This is the mail archive of the
mailing list for the GCC project.
Calculating elements addresses from arrays once
- From: Gruik <gruikmusic at ifrance dot com>
- To: gcc at gcc dot gnu dot org
- Date: Thu, 20 May 2004 11:45:09 +0200
- Subject: Calculating elements addresses from arrays once
- If-filter0: N
I was just wondering something.
Let's see my simplified code :
typedef struct _abc
void my_func ()
abc my_array ;
for (i = 0; i < n; i++)
if (my_array[i].a == value)
my_array[i].c = my_array[i].b + 15;
//other operations accessing fields of my_array[i]
You can see my_array[i] is often accessed.
I know that in assembly (and so in machine code), to write in
my_array[i], the computer has to :
1) Retrieve my_array address
2) Retrieve i value
3) Adding i to my_array address
4) Writing to the obtained address
My question is : do you think GCC, in this kind of loops, performs a
little optimisation by saving somewhere the address of my_array[i] ?
If not, do you think the following code does a significant optimisation ?
void my_func ()
abc * current_abc;
for (current_abc = my_array, i = 0; i < n; current_abc++, i++)
if (current_abc->a == value)
current_abc->c = current_abc->b + 15;
//other operations accessing fields of *current_abc
Note : in my real case, i have much more fields in my struct, and there
are much more accesses to the current element. Using temporary variables
should not be very efficient in my case.
Note2 : it's for a PHP module and i want it to become the fastest possible
Envie de discuter en "live" avec vos amis ? Télécharger MSN Messenger
http://www.ifrance.com/_reloc/m la 1ère messagerie instantanée de France