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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