linux-kernel - Re: [PATCH] x86: Create clflush() inline, remove hardcoded wbinvd

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Date:	Sat, 21 Jul 2007 15:24:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
CC:	Andi Kleen <andi@...stfloor.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Create clflush() inline, remove hardcoded wbinvd

H. Peter Anvin wrote:
> Muli Ben-Yehuda wrote:
>> Ok, let's try again:
>>
>> You're changing this (pageattr.c)
>>
>>                 asm volatile("clflush (%0)" :: "r" (adr + i));
>>
>> into this:
>>
>> 		asm volatile("clflush %0" : "+m" (*(char __force*)(adr + i)));
>>
>> The original one calls clflush with (adr + i), the new one with (*(adr
>> + i)). Are these calls equivalent?
> 
> Yes, they are.  The parentheses which are part of the old assembly
> string has the same effect as the asterisk operator in C.
> 
> The difference between the two is that the latter form allows the C
> compiler to select the addressing mode, which allows the full range of
> addressing modes, whereas the former forces it to use a single register
> indirect.
> 

Just to be absolutely obvious about it:

: tazenda 15 ; cat demo.c
#define __force

static inline void clflush1(volatile void *__p)
{
        asm volatile("clflush %0" : "+m" (*(char __force *)__p));
}

static inline void clflush2(volatile void *__p)
{
        asm volatile("clflush (%0)" :: "r" (__p));
}

void demo(void *q)
{
        clflush1(q);
        clflush2(q);
}
: tazenda 16 ; gcc -m32 -O3 -S demo.c
: tazenda 17 ; cat demo.s
        .file   "demo.c"
        .text
        .p2align 4,,15
.globl demo
        .type   demo, @function
demo:
        pushl   %ebp
        movl    %esp, %ebp
        movl    8(%ebp), %eax
#APP
        clflush (%eax)
        clflush (%eax)
#NO_APP
        popl    %ebp
        ret
        .size   demo, .-demo
        .ident  "GCC: (GNU) 4.1.2 20070626 (Red Hat 4.1.2-13)"
        .section        .note.GNU-stack,"",@progbits
-
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