lshift & rshift

**robertwessel2@yahoo.com** · Sep 27 '07, 10:45 AM

Re: lshift & rshift

On Sep 27, 5:23 am, "ie...@free .fr" <ie...@free.frw rote:

On Sep 27, 12:15 pm, "ie...@free .fr" <ie...@free.frw rote:
>

Can you post the snapshot of the internal of a*2 and a+a that you
got ?

>

I can't, it was in past. But I'm sure. I asked a friend, and he
explain me that on some architecture (like the HP adding machine)
there wasn't a multiplication in the CPU, so the multiplication was a
software and it was better to use + than *. And it was not a lshift, I
swear it.

>
Finnaly I can, here you are :
I made gcc -S mult.c to get the assembly code.
>
****mult.c****
**************
int mult_by_7 (int a)
{
return a * 7;
>
}
>
int mult_by_2 (int a)
{
return a * 2;
>
}
>
****mult.s****
**************
.file "mult.c"
.text
.globl mult_by_7
.type mult_by_7, @function
mult_by_7:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
movl %edx, %eax
sall $3, %eax
subl %edx, %eax
popl %ebp
ret
.size mult_by_7, .-mult_by_7
.globl mult_by_2
.type mult_by_2, @function
mult_by_2:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
addl %eax, %eax
popl %ebp
ret
.size mult_by_2, .-mult_by_2
.ident "GCC: (GNU) 4.1.2 (Ubuntu 4.1.2-0ubuntu4)"
.section .note.GNU-stack,"",@progb its
>
I don't understand the first (assembly language is a little bit new
for me).
Bu for mult_by_2 you can see the addl %eax,%eax
so it changed a * 2 by a + A.
>
Or perhaps I don't understand what does this assembly code.

The first routine compiles the (a*7) to ((a*8-a), with the
multiplication by eight done via a shift. In any event it looks like
you have not, as suggested, turned up the optimization level for GCC,
so he's generated fairly lame code. Try it with -O2 and see if that
doesn't reduce to a mov/lea/sub/ret sequence.

**Army1987** · Sep 27 '07, 10:55 AM

Re: lshift & rshift

On Thu, 27 Sep 2007 01:26:53 -0700, iesvs@free.fr wrote:

Hello guys,
every time a rode a doc or a book about the language C I saw that
operators << and >exist. But each time they said that << translate
the digit to the left (and >...) but no one said if a << 1 mean
every time a * 2, because there is the problem with the way the
integer are stored. So I never use thoses operators because I fear
strange behaviour on different architecture. Can I use them and be
sure that a << 1 means a * 2 on every computer ? Is is in the ansi
standard of language C ?

If the first operand is positive, it is.

--
Army1987 (Replace "NOSPAM" with "email")
A hamburger is better than nothing.
Nothing is better than eternal happiness.
Therefore, a hamburger is better than eternal happiness.

**iesvs@free.fr** · Sep 27 '07, 10:55 AM

Re: lshift & rshift

Looks like it. The other one looks likes a "a<<3 - a" to me, but that's only

a guess...

No you're right. It's just I'm not able to read assembly code.
I can't understand the result of my test. Maybe you're right.
I don't know.

**robertwessel2@yahoo.com** · Sep 27 '07, 10:55 AM

Re: lshift & rshift

On Sep 27, 5:16 am, "ie...@free .fr" <ie...@free.frw rote:

It's been demonstrated repeatedly over the last few decades that
programmers are absolutely *terrible* at identifying hotspots in their
code.

>
Like they're terrible to produce a correct code.

Indeed. And please be sure to remember to include yourself in that
category.

**Richard Heathfield** · Sep 27 '07, 11:35 AM

Re: lshift & rshift

pete said:

Richard Heathfield wrote:

>>

<snip>

>ch <<= 1; /* ch is now 128 */
>ch <<= 1; /* ch is now 0, even though 2 * 128 is 256. */

>
That example doesn't show a difference
between shifting and a doubling.

I agree. It does, however, show that shifting doesn't always double the
number. The fact that doubling doesn't always double the number is another
issue. :-)

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**Richard Bos** · Sep 27 '07, 12:45 PM

Re: lshift & rshift

"iesvs@free .fr" <iesvs@free.frw rote:

Which is where a profiler comes in...

>
No. I know when I programm if * 2 will be used 10 times or 10000....
I know from the beginning where are the important and no important
part. I don't need a software to say me these kind of things, moreover
there are more dumbs than me (a computer beat myself, ha ha ha).

You overestimate yourself. That's ok; nearly everybody does, until he
gets a taste of what a _real_ profiler and debugging suite can do. But
it isn't wise to refuse to learn better.

Richard

**karthikbalaguru** · Sep 27 '07, 12:45 PM

Re: lshift & rshift

On Sep 27, 3:30 pm, "ie...@free .fr" <ie...@free.frw rote:

I also try a << 1
and it converted it into a + a

What is the optimisation level you have ?
Does it treat a<<1 and a*2 as a+a for all the optimisation levels ?
What is the architecture you use ?

