linux-kernel - Re: [PATCH] x86: collapse the various size-dependent percpu accessors together

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Date:	Wed, 04 Jun 2008 13:09:18 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mike Travis <travis@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: collapse the various size-dependent percpu	accessors
 together

Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> Jeremy Fitzhardinge wrote:
>>     
>>> We can use gcc's %z modifier to emit the appropriate size suffix for
>>> an instruction, so we don't need to duplicate the asm statement for
>>> each size.
>>>       
>> Nah, it's a disaster.  Drop this one.
>>     
>
> hm, what's the problem with it? What you are trying to do here looks 
> like a nice cleanup - assuming it results in the same instructions 
> emitted ;-)

Yes, would have been lovely.  But gcc emits junk:

  CC      arch/x86/xen/enlighten.o
{standard input}: Assembler messages:
{standard input}:637: Error: no such instruction: `movll %gs:per_cpu__xen_vcpu(%rip),%rax'
{standard input}:655: Error: no such instruction: `movll %gs:per_cpu__xen_vcpu(%rip),%rax'
{standard input}:671: Error: no such instruction: `movll %gs:per_cpu__xen_vcpu(%rip),%rax'
{standard input}:682: Error: no such instruction: `movll %gs:per_cpu__xen_vcpu(%rip),%rax'
{standard input}:783: Error: no such instruction: `movll %gs:per_cpu__pda+8(%rip),%rbx'
{standard input}:834: Error: no such instruction: `movll %gs:per_cpu__xen_mc_irq_flags(%rip),%rdi'
{standard input}:901: Error: no such instruction: `movll %gs:per_cpu__pda+8(%rip),%rbx'
{standard input}:978: Error: no such instruction: `movll %gs:per_cpu__xen_mc_irq_flags(%rip),%rdi'
{standard input}:1064: Error: no such instruction: `movll %gs:per_cpu__pda+8(%rip),%rbx'
{standard input}:1110: Error: no such instruction: `movll %gs:per_cpu__xen_mc_irq_flags(%rip),%rdi'
...
  CC      arch/x86/vdso/vclock_gettime.o
{standard input}: Assembler messages:
{standard input}:75: Error: suffix or operands invalid for `movs'
(all over the place)

I tried a version to do 64-bit accesses with an explicit "movq" to solve 
the "movll" problem, but it generates "movs" on occasion and that was 
the point I gave up.

    J
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