Hello.
Borislav Petkov wrote:
I still don't undestand why you assume that such variable will be
alloceted on stack -- gcc has 3 registers available for local variables
(which doesn't have to save across function calls). However, the
register variables have to take stack space indeed as they need to be
saved on funciton entry... though I'm not sure that gcc will necessary
put such variable in one of those 3 registers if it figures out that
there are no function calls going to happen during its life time.
and the code is more readable. A win-win situation, I'd say :).
You haven't presented the code which gets generated when the local
variable is used, so it's impossible to compare.
Here's another example from ide-disk.c where you have stack variables cashing
those flags checks:
They are caching the result of !! in 'u8' variables -- which is not
the same as cahing the flags. I suspect gcc avoid putting byte-sized
variables into registers...
<ide-disk.c>
static ide_startstop_t __ide_do_rw_disk(ide_drive_t *drive, struct request *rq,
sector_t block)
{
ide_hwif_t *hwif = drive->hwif;
u16 nsectors = (u16)rq->nr_sectors;
u8 lba48 = !!(drive->dev_flags & IDE_DFLAG_LBA48);
u8 dma = !!(drive->dev_flags & IDE_DFLAG_USING_DMA);
ide_task_t task;
struct ide_taskfile *tf = &task.tf;
ide_startstop_t rc;
if ((hwif->host_flags & IDE_HFLAG_NO_LBA48_DMA) && lba48 && dma) {
if (block + rq->nr_sectors > 1ULL << 28)
dma = 0;
else
lba48 = 0;
}
</ide-disk.c>
Corresponding asm (this time i386 but I don't think it matters since we
need at least one arch to prove my point).
<ide-disk.s>
.LVL48:
.loc 1 94 0
movl %eax, %edx # D.32119, tmp93
.loc 1 95 0
shrl %eax # D.32119
Where's the shift count I wonder?
andb $1, %al #,
movb %al, -58(%ebp) #, dma
.LVL49:
.loc 1 100 0
movl -52(%ebp), %eax # hwif,
.loc 1 94 0
shrl $21, %edx #, tmp93
andb $1, %dl #,
movb %dl, -57(%ebp) #, lba48
.LVL50:
.loc 1 100 0
testb $4, 90(%eax) #, <variable>.host_flags
je .L37 #,
testb %dl, %dl #
je .L37 #,
cmpb $0, -58(%ebp) # dma
movb $1, -57(%ebp) #, lba48
.LVL51:
</ide-disk.s>
Now look at the last lines at labels .LVL48 and .LVL49 - they both save
those 1-byte u8's called dma and lba48 on the stack at -57(%ebp) and
-58(%ebp), respectively. And guess what, later on label LVL50 they get
accessed in the check.
If you look better, you'll see that the copy of 'lba48' in the %dl
register gets used.
And several times more later, which in most sane
architectures still means cache accesses but when you have registers its
even faster :).
Didn't quite get that statement.
Well, this example wasn't very convincing...
MBR. Sergei
--
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