It is dependent on your architecture and compiler you use.

Karthik Balaguru

**Joachim Schmitz** · Sep 27 '07, 12:55 PM

Re: lshift & rshift

"karthikbalagur u" <karthikbalagur u79@gmail.comsc hrieb im Newsbeitrag
news:1190897100 .262252.150400@ 19g2000hsx.goog legroups.com...

On Sep 27, 3:30 pm, "ie...@free .fr" <ie...@free.frw rote:

>I also try a << 1
>and it converted it into a + a

>
What is the optimisation level you have ?
Does it treat a<<1 and a*2 as a+a for all the optimisation levels ?
What is the architecture you use ?

I've checked with gcc on a pentium running Linux. It generates identical
code for a*2 and a<<1 in all optimization levels and identical code for a+a
with -O1 and higher (and only slightly different code with -O0).

Bye, Jojo

**Mark Bluemel** · Sep 27 '07, 12:55 PM

Re: lshift & rshift

iesvs@free.fr wrote:

I also try a << 1
and it converted it into a + a
>

Which presumably means that the designer of the compiler knew that it
was cheaper (fewer cycles) on the specific target architecture to add
"a" to itself than to use some other mechanism to achieve a 1-place left
shift.

If you don't want to trust the compiler writers to do a decent job of
optimisation, I suggest you move to writing assembler...

**iesvs@free.fr** · Sep 27 '07, 01:15 PM

Re: lshift & rshift

If you don't want to trust the compiler writers to do a decent job of

optimisation, I suggest you move to writing assembler...

No, no, I trust them. It's their work, not mine.
In beginning my question was theoretical.
The real meaning was, is << a left shift or something who shift the
numbers from the less important bit to the most important.
And people said it's the second, not the first.

**karthikbalaguru** · Sep 27 '07, 05:35 PM

Re: lshift & rshift

On Sep 27, 6:07 pm, "ie...@free .fr" <ie...@free.frw rote:

If you don't want to trust the compiler writers to do a decent job of
optimisation, I suggest you move to writing assembler...

>
No, no, I trust them. It's their work, not mine.
In beginning my question was theoretical.
The real meaning was, is << a left shift or something who shift the
numbers from the less important bit to the most important.
And people said it's the second, not the first.

Onceagain,
You have not replied to the below questions -
1) What is the optimisation level you have ?
2) Does it treat a<<1 and a*2 as a+a for all the optimisation
levels ?
3) What is the architecture you use ?

Karthik Balaguru

**iesvs@free.fr** · Sep 27 '07, 05:45 PM

Re: lshift & rshift

I just need a precision about an operator. I saw many code with the
operator << and didn't know if it was portable.

Today I don't care if "<< 1" is faster than "* 2" or "+".

**karthikbalaguru** · Sep 27 '07, 05:45 PM

Re: lshift & rshift

On Sep 27, 10:36 pm, "ie...@free .fr" <ie...@free.frw rote:

I just need a precision about an operator. I saw many code with the
operator << and didn't know if it was portable.
>
Today I don't care if "<< 1" is faster than "* 2" or "+".

You have not yest answered these questions which will clarify your
doubts .

1) What is the optimisation level you have ?
2) Does it treat a<<1 and a*2 as a+a for all the optimisation
levels ?
3) What is the architecture you use ?
4) Also tell about the compiler and programming language you use ?

Karthik Balaguru

**Joachim Schmitz** · Sep 27 '07, 06:05 PM

Re: lshift & rshift

"karthikbalagur u" <karthikbalagur u79@gmail.comsc hrieb im Newsbeitrag
news:1190915081 .349773.130800@ g4g2000hsf.goog legroups.com...

On Sep 27, 10:36 pm, "ie...@free .fr" <ie...@free.frw rote:

>I just need a precision about an operator. I saw many code with the
>operator << and didn't know if it was portable.
>>
>Today I don't care if "<< 1" is faster than "* 2" or "+".

>
You have not yest answered these questions which will clarify your
doubts .
>
1) What is the optimisation level you have ?

As he said earlier (implicitly): no optimization

2) Does it treat a<<1 and a*2 as a+a for all the optimisation
levels ?

Yes (I've checked)

3) What is the architecture you use ?

From the assembly he posted: Intel ix86

4) Also tell about the compiler and programming language you use ?

gcc and C

Bye, Jojo

**iesvs@free.fr** · Sep 27 '07, 06:45 PM

Re: lshift & rshift

1) What is the optimisation level you have ?

>
As he said earlier (implicitly): no optimization

No, I used to use -O3, but when I post my example I used no
optimization.

2) Does it treat a<<1 and a*2 as a+a for all the optimisation
levels ?

>
Yes (I've checked)

Ha, interesting.

3) What is the architecture you use ?

>
From the assembly he posted: Intel ix86

686 to be exact (P4 I think).

>

4) Also tell about the compiler and programming language you use ?

>
gcc and C

We're in a C forum.

lshift & rshift

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment