Arjan,
I can experiment with any constraints if you suggest which one.
>From our experiments with gcc, it compares asm strings (sic!!!) to find matches
to be merged! Sigh...
Below are 2 programs which differ in one space in read_cr3_b() asm statement.
The first one compiles incorrectly, while 2nd one - correctly.
My personal feeling is that comparing asm strings is simply a "misfeature".
-------------------------- cut ----------------------------
static inline unsigned long read_cr3_a(void)
{
unsigned long cr3;
asm("movq %%cr3,%0" : "=r" (cr3));
return cr3;
}
static inline unsigned long read_cr3_b(void)
{
unsigned long cr3;
asm("movq %%cr3,%0" : "=r" (cr3));
return cr3;
}
static inline void write_cr3(unsigned long val)
{
asm volatile("movq %0,%%cr3" :: "r" (val) : "memory");
}
void main()
{
unsigned long c;
c = read_cr3_a();
write_cr3(c | 0x80);
c = read_cr3_b();
write_cr3(c | 0x100);
}
-------------------------- cut ----------------------------
-------------------------- cut ----------------------------
static inline unsigned long read_cr3_a(void)
{
unsigned long cr3;
asm("movq %%cr3,%0" : "=r" (cr3));
return cr3;
}
static inline unsigned long read_cr3_b(void)
{
unsigned long cr3;
asm("movq %%cr3,%0" : "=r" (cr3));
return cr3;
}
static inline void write_cr3(unsigned long val)
{
asm volatile("movq %0,%%cr3" :: "r" (val) : "memory");
}
void main()
{
unsigned long c;
c = read_cr3_a();
write_cr3(c | 0x80);
c = read_cr3_b();
write_cr3(c | 0x100);
}
-------------------------- cut ----------------------------
Kirill
Arjan van de Ven wrote:
> On Tue, 02 Oct 2007 18:08:32 +0400
> Kirill Korotaev <[email protected]> wrote:
>
>
>>Some gcc versions (I checked at least 4.1.1 from RHEL5 & 4.1.2 from
>>gentoo) can generate incorrect code with read_crX()/write_crX()
>>functions mix up, due to cached results of read_crX().
>>
>
>
> I'm not so sure volatile is the right answer, as compared to giving the
> asm more strict contraints....
>
> asm volatile tends to mean something else than "the result has
> changed"....
>
>
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