2011/7/14 Ian Lance Taylor <iant@xxxxxxxxxx>:
> Parmenides <mobile.parmenides@xxxxxxxxx> writes:
>
>>>> 2. "and not optimize stores or loads to that memory"
>>>> Except caching memory values in registers, is there any other
>>>> optimizaiton for stores or loads to memory?
>>>
>>> Not in this case, I think it's just another way of saying the same
>>> thing.
>>>
>>
>> I think the reordering instructions involving memory operations but
>> not stores or loads might count as some optimization. It seems that a
>> "memory" will prevent gcc from this kind of optimization. If so, would
>> the manual give some statements about it.
>
> I'm sorry, I don't understand what you mean.
For the purpose of understanding some gcc's features, without ideas of
details underlying gcc, I have to code some examples in C and compile
them into assembly code, then observe them to get some ideas. Memory
values caching in registers is one optimization taken by gcc,
reordering instructions is another. A "memory" clobber in an inline
assembly may have influence on the both. I have coded an example in C
to try to understand the former.
int s = 0;
int tst(int lim)
{
int i;
for (i = 1; i < lim; i++)
s = s + i;
asm volatile(
"nop"
);
s = s * 10;
return s;
}
To compile the C souce, the following command is excuted.
gcc -S -O tst.c
The corresponding assembly code is as follows:
tst:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %ecx
cmpl $1, %ecx
jle .L2
movl s, %edx
movl $1, %eax
.L4:
addl %eax, %edx
incl %eax
cmpl %eax, %ecx
jne .L4
movl %edx, s <--- After the loop, s is write back into memory.
.L2:
movl s, %eax <--- Before the evaluating 's = s * 10', s
is reload into register.
leal (%eax,%eax,4), %eax
addl %eax, %eax
movl %eax, s
popl %ebp
ret
So, the "memory" clobber have prevented the optimization. But for the
latter case, namely reordering instructions, I can not obtain an
example like the above to illustrate how "memory" clobber prevent
reordering instructions. I don't know some circumstances under which
gcc will do reodering. Without them, I can not observe the effect of
the "memory" clobber.
