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Date:	Sun, 14 Jul 2013 12:41:06 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Tim Northover <t.p.northover@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ramkumar Ramachandra <artagnon@...il.com>,
	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 12:30 PM, Tim Northover wrote:
>> And that is why I think you should just consider "bt $x,y" to be
>> trivially the same thing and not at all ambiguous. Because there is
>> ABSOLUTELY ZERO ambiguity when people write
>>
>>    bt $63, mem
>>
>> Zero. Nada. None. The semantics are *exactly* the same for btl and btq
>> in this case, so why would you want the user to specify one or the
>> other?
> I don't think you've actually tested that, have you? (x86-64)
>
> int main() {
>   long val = 0xffffffff;
>   char res;
>
>   asm("btl $63, %1\n\tsetc %0" : "=r"(res) : "m"(val));
>   printf("%d\n", res);
>
>   asm("btq $63, %1\n\tsetc %0" : "=r"(res) : "m"(val));
>   printf("%d\n", res);
> }

Blerk.  It doesn't undermine the original point - that gas can
unambiguously choose the right operation size for a constant bit offset
- but yes, the operation size is meaningful in the case of a immediate
bit offset. Its pretty nasty of Intel to hide that detail in Table 3-2,
far from the instructions which use it...

    J

>
> Tim.
>

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