Re: Efficiency
- From: "almas" <almas@xxxxxxxxxxxx>
- Date: Sun, 28 Oct 2007 01:30:15 +0200
Lods ans stosb use many clock of computer
a mov ax,si use less time
So, it use more bytes, but it is more speed.
"Dirk Wolfgang Glomp" <dirk@xxxxxxxxxxxxxxxxxxx> a écrit dans le message de
news: 1kjl6gfgwyb8c$.16mxth2njz8mc.dlg@xxxxxxxxxxxxx
Am Fri, 26 Oct 2007 09:49:20 -0700 schrieb KJH:
Let's say I want to write compact and fast code.
I'm thinking about something like this:
; proc returns ZF set if char is 'a' or 'z'
myproc proc
cmp al,'a'
je @@1
cmp al,'z'
@@1: ret
myproc endp
compared to C version:
int myfunc(int c)
{
if (c == 'a' || c == 'z')
return 1;
return 0;
}
What is the penalty? Okay, these are very basic and small routines,
Yes, i never use subroutines only for a few bytes of code.
but I guess that modern C compilers are more aware about optimal
alignment
Shure, more than older C compilers.
and also it seems that modern Pentiums and above really like
their instructions more RISC-like (aka boring instr set).
Nice to generate a blended code to speedup many different CPUs.
I am used to instructions like lodsb, stosb, inc, dec etc... I'm
adapted to that kind of mental image, lodsb for example in my mind is
a very convenient instruction to load a byte from memory pointed by
DS:ESI. I can visualize it. Also loop instruction is nice, it's
compact (in a conceptual way), but I don't know...
Why is it so that nowadays:
mov eax,[esi]
mov [edi],eax
is more efficient than
lodsd
stosd
?
If these special registers are not free to use it for lods/stos/movs or
loop -instructions, they must be store and reload. Else these values
will be loose. So it is sometimes easier to use other register to do
this job without those depencys.
ABOVE:
mov eax,[ecx]
mov [edx],eax
dec esi
jnz ABOVE
I just don't get it.
But to my actual question, is it likely that I'm going to take a speed
hit or cache misses by using these old asm constructs vs. more modern
C-like constructs? Code compactness doesn't necessarily translate to
speed, no?
Hopefully somebody can make some sense what I'm talking about, I'm not
sure if I express myself clearly :)
You can recieve all these stuff to optimize a code from intel/amd.
Like "248966.pdf"
Intel® 64 and IA-32 Architectures
Optimization Reference Manual
Dirk
.
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- From: Dirk Wolfgang Glomp
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