Re: Question: What is the impact of changing .cpp AudioUnitEffectsource to .mm

Subject: Re: Question: What is the impact of changing .cpp AudioUnitEffectsource to .mm
From: "Paul Sanders" <email@hidden>
Date: Mon, 21 Jun 2010 08:45:44 +0100

OK, thanks very much. It looks like method resolution is the only time objc_msgSend can block so it is possible to use objC calls in an IOProc if one takes a bit of care, which is what I wanted to establish. I imagine there are people out there who want to know this.

Regards,

Paul Sanders.

----- Original Message -----

From: Stephen F. Booth

To: Paul Sanders

Cc: email@hidden

Sent: Sunday, June 20, 2010 11:04 PM

Subject: Re: Question: What is the impact of changing .cpp AudioUnitEffectsource to .mm

objc_msgSend can block when it is called for the first time for a particular class/selector combination (see http://www.friday.com/bbum/2010/02/04/objc_msgsend-tour-part-4-method-lookup-some-odds-and-ends/). When this path of objc_msgSend is taken the method is resolved and cached (as you said), and the relevant +initialize methods are called where a lock is taken to ensure thread-safety (see http://www.friday.com/bbum/2009/09/06/iniailize-can-be-executed-multiple-times-load-not-so-much/). So depending on how your Objective-C classes are written the hit could be minimal or substantial.

I don't know if this is the only time that objc_msgSend might block, but if you are an assembly wizard (I'm not!) the code is available at http://www.opensource.apple.com/source/objc4/objc4-437.1/runtime/Messengers.subproj/objc-msg-x86_64.s

Stephen

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References:
	>Question: What is the impact of changing .cpp AudioUnitEffect source to .mm (From: Motti Shneor <email@hidden>)
	>Re: Question: What is the impact of changing .cpp AudioUnitEffect source to .mm (From: Brian Willoughby <email@hidden>)
	>Re: Question: What is the impact of changing .cpp AudioUnitEffect source to .mm (From: Kyle Sluder <email@hidden>)
	>Re: Question: What is the impact of changing .cpp AudioUnitEffectsource to .mm (From: "Paul Sanders" <email@hidden>)
	>Re: Question: What is the impact of changing .cpp AudioUnitEffectsource to .mm (From: "Stephen F. Booth" <email@hidden>)

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