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Re: Garbage collector vs variable lifetime

On Jun 6, 2008, at 5:36 PM, Quincey Morris wrote:
Thanks, and to Shawn for the same suggestion. It's a pragmatic solution I can deal with.
A little inner voice insists on asking, though, how we know some future version of the compiler might not optimize '[data self]' upwards before the loop, if it decides that nothing inside the loop references anything non-local:

	NSData* data = <get it from somewhere>;
	const unsigned char* bytes = [data bytes];
	NSUInteger count = [data length];
	NSUInteger largestByte = 0;
	for (NSUInteger i = 0; i < count; i++)
		largestByte = MAX (largestByte , bytes [i]);
	[data self];

TBH, I don't see any real solution without making the compiler aware of the GC consequences of its optimizations.

In this case, that wouldn't be a GC issue. Every invocation of an Objective-C -- every Obj-C call site -- is dynamically bound and, thus, dynamically dispatched. Thus, the call site can't be moved without risk that the application behavior will change.

For example, it would be possible to cause the above to change what [data self] means when a particular bytes[i] is read.

b.bum

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References: 
 >Garbage collector vs variable lifetime (From: Quincey Morris <email@hidden>)
 >Re: Garbage collector vs variable lifetime (From: Bill Bumgarner <email@hidden>)
 >Re: Garbage collector vs variable lifetime (From: Ricky Sharp <email@hidden>)
 >Re: Garbage collector vs variable lifetime (From: Bill Bumgarner <email@hidden>)
 >Re: Garbage collector vs variable lifetime (From: Quincey Morris <email@hidden>)



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