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On Feb 21, 8:45*pm, Sherm Pendley <sherm.pend...@gmail.com> wrote:
> aleksa <aleks...@gmail.com> writes:
> > On Feb 21, 8:05*pm, Keith Thompson <ks...@mib.org> wrote:
> >> * * #define ARBITRARY_DATA 42
> >> * * *p = ARBITRARY_DATA;
>
> > I was actually trying to avoid that approach,
> > because it requires the CPU to load a register with a
> > specific value, which isn't really necessary.
>
> Not necessarily - all of the assembler languages I'm familiar with have
> an instruction to store a literal value, and I assume any decent compiler
> would emit such an instruction if given the above code.

Yes, I've forgotten about that...
But that is usually a 1 byte longer instruction

On Feb 21, 8:45*pm, Sherm Pendley <sherm.pend...@gmail.com> wrote:
> aleksa <aleks...@gmail.com> writes:
> > On Feb 21, 8:05*pm, Keith Thompson <ks...@mib.org> wrote:
> >> * * #define ARBITRARY_DATA 42
> >> * * *p = ARBITRARY_DATA;
>
> > I was actually trying to avoid that approach,
> > because it requires the CPU to load a register with a
> > specific value, which isn't really necessary.
>
> Not necessarily - all of the assembler languages I'm familiar with have
> an instruction to store a literal value, and I assume any decent compiler
> would emit such an instruction if given the above code.

Yes, I've forgotten about that...
But that is usually a 1 byte longer instruction

And I'm not sure if ARM can do that, especially if its
base + offset = data

On 2/21/2011 2:36 PM, aleksa wrote:
> On Feb 21, 8:05 pm, Keith Thompson<ks...@mib.org> wrote:
>> #define ARBITRARY_DATA 42
>> *p = ARBITRARY_DATA;
>
> I was actually trying to avoid that approach,
> because it requires the CPU to load a register with a
> specific value, which isn't really necessary.
>
> As I said to Jens:
> "Writing whatever value is stored at that moment in whatever CPU
> register",
> without the need to initialize it first.

There's no way to do this in C. There may be ways to do it in
a particular implementation of C, with a particular set of compiler
options, under a particular set of conditions that enable or prevent
particular optimizations, if you're a left-handed seventh son of a
seventh son programming in an abandoned churchyard by the light of
the second full Moon of an odd-numbered month.

(In short, if you're counting the individual instructions that
the machine executes to carry out the intent of your code, C is not
the way to express that intent. Wrong tool; use another.)

--
Eric Sosman
d

Keith Thompson <kst-> writes:

> Yes, but I'd be more comfortable with something like this:
>
> #define ARBITRARY_DATA 42
> *p = ARBITRARY_DATA;

It just occurred to me, that noone have mentioned the volatile
keyword yet. Especially this case, where the same value is written
repeatedly, is warranting a volatile declaration of p. Otherwise te
compile may end up optimizing any but the first write into oblibion.

--
/Wegge

Leder efter redundant peering af dk.*,linux.debian.*

On Feb 21, 9:36*pm, Anders Wegge Keller <we...@wegge.dk> wrote:

> *It just occurred to me, that noone have mentioned the volatile
> keyword yet.

Hardware registers are to be defined as volatile, yes.
(not that it changes the original question, though)

On Feb 21, 6:42*pm, Ben Pfaff <b...@cs.stanford.edu> wrote:
> This may work:
>
> * * unsigned char x;
>
> * * *p = x;

It doesn't as I would like.
My compiler stores 0 in reg, then reg to memory.
(as opposed to just storing a reg to memory)

aleksa <> writes:
> On Feb 21, 6:42Â*pm, Ben Pfaff <b...@cs.stanford.edu> wrote:
>> This may work:
>>
>> Â* Â* unsigned char x;
>>
>> Â* Â* *p = x;
>
> It doesn't as I would like.
> My compiler stores 0 in reg, then reg to memory.
> (as opposed to just storing a reg to memory)

Does your compiler generate code to store 0, or does the register
just happen to contain 0?

--
Keith Thompson (The_Other_Keith) kst- <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"

On Feb 21, 10:14*pm, Keith Thompson <ks...@mib.org> wrote:

> Does your compiler generate code to store 0, or does the register
> just happen to contain 0?

The first one.

Keith Thompson wrote:
> aleksa <> writes:
>> On Feb 21, 6:42 pm, Ben Pfaff <b...@cs.stanford.edu> wrote:
>>> This may work:
>>>
>>> unsigned char x;
>>>
>>> *p = x;
>> It doesn't as I would like.
>> My compiler stores 0 in reg, then reg to memory.
>> (as opposed to just storing a reg to memory)
>
> Does your compiler generate code to store 0, or does the register
> just happen to contain 0?
>
It might be useful to give x some value.......

Sjouke Burry <> writes:

> Keith Thompson wrote:
>> aleksa <> writes:
>>> On Feb 21, 6:42 pm, Ben Pfaff <b...@cs.stanford.edu> wrote:
>>>> This may work:
>>>>
>>>> unsigned char x;
>>>>
>>>> *p = x;
>>> It doesn't as I would like.
>>> My compiler stores 0 in reg, then reg to memory.
>>> (as opposed to just storing a reg to memory)
>>
>> Does your compiler generate code to store 0, or does the register
>> just happen to contain 0?
>>
> It might be useful to give x some value.......

The whole point was to not give x a value; please read back to
the beginning of the thread.
--
char a[]="\n .CJacehknorstu";int putchar(int);int main(void){unsigned long b[]
={0x67dffdff,0x9aa9aa6a,0xa77ffda9,0x7da6aa6a,0xa6 7f6aaa,0xaa9aa9f6,0x11f6},*p
=b,i=24;for(;p+=!*p;*p/=4)switch(0[p]&3)case 0:{return 0;for(p--;i--;i--)case+
2:{i++;if(i)break;else default:continue;if(0)case 1utchar(a[i&15]);break;}}}