linux-ext4 - Re: [PATCH 1/1] x86: fix text

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Message-ID: <20080425203717.GB25950@Krystal>
Date:	Fri, 25 Apr 2008 16:37:17 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Jiri Slaby <jirislaby@...il.com>,
	David Miller <davem@...emloft.net>, zdenek.kabelac@...il.com,
	rjw@...k.pl, paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org, herbert@...dor.apana.org.au,
	penberg@...helsinki.fi, clameter@....com,
	linux-kernel@...r.kernel.org, pageexec@...email.hu,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 1/1] x86: fix text_poke

* H. Peter Anvin (hpa@...or.com) wrote:
> Mathieu Desnoyers wrote:
>> This epilogue should then be used on both branches of the condition,
>> like this :
>> if (unlikely(imv_cond(var))) {
>>   imv_cond_end();
>>   ...
>> } else {
>>   imv_cond_end();
>>    ...
>> }
>> Where imv_cond_end() would look like this :
>> +/*
>> + * Puts a test and branch make sure the %al register and ZF are not live
>> + * anymore.
>> + * All asm statements clobbers the flags, but add "cc" clobber just to be 
>> sure.
>> + * Clobbers %al.
>> + */
>> +#define imv_cond_end()                                                 \
>> +       do {                                                            \
>> +               asm (".section __imv_cond_end,\"a\",@progbits\n\t"      \
>> +                               _ASM_PTR "1f\n\t"                       \
>> +                               ".previous\n\t"                         \
>> +                               "1:\n\t"                                \
>> +                               : : : "a", "cc");                       \
>> +       } while (0)
>> +
>
> As far as this is concerned, all you accomplish here is that gcc, if it 
> wants to re-use the %al value, will copy it into another register before 
> doing your imv_conv_end().
>

Exactly, and by doing so, it will have to add instructions (mov, push..)
in the instruction pattern I am looking for and therefore I will detect
this and fall back on standard immediate values.

Mathieu

> 	-hpa

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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