>
> Didn't quite get that statement.
> Well, this example wasn't very convincing...
Oh, I'm really getting tired of this!
Here's your final example:
A stack variable caching the flags check. REMEMBER, no write accesses to
it!:
<ide-cd.c>
int ide_cd_read_toc(ide_drive_t *drive, struct request_sense *sense)
{
int stat, ntracks, i;
struct cdrom_info *info = drive->driver_data;
struct cdrom_device_info *cdi = &info->devinfo;
struct atapi_toc *toc = info->toc;
struct {
struct atapi_toc_header hdr;
struct atapi_toc_entry ent;
} ms_tmp;
long last_written;
unsigned long sectors_per_frame = SECTORS_PER_FRAME;
u8 tmp = drive->atapi_flags & IDE_AFLAG_TOCTRACKS_AS_BCD;
</ide-cd.c>
later you access the _same_ variable in an if-check:
<ide-cd.c>
if (tmp) {
toc->hdr.first_track = bcd2bin(toc->hdr.first_track);
toc->hdr.last_track = bcd2bin(toc->hdr.last_track);
}
</ide-cd.c>
Resulting assembly snippets:
<ide-cd.s>
.LCFI166:
.loc 1 1202 0
movl %edx, -52(%ebp) # sense, sense
.loc 1 1213 0
movl 500(%edi), %ecx # <variable>.atapi_flags,
.loc 1 1204 0
movl 24(%eax), %eax # <variable>.driver_data,
.LVL399:
movl %eax, -40(%ebp) #, info
.LVL400:
.loc 1 1206 0
movl 16(%eax), %esi # <variable>.toc, toc
.LVL401:
.loc 1 1212 0
movl $4, -20(%ebp) #, sectors_per_frame
.LVL402:
.loc 1 1213 0
movl %ecx, -48(%ebp) #, D.31862
</ide-cd.s>
you see here the ->atapi_flags, dereferenced above, being saved on the
stack. Here's the asm for the check later:
<ide-cd.s>
.LVL415:
jne .L283 #,
.loc 1 1256 0
movb -48(%ebp), %al # D.31862,
.LVL416:
andb $16, %al #,
movb %al, -33(%ebp) #, tmp
.LVL417:
je .L287 #,
.loc 1 1257 0
</ide-cd.s>
So, the ->atapi_flags is moved into 8bit %al from the stack! Then the
$16 is the IDE_AFLAG_TOCTRACKS_AS_BCD, so the binary & against the
->atapi_flags is done actually here. Then the tmp variable is SAVED ON
THE STACK at -33(%ebp) and it is dereferenced AGAIN in the next check:
.LVL426:
jne .L283 #,
.loc 1 1297 0
cmpb $0, -33(%ebp) # tmp
je .L290 #
If you still don't see it, go play with your own assembler and do some
code staring on your own which doesn't involve me!
--
Regards/Gruss,
Boris.
--
To unsubscribe from this list: send the line "unsubscribe linux-ide" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
