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

From: "H. Peter Anvin" <hpa@linux.intel.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>, gcc <gcc@gcc.gnu.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	David Daney <ddaney.cavm@gmail.com>,
	Behan Webster <behanw@converseincode.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Herbert Xu <herbert@gondor.hengli.com.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections
Date: Mon, 05 Aug 2013 21:28:32 -0700	[thread overview]
Message-ID: <52007B70.9070504@linux.intel.com> (raw)
In-Reply-To: <20130806041437.GA30449@Krystal>

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