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

From: "H. Peter Anvin" <hpa@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>, gcc <gcc@gcc.gnu.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	David Daney <ddaney.cavm@gmail.com>,
	Behan Webster <behanw@converseincode.com>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections
Date: Mon, 12 Aug 2013 10:47:37 -0700	[thread overview]
Message-ID: <52091FB9.4080805@linux.intel.com> (raw)
In-Reply-To: <20130812160950.GT3008@twins.programming.kicks-ass.net>

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