making a large lookup table

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tthsqe

Joined: 20 May 2009
Posts: 730

tthsqe

I would like to make a lookup table for around 1000-2000 quad words.
I of course would not fill in this table by hand, but have the program fill it in first. What is the best way to do this?

28 Jun 2009, 22:35

Borsuc

Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania

Borsuc

Fill it with what?

EDIT: by "quad words" you mean "english words with 4 characters" or "quadwords" ("word" is 16-bits number)?

29 Jun 2009, 00:19

tthsqe

Joined: 20 May 2009
Posts: 730

tthsqe

quad word = 64 bit integer
The look up table should look like

[starting address ] = 0xFFFFFFFFFFFFFFFF
[starting address + 8] = 0xFFFFFFFFFFFFFFFF
[starting address +16] = 0xFFFFFFFFFFFFFFFF
...
[starting address + bignum*8]

I don't want the program to have a chance of overwriting anything that happens to be at these latter addresses.

29 Jun 2009, 02:38

tthsqe

Joined: 20 May 2009
Posts: 730

tthsqe

I think I got it:

table db 1000 dup (0,0,0,0,0,0,0,0)

Will this set aside 1000 8-byte memory locations for table?
Is there a better way to do this to allow for faster lookup times?

29 Jun 2009, 03:36

sinsi

Joined: 10 Aug 2007
Posts: 707
Location: Adelaide

sinsi

Code:

table rq 1000

29 Jun 2009, 04:00

tthsqe

Joined: 20 May 2009
Posts: 730

tthsqe

hm...

Code:

table dq 1000

is not working but,

Code:

table  dq 1000 dup 0

is.

29 Jun 2009, 06:40

sinsi

Joined: 10 Aug 2007
Posts: 707
Location: Adelaide

sinsi

"rq", not "dq" - reserve quadword(s)

29 Jun 2009, 06:53

Borsuc

Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania

Borsuc

I'm confused. You said you wanted 0xFFFFFFFFFFFFFFFF, not 0, so why use dup(0) in your examples? Use dup(-1) instead.

30 Jun 2009, 02:29

bitRAKE

Joined: 21 Jul 2003
Posts: 3024
Location: vpcmipstrm

bitRAKE

Code:

table rq .size
  .size = 1000
.
.
macro table.init {
  lea rdi,[table]
  mov ecx,table.size
  or rax,-1
  rep stosq
}

...uses less space in the executable if table is at the end of a section or in an uninitialized section. Of course, a couple more bytes can be saved in the code with:

Code:

lea rdi,[table]
mov ecx,table.size * 8
mov al,-1
rep stosb

30 Jun 2009, 09:05

tthsqe

Joined: 20 May 2009
Posts: 730

tthsqe

Quote:

I'm confused. You said you wanted 0xFFFFFFFFFFFFFFFF, not 0, so why use dup(0) in your examples? Use dup(-1) instead.

don't be confused. The intial contents of the table is irrelevent - it is initialized by the program and then used as a lookup table.

02 Jul 2009, 02:31

tthsqe

Joined: 20 May 2009
Posts: 730

tthsqe

If the start of the table is not a address that is a multiple of 8 (or 4?), will there be a penalty for misaligned memory accesses? Is there a way to fix this in fasm? The values are looked up in a performance sensitive loop.

02 Jul 2009, 02:38

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4633
Location: Argentina

LocoDelAssembly

If the memory access crosses a cache line boundary you'll get a penalty (not sure what about a misaligned access that doesn't crosses a line on newer processors). Since some elements in the table will obviously cross a cache line (and probably a page boundary too), it is better to align always.

Well, if what you want is a LUT of quad words then this is the way

Code:

align 8 ; Perhaps could be better to use 16, 32, or 64 (the latter is the cache line size of an Athlon64)
LUT rq LUT_SIZE_IN_QWORDS

02 Jul 2009, 02:56

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