

Identical! Ah, that’s nice. Smoke ‘em if you got ‘em.

In conclusion, feel free to sprinkle, pepper, dash, or even drown your code with local variables. And with the engineering hours of debugging time you save, get me a nice (not free) present. I’m partial to flavored coffee and unflavored MacBooks.

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