On Sun, Apr 18, 2010 at 10:58 PM, Kyle Sluder <kyle.slu...@gmail.com> wrote:
> On Apr 18, 2010, at 6:52 PM, Michael Ash <michael....@gmail.com> wrote:
>
>> If by "indirected through the runtime" you mean "accesses one global
>> variable to get an offset". It does not, as one might take your
>> comment to indicate, actually call any runtime functions.
>
> If that's the case, how can the language support reordering instance
> variables? The only way I can see o support taking the address of an
> instance variable is to convert &ivar into a runtime call.
Why not take a look for yourself?
$ cat ivartest.m
#import <Cocoa/Cocoa.h>
@interface Foo : NSObject { id ivar; } @end
@implementation Foo
- (void)testMethod {
ivar = nil;
}
@end
$ gcc -c ivartest.m
$ otool -tV ivartest.o
ivartest.o:
(__TEXT,__text) section
-[Foo testMethod]:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp,%rbp
0000000000000004 movq %rdi,0xf8(%rbp)
0000000000000008 movq %rsi,0xf0(%rbp)
000000000000000c movq 0xf8(%rbp),%rdx
0000000000000010 movq _OBJC_IVAR_$_Foo.ivar(%rip),%rax
0000000000000017 movq (%rax),%rax
000000000000001a leaq (%rdx,%rax),%rax
000000000000001e movq $-[Foo testMethod],(%rax)
0000000000000025 leave
0000000000000026 ret
As you can see, the offset of the ivar itself is referenced as a
linker symbol which will then get resolved at load time. And you can
see that no function calls take place.
Mike
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