Array question

[sanjay]

Hello i have this code,

void f () {
static const int array[2][4] = {
{0,1,2,3},
{4,5,6,7}};
int a_bit;

asm(
"movl [array_ptr + 2*column + row], %%eax \n"
"movl %%eax, %0 \n"
: "=g" (a_bit)
: "g" (array[0][0])
: "ax", "memory"
)

}

but this wont work. What I want that is fast f(), thats why `static' is
(is it ok?), and the f() shoud do:
`a_bit = array[column][row];'
but I don't know what should I write in place of:
`[array_ptr + 2*column + row]'.

Can u advise thanx.

[Keith Thompson]

Re: Array question

sanjay <nospam@nospam. comwrites:

Hello i have this code,
>
void f () {
static const int array[2][4] = {
{0,1,2,3},
{4,5,6,7}};
int a_bit;
>
asm(
"movl [array_ptr + 2*column + row], %%eax \n"
"movl %%eax, %0 \n"
: "=g" (a_bit)
: "g" (array[0][0])
: "ax", "memory"
)
>
}
>
but this wont work. What I want that is fast f(), thats why `static'
is (is it ok?), and the f() shoud do:
`a_bit = array[column][row];'
but I don't know what should I write in place of:
`[array_ptr + 2*column + row]'.

If you want f() to do
a_bit = array[column][row];
why not just write
a_bit = array[column][row];
? For such a simple operation, the code generated by the compiler is
likely to be as fast as anything you can write manually in assembly
language (and more likely to be correct).

If you want to know what the assembly language for the assignment
should look like, you can always write it in straight C and ask the
compiler to generate an assembly listing. ("-S" is a common option
for this.)

But in most cases, there's no good reason you should care. If the
code works and is not *measurably* too slow, just don't worry about
it.

Having said all that, if you still insist on using inline assembly,
you'll need to ask elsewhere. Standard C doesn't provide this
feature. If you're using gcc, try gnu.gcc.help; if you're using
something else, you'll need to find a newsgroup that covers your
compiler or system.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Eric Sosman]

Re: Array question

sanjay wrote:

Hello i have this code,
>
void f () {
static const int array[2][4] = {
{0,1,2,3},
{4,5,6,7}};
int a_bit;
>
asm(
"movl [array_ptr + 2*column + row], %%eax \n"
"movl %%eax, %0 \n"
: "=g" (a_bit)
: "g" (array[0][0])
: "ax", "memory"
)
>
}
>
but this wont work. What I want that is fast f(), thats why `static' is
(is it ok?), and the f() shoud do:
`a_bit = array[column][row];'
but I don't know what should I write in place of:
`[array_ptr + 2*column + row]'.
>
Can u advise thanx.

Write

a_bit = array[column][row];

.... and remember to define `column' and `row' somewhere.

--
Eric.Sosman@sun .com

[Antoninus Twink]

Re: Array question

On 21 May 2008 at 21:43, sanjay wrote:

What I want that is fast f(), thats why `static' is (is it ok?), and
the f() shoud do:
`a_bit = array[column][row];'
but I don't know what should I write in place of:
`[array_ptr + 2*column + row]'.

Why don't you just do
asm ("movl %1,%0" : "=r" (a_bit) : "m" (array[column][row]));

But really, do you honestly think your compiler will do a bad job of
translating this assignment statement??

You also seem to have the row and column indexes backwards compared to
most other people in the world...