Maybe the full code of the for loop, as well as the number of
iterations would help us help you.
Brian
On Tue, Feb 1, 2011 at 9:06 AM, isuru herath <isuru81@xxxxxxxxx> wrote:
> Hi Brian,
>
> Well, this is related with my research. I am studying cache behavior. I am interested in allocating certain variables in a known physical address range. The way I follow to do this is to allocate them in a structure and then allocate space for this structure in the address space I am interested in. Later in the code I access these variable via a pointer to that structure. This introduces another cache access(which is the access to pointer). So I am looking for another way to allocate these variables so that it doesn't introduces another access.
>
> regards,
> Isuru
>
> --- On Tue, 2/1/11, Brian Budge <brian.budge@xxxxxxxxx> wrote:
>
>> From: Brian Budge <brian.budge@xxxxxxxxx>
>> Subject: Re: Allocate a variable in a known physical location
>> To: "isuru herath" <isuru81@xxxxxxxxx>
>> Cc: gcc-help@xxxxxxxxxxx, Cenedese@xxxxxxxx
>> Date: Tuesday, February 1, 2011, 8:43 AM
>> So you are counting the number of
>> dereferences/loads?
>>
>> What optimization level are you using? Depending on
>> your code, you
>> may also need to specify that these addresses cannot alias
>> one
>> another, as the potentially aliasing variables may require
>> more loads,
>> depending on how you use the pointers.
>>
>> Is this for an experiment, or for real usable code?
>>
>> Brian
>>
>> On Tue, Feb 1, 2011 at 7:53 AM, isuru herath <isuru81@xxxxxxxxx>
>> wrote:
>> > Hi Fabi,
>> >
>> > Thanks for the reply. I tried that, but still numbers
>> don't change. Let me
>> > describe the scenario.
>> >
>> > My code without any modification I got 201557258
>> accesses. I needed to
>> > allocate those i and j variables in a separate area of
>> memory. To do that
>> > I follow the method described earlier(using a
>> structure). Therefore I got
>> > accesses in that separate area. I got 100893832
>> accesses in that area, but
>> > my total accesses are increased to 302450960. I
>> thought this is because
>> > every time I access variable i or j, I have to access
>> poniter p first. No
>> > I tried Fabi's suggestion. code shown below
>> >
>> > int* p_i = &(p->i);
>> > int* p_j = &(p->j);
>> > int* p_k = &(p->k);
>> >
>> > for (*p_k=0; *p_k < *p_mat_size; (*p_k)++)
>> > ...
>> > ...
>> >
>> > Still I got total access as 302450960. Could somebody
>> help me to
>> > understand this.
>> >
>> > Any help/advice is greatly appreciated.
>> >
>> > regards,
>> > Isuru
>> >
>> >> Once you have p->i, you can also do int*
>> pi=&(p->i);
>> >> So *pi=1 will only be one access.
>> >
>> >> bye Fabi
>> >
>> >
>> > --- On Tue, 2/1/11, isuru herath <isuru81@xxxxxxxxx>
>> wrote:
>> >
>> >> From: isuru herath <isuru81@xxxxxxxxx>
>> >> Subject: Re: Allocate a variable in a known
>> physical location
>> >> To: gcc-help@xxxxxxxxxxx
>> >> Cc: david@xxxxxxxxxxxxxxx
>> >> Date: Tuesday, February 1, 2011, 3:07 AM
>> >> Hi David,
>> >>
>> >> Thanks a lot for the reply. The address 0x10001000
>> is a
>> >> physical address
>> >> and not a virtual address. I thought we can only
>> do this
>> >> type casting with
>> >> virtual addresses. Anyway I tried the method you
>> suggested
>> >> and I got a
>> >> segmentation fault.
>> >>
>> >> I use mmap to map those physical addresses to
>> virtual
>> >> addresses, because
>> >> OS(linux) in unaware of this other piece of memory
>> which
>> >> uses physical
>> >> address range 0x10001000 to 0x10101000.
>> >>
>> >> In my example, when I use my method to access i
>> via pointer
>> >> p (p->i), it
>> >> first accesses p and then accesses i. But this
>> introduces
>> >> unnecessary
>> >> access p. Therefore I was wondering how to
>> allocate i in
>> >> the above
>> >> physical region.(Please note that I cant use any
>> >> optimization -O2, -O3)
>> >>
>> >> I was looking in section attribute, but still
>> couldn't
>> >> figure out how to
>> >> use it, also I am not sure it is the correct way
>> to do
>> >> this.
>> >>
>> >> any help/suggestion is greatly appreciated.
>> >>
>> >> regards,
>> >> Isuru
>> >>
>> >> > I don't know what OS you are using, or what
>> you want
>> >> to do with mmap.
>> >> > But if you have struct that you want to
>> access at a
>> >> particular address,
>> >> > the easiest way is with a bit of
>> typecasting:
>> >>
>> >> > struct my *p = (struct my*) 0x10001000;
>> >>
>> >> > Then when you access p->j, for example,
>> the
>> >> generated code will use the
>> >> > absolute address 0x10001004 (for 32-bit
>> ints).
