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

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Message-ID: <52007B70.9070504@linux.intel.com>
Date:	Mon, 05 Aug 2013 21:28:32 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On 08/05/2013 09:14 PM, Mathieu Desnoyers wrote:
>>
>> For unconditional jmp that should be pretty safe barring any fundamental
>> changes to the instruction set, in which case we can enable it as
>> needed, but for extra robustness it probably should skip prefix bytes.
> 
> On x86-32, some prefixes are actually meaningful. AFAIK, the 0x66 prefix
> is used for:
> 
> E9 cw   jmp rel16   relative jump, only in 32-bit
> 
> Other prefixes can probably be safely skipped.
> 

Yes.  Some of them are used as hints or for MPX.

> Another question is whether anything prevents the assembler from
> generating a jump near (absolute indirect), or far jump. The code above
> seems to assume that we have either a short or near relative jump.

Absolutely something prevents!  It would be a very serious error for the
assembler to generate such instructions.

	-hpa




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