linux-kernel - Re: [LLVMdev] [PATCH] x86/asm: avoid mnemonics without type suffix

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Message-ID: <51E44204.4000802@zytor.com>
Date:	Mon, 15 Jul 2013 11:40:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Tim Northover <t.p.northover@...il.com>,
	Ramkumar Ramachandra <artagnon@...il.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andi Kleen <ak@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	LLVMdev <llvmdev@...uiuc.edu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [LLVMdev] [PATCH] x86/asm: avoid mnemonics without type suffix

On 07/14/2013 12:49 PM, Linus Torvalds wrote:
> On Sun, Jul 14, 2013 at 12:30 PM, Tim Northover <t.p.northover@...il.com> wrote:
>>
>> I don't think you've actually tested that, have you? (x86-64)
> 
> Oh, you're right, for constants > 5 bits you have that other thing
> going on. I didn't think about the fact that the constant changed in
> the middle of the thread (it started out as 1).
> 
> We use the gcc constraint "I" (0-31) in the kernel for this reason.
> 
>              Linus

This is also why the Intel manuals point out that "some assemblers" can
take things like:

	bt[l] $63,(%rsi)

... and turn it into:

	btl $31,4(%rsi)

This is definitely the friendly thing to do toward the human programmer.
 Unfortunately gas doesn't, nor does e.g. NASM.

	-hpa


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