linux-kernel - Re: [RFC] gcc feature request: Moving blocks into sections

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Message-ID: <52091FB9.4080805@linux.intel.com>
Date:	Mon, 12 Aug 2013 10:47:37 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On 08/12/2013 09:09 AM, Peter Zijlstra wrote:
>>
>> On the majority of architectures, including x86, you cannot simply copy
>> a piece of code elsewhere and have it still work.
> 
> I thought we used -fPIC which would allow just that.
> 

Doubly wrong.  The kernel is not compiled with -fPIC, nor does -fPIC
allow this kind of movement for code that contains intramodule
references (that is *all* references in the kernel).  Since we really
doesn't want to burden the kernel with a GOT and a PLT, that is life.

>> You end up doing a
>> bunch of the work that a JIT would do anyway, and would end up with
>> considerably higher complexity and worse results than a true JIT.  
> 
> Well, less complexity but worse result, yes. We'd only poke the specific
> static_branch sites with either NOPs or the (relative) jump target for
> each of these branches. Then copy the result.

Once again, you can't "copy the result".  You end up with a full
disassembler.

	-hpa

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