>> >>
>> >> > mvh.,
>> >>
>> >> > David
>> >>
>> >> --- On Mon, 1/31/11, isuru herath <isuru81@xxxxxxxxx>
>> >> wrote:
>> >>
>> >> > From: isuru herath <isuru81@xxxxxxxxx>
>> >> > Subject: Re: Allocate a variable in a known
>> physical
>> >> location
>> >> > To: "Ian Lance Taylor" <iant@xxxxxxxxxx>
>> >> > Cc: gcc-help@xxxxxxxxxxx
>> >> > Date: Monday, January 31, 2011, 1:01 PM
>> >> > Hi Ian,
>> >> >
>> >> > Thanks a lot for your quick response and I am
>> sorry
>> >> for not
>> >> > explaining the
>> >> > problem correctly.
>> >> >
>> >> > I have a separate piece of memory for which I
>> have
>> >> given
>> >> > physical address
>> >> > range 0x10001000 to 0x10101000. I want to
>> allocate
>> >> > variables in this
>> >> > address range. To achieve this I create a
>> structure
>> >> with
>> >> > variables I need
>> >> > to allocate there. For example if I need to
>> allocate i
>> >> and
>> >> > j in the above
>> >> > address range, I define a structure like
>> following.
>> >> >
>> >> > struct my
>> >> > {
>> >> > int i;
>> >> > int j;
>> >> > };
>> >> >
>> >> > and then allocate memory for the structure
>> using mmap
>> >> like
>> >> > below.(bear with
>> >> > me if syntax are wrong).
>> >> >
>> >> > struct my *p = mmap(........);
>> >> >
>> >> > when ever I need to access i, j in my code I
>> access
>> >> them
>> >> > via pointer p like
>> >> > following.
>> >> >
>> >> > p->i or p->j
>> >> >
>> >> > All what I need is to allocate i and j in the
>> above
>> >> address
>> >> > range. Due to
>> >> > lack of my knowledge in compiler and gcc
>> this is
>> >> how
>> >> > I did it. The
>> >> > drawback of this is that to access i, it has
>> to access
>> >> p
>> >> > first. This
>> >> > introduces an unnecessary access to my
>> statistics.
>> >> > Therefore if I could
>> >> > allocate i and j without using the above
>> method I
>> >> thought
>> >> > my problem will
>> >> > be solved.
>> >> >
>> >> > As you mentioned in your reply can I use
>> section
>> >> attribute
>> >> > to achieve this or do you have any other
>> suggestion.
>> >> >
>> >> > Any help/advice is greatly appreciated.
>> >> >
>> >> > regards,
>> >> > Isuru
>> >> >
>> >> > --- On Mon, 1/31/11, Ian Lance Taylor <iant@xxxxxxxxxx>
>> >> > wrote:
>> >> >
>> >> > > From: Ian Lance Taylor <iant@xxxxxxxxxx>
>> >> > > Subject: Re: Allocate a variable in a
>> known
>> >> physical
>> >> > location
>> >> > > To: "isuru herath" <isuru81@xxxxxxxxx>
>> >> > > Cc: gcc-help@xxxxxxxxxxx
>> >> > > Date: Monday, January 31, 2011, 11:21
>> AM
>> >> > > isuru herath <isuru81@xxxxxxxxx>
>> >> > > writes:
>> >> > >
>> >> > > > I need to allocate a variable in a
>> known
>> >> > physical
>> >> > > location, let's say I need
>> >> > > > to allocate void *p in location
>> >> 0x10001000. I
>> >> > > was using mmap to to do this,
>> >> > > > but in that manner I can only
>> allocate
>> >> p[0],
>> >> > > p[1]...p[n] in that physical
>> >> > > > address range. Therefore when I
>> access
>> >> p[i],
>> >> > accesses
>> >> > > to p results in
>> >> > > > outside {0x10001000,
>> 0x10001000+offset}
>> >> and
>> >> > p[i]
>> >> > > results as an access in
>> >> > > > the range I am interested in.
>> >> > >
>> >> > > I don't understand the last sentence
>> there.
>> >> > >
>> >> > > > I was wondering is there a was for
>> me to
>> >> force
>> >> > > > to allocate variable p in that
>> address range
>> >> or I
>> >> > am
>> >> > > looking for something
>> >> > > > totally unrealistic. Because of the
>> nature
>> >> of my
>> >> > > research I can use any
>> >> > > > optimization(-O2, O3).
>> >> > >
>> >> > > If you don't want to use mmap, the
>> simplest way
>> >> to put
>> >> > a
>> >> > > variable at a
>> >> > > specific location is to put it in a
>> specific
>> >> section
>> >> > using
>> >> > > __attribute__
>> >> > > ((section ("..."))) and then put that
>> section at
>> >> a
>> >> > specific
>> >> > > address
>> >> > > using a linker script.
>> >> > >
>> >> > > Ian
>> >> > >
>> >> >
>> >> >
>> >> >
>> >> >
>> >>
>> >>
>> >>
>> >>
>> >
>> >
>> >
>> >
>>
>
>
>
>
