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
> >> > >
> >> >
> >> >
> >> >
> >> >
> >>
> >>
> >>
> >>
> >
> >
> >
> >
>
