mixing C and assembly

In article <seCdnTwTA7tRdZLVnZ2dnUVZ_gmdnZ2d@visi>,
Grant Edwards  <grante@visi.com> wrote:
<SNIP>
>
>For example, two ways to sum the bytes in a buffer:
>
>  unsigned char buf[128];
>  unsigned sum = 0;
>
>  unsigned i;
>  for (i=0; i<sizeof buf; ++i)
>    sum += buf[i];
>
>  unsigned char *p;
>  p = buf;
>  while (p < buf + (sizeof buf))
>    sum += *p++;
>
>Those two blocks of code do the same thing.  On some
>target/toolchain combinations, they're both about the same
>number of bytes/clocks.  On some platforms I've used the first
>was significantly smaller/faster.  On others, the second was
>significantly smaller/faster.

Going from the first to the second is a standard optimisation
technique called strength reduction (with regard to an address
calculation).

>
>In the fist case, are you better off with an index that's an
>"unsigned" or an "unsigned char" or an "int"?  They can be
>significatly different.  Always pick something that will work
>on as many platforms as possible, but if there are multiple
>"correct" ways to do something, you might as well pick the one
>that's going to generate the smallest/fastest code for the
>target.

A good compiler knows that those are equivalent and do the
picking for you.

>
>If you want to be more than an amateur doing trivial projects,
>you've got to know your target's instruction set and know what
>code the compiler is going to generate each time you write a
>line of C.
>
>I always set up my makefile so that the toolchain produces a
>mixed C/assembly listing for each module. Anytime I'm not sure
>what the compiler is going to do with a particular construct, I
>look at the listing and see.

That is a good advice. You can learn something. However ...
Did you look at some Itanium code or SEAforth code lately?
Did you learn something or did you awake with a headache?

>--
>Grant Edwards                   grante             Yow! I invented skydiving

Groetjes Albert

--
-- 
Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- like all pyramid schemes -- ultimately falters.
albert@spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst

On 2008-05-09, Albert van der Horst <albert@spenarnc.xs4all.nl> wrote:

> A good compiler knows that those are equivalent and do the
> picking for you.

True.  One doesn't always have the pleasure of using a good
compiler. :)

> That is a good advice. You can learn something. However ...
> Did you look at some Itanium code or SEAforth code lately?

Can't say that I have.

> Did you learn something or did you awake with a headache?

What I'm usually looking at are pretty standard CISC micros
like the H8 or MSP430, or vanilla RISC CPUs like the ARM or
NIOS2.

Never had a chance to embed an Itanium in anything.

-- 
Grant Edwards                   grante             Yow! Here I am in the
                                  at               POSTERIOR OLFACTORY LOBULE
                               visi.com            but I don't see CARL SAGAN
                                                   anywhere!!

Grant Edwards wrote:
> On 2008-05-09, Albert van der Horst <albert@spenarnc.xs4all.nl> wrote:
> 
>> A good compiler knows that those are equivalent and do the
>> picking for you.
> 
> True.  One doesn't always have the pleasure of using a good
> compiler. :)
> 
>> That is a good advice. You can learn something. However ...
>> Did you look at some Itanium code or SEAforth code lately?
> 
> Can't say that I have.
> 
>> Did you learn something or did you awake with a headache?
> 
> What I'm usually looking at are pretty standard CISC micros
> like the H8 or MSP430, or vanilla RISC CPUs like the ARM or
> NIOS2.
> 
> Never had a chance to embed an Itanium in anything.
> 

I was once involved in a prototype for a very large power supply. 
Because of its odd design, it had a high minimum output current, so in 
idle we had to burn off about 800W.  We used a panel oven, but an 
embedded Itanium would have given a more compact solution.

On 2008-05-09, David Brown <david@westcontrol.removethisbit.com> wrote:

>> Never had a chance to embed an Itanium in anything.
>
> I was once involved in a prototype for a very large power
> supply.  Because of its odd design, it had a high minimum
> output current, so in idle we had to burn off about 800W.  We
> used a panel oven, but an embedded Itanium would have given a
> more compact solution.

The worlds first power supply that runs protien folding
simulations while idle...

-- 
Grant Edwards                   grante             Yow! Now, let's SEND OUT
                                  at               for QUICHE!!
                               visi.com            

Grant Edwards wrote:
> On 2008-05-09, David Brown <david@westcontrol.removethisbit.com> wrote:
> 
>>> Never had a chance to embed an Itanium in anything.
>> I was once involved in a prototype for a very large power
>> supply.  Because of its odd design, it had a high minimum
>> output current, so in idle we had to burn off about 800W.  We
>> used a panel oven, but an embedded Itanium would have given a
>> more compact solution.
> 
> The worlds first power supply that runs protien folding
> simulations while idle...
> 

"Protein folding" - that's like cooking a steak on top of the box?