Hello.
Borislav Petkov wrote:
or similar instead of wasting stack space?
It doesn't necessarily waste stack space. Haven't you heard about
compiler
putting local vairables into registers?
Yes, have you heard of unnecessary register spilling?
No -- only about stack spilling on CPUs "caching" the top of stack in their
register file (like SPARC).
Linux runs not only on x86 and many RISCs can store several local variables
in the dedicated registers -- it's the part of say MIPS ABIs...
Oh, it's really the same on x86, only there are only 3 registers
dedicated to that. RISCs typically have more, however x86_64 wiuth 16
its registers is close to that already. However, you're right -- these
registers need saving at the function entry, so they effectively take
the stack space.
It'll also read better in the if() check:
if (drv_can_irq_interrupt(drive)) { ...
It's faster to checj a local variable than to dereference drv several
times
-- unless gcc optimizes that away (by creating an implicit local variable
:-).
Let's look at an example:
In ide-cd.c:cdrom_newpc_intr() you have the following code snippet:
<ide-cd.c>
799 thislen = blk_fs_request(rq) ? len : rq->data_len;
800 if (thislen > len)
801 thislen = len;
802
803 ide_debug_log(IDE_DBG_PC, "%s: DRQ: stat: 0x%x, thislen: %d\n",
804 __func__, stat, thislen);
805
806 /* If DRQ is clear, the command has completed. */
807 if ((stat & ATA_DRQ) == 0) {
808 if (blk_fs_request(rq)) {
</ide-cd.c>
Now watch the blk_fs_request() thing.
Here's what my gcc¹ spits out:
Thsi code is somewhat confused. Also, I was of a better opinion of gcc...
<ide-cd.s>
.LVL174:
.loc 1 799 0
movl 76(%r12), %ecx # <variable>.cmd_type, prephitmp.1128
cmpl $1, %ecx #, prephitmp.1128
movl %ecx, %r8d # prephitmp.1128, prephitmp.1047
je .L225 #,
Now where is that label?
.LVL175:
.loc 1 800 0
movzwl -44(%rbp), %r15d # len, thislen
Oh, that AT&T syntax... it took me a while to realize that it's a
movzx insn. :-)
.LVL176:
.loc 1 799 0
movl 280(%r12), %edx # <variable>.data_len, thislen.1129
.LVL177:
.loc 1 800 0
cmpl %r15d, %edx # thislen, thislen.1129
movl %r15d, %ebx # thislen, thislen.1163
jg .L145 #,
.LVL178:
movl %edx, %r15d # thislen.1129, thislen
I wonder why it doesn't generate cmovng ISO jg and mov...
.LVL179:
.L145:
.loc 1 807 0
testb $8, -64(%rbp) #, stat
jne .L147 #,
.LVL180:
.loc 1 808 0
cmpl $1, %ecx #, prephitmp.1128
je .L226 #,
.loc 1 825 0
cmpl $2, %ecx #, prephitmp.1128
.p2align 4,,3
.p2align 3
je .L152 #,
.LBB408:
</ide-cd.s>
and at label .LVL174 you see the blk_fs_request() check from line
799 above. Later, at label .LVL180 you see the next blk_fs_request() check from
line 808 and this is cached in %ecx so gcc is smart enough to do that. So,
Yes, CSE optimization does work...
actually you get the same thing/or even better with variables in registers
instead of on stack
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.
MBR, Sergei
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