Nat!'s Web Journal

#import <Foundation/Foundation.h>

@interface Foo : NSObject 
{
   long  a;
   long  b;
   long  c;
}

- (long) abc;

@end

@implementation Foo

- (long) abc
{
   return( self->a + self->b + self->c);
}

@end

And this is the solution:

#import <Foundation/Foundation.h>

@interface Foo : NSObject 
{
   struct 
   {
      long  a;
      long  b;
      long  c;
   } abc;
}

- (long) abc;

@end

@implementation Foo

- (long) abc
{
   return( self->abc.a + self->abc.b + self->abc.c);
}

@end

So why is this faster ? A look at the disassembled code shows, that because of the struct, there is no need to fetch an offset for each value. That ivars are dynamically offset is a new "feature" of the 64 bit runtime.

Before:

	pushq	%rbp
	movq	%rsp, %rbp

	movq	_OBJC_IVAR_$_Foo.a(%rip), %rax
	movq	(%rdi,%rax), %rax
	movq	_OBJC_IVAR_$_Foo.b(%rip), %rdx
	addq	(%rdi,%rdx), %rax
	movq	_OBJC_IVAR_$_Foo.c(%rip), %rdx
	addq	(%rdi,%rdx), %rax

	leave
	ret

After:

	pushq	%rbp
	movq	%rsp, %rbp

	addq	_OBJC_IVAR_$_Foo.x(%rip), %rdi

	movq	(%rdi), %rax
	addq	8(%rdi), %rax
	addq	16(%rdi), %rax

	leave
	ret

I don't know, why gcc doesn't optimize the stack frame away in these simple cases.