memory allocators & proper alignment...

[Chris Thomasson]

Here is some info on a C++ allocator prototype I am working on:

http://groups.google.com/group/comp....eee1f61fdbb52c

Any tried-and-true techniques for calculating the correct alignment of any
C++ type the user can throw at it? For the initial code, I was assuming that
the alignment(T) == sizeof(T)... Now that I am so close to being able to
release this thing, I wanted to be able to stitch up this loose end.

All of the allocators I have worked on were always uses by entities which
were private to a library implementation. .. Any they only needed to align
structures on level-2 cache-line boundaries. So, I didn't need to align for
the type, I only align for the cache line.

Now I need to figure out how to get the correct alignment of any C++ type a
user can come up with...

Help!
:^)

--
Chris M. Thomasson
http://appcore.home.comcast.net

[Chris Thomasson]

"Chris Thomasson" <cr*****@comcas t.netwrote in message
news:mJ******** *************** *******@comcast .com...

Here is some info on a C++ allocator prototype I am working on:

http://groups.google.com/group/comp....eee1f61fdbb52c

[...]

Now I need to figure out how to get the correct alignment of any C++ type
a user can come up with...

I think I will just go ahead and post the code today or tomorrow.

[John Harrison]

Chris Thomasson wrote:

Here is some info on a C++ allocator prototype I am working on:

http://groups.google.com/group/comp....eee1f61fdbb52c
Any tried-and-true techniques for calculating the correct alignment of
any C++ type the user can throw at it? For the initial code, I was
assuming that the alignment(T) == sizeof(T)... Now that I am so close to
being able to release this thing, I wanted to be able to stitch up this
loose end.

All of the allocators I have worked on were always uses by entities
which were private to a library implementation. .. Any they only needed
to align structures on level-2 cache-line boundaries. So, I didn't need
to align for the type, I only align for the cache line.

Now I need to figure out how to get the correct alignment of any C++
type a user can come up with...

Help!
:^)

There is no infallible way to compute that in standard C++. I recall
reading about techniques that almost always work. This article seems to
describe one of those (although it doesn't ring any bells for me).

http://www.monkeyspeak.com/alignment/

john

[Gianni Mariani]

On May 17, 11:02 am, "Chris Thomasson" <cris...@comcas t.netwrote:

"Chris Thomasson" <cris...@comcas t.netwrote in message

news:mJ******** *************** *******@comcast .com...

Here is some info on a C++ allocator prototype I am working on:

http://groups.google.com/group/comp..../thread/beeee1...

[...]

Now I need to figure out how to get the correct alignment of any C++ type
a user can come up with...

one way:
template <typename T>
struct alignof_static
{
struct Helper1 { T v; };
struct Helper2 { char c; Helper1 v; };

static const unsigned value = sizeof(Helper2)-sizeof(Helper1) ;
};
template <typename T>
unsigned alignof()
{
return alignof_static< T>::value;
}

#ifdef __GNUC__
struct X
{
int a;
} __attribute__ ((aligned (8)));
#else
typedef int X;
#endif
#include <iostream>
int main()
{
std::cout << alignof<char>() << "\n";
std::cout << alignof<short>( ) << "\n";
std::cout << alignof<int &>() << "\n";
std::cout << alignof<char &>() << "\n";
std::cout << alignof<X>() << "\n";
}

[Gianni Mariani]

On May 17, 11:02 am, "Chris Thomasson" <cris...@comcas t.netwrote:

"Chris Thomasson" <cris...@comcas t.netwrote in message

news:mJ******** *************** *******@comcast .com...

Here is some info on a C++ allocator prototype I am working on:

http://groups.google.com/group/comp..../thread/beeee1...

[...]

Now I need to figure out how to get the correct alignment of any C++ type
a user can come up with...

I think I will just go ahead and post the code today or tomorrow.

I'm not sure why my previous attempt at posting this didn't show up -
if this is a second post - apologies in advance.

template <typename T>
struct alignof_static
{
struct Helper1 { T v; };
struct Helper2 { char c; Helper1 v; };

static const unsigned value = sizeof(Helper2)-sizeof(Helper1) ;
};
template <typename T>
unsigned alignof()
{
return alignof_static< T>::value;
}

#ifdef __GNUC__
struct X
{
int a;
} __attribute__ ((aligned (8)));
#else
typedef int X;
#endif
#include <iostream>
int main()
{
std::cout << alignof<char>() << "\n";
std::cout << alignof<short>( ) << "\n";
std::cout << alignof<int &>() << "\n";
std::cout << alignof<char &>() << "\n";
std::cout << alignof<X>() << "\n";
}

[James Kanze]

On May 17, 11:34 am, Gianni Mariani <gi3nos...@mari ani.wswrote:

On May 17, 11:02 am, "Chris Thomasson" <cris...@comcas t.netwrote:

"Chris Thomasson" <cris...@comcas t.netwrote in message

news:mJ******** *************** *******@comcast .com...

Here is some info on a C++ allocator prototype I am working on:

>http://groups.google.com/group/comp..../thread/beeee1....

[...]

Now I need to figure out how to get the correct alignment
of any C++ type a user can come up with...

one way:
template <typename T>
struct alignof_static
{
struct Helper1 { T v; };
struct Helper2 { char c; Helper1 v; };
static const unsigned value = sizeof(Helper2)-sizeof(Helper1) ;

Just curious, but why Helper1? I would have expected:

struct Helper { char c; T v ; } ;
static const size_t value = sizeof( Helper ) - sizeof( T ) ;

Word addressed machines will typically require all struct's to
be aligned at word boundaries; only character types are exempt
from the requirement. But since the above seems so obvious, I
presume that there is some reason I haven't seen for not using
it.

--
James Kanze (Gabi Software) email: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Gianni Mariani]

James Kanze wrote:

On May 17, 11:34 am, Gianni Mariani <gi3nos...@mari ani.wswrote:

>On May 17, 11:02 am, "Chris Thomasson" <cris...@comcas t.netwrote:

>>"Chris Thomasson" <cris...@comcas t.netwrote in message

>>news:mJ****** *************** *********@comca st.com...

>>>Here is some info on a C++ allocator prototype I am working on:

>>>http://groups.google.com/group/comp..../thread/beeee1...

>>[...]
Now I need to figure out how to get the correct alignment
of any C++ type a user can come up with...

>one way:
template <typename T>
struct alignof_static
{
struct Helper1 { T v; };
struct Helper2 { char c; Helper1 v; };
static const unsigned value = sizeof(Helper2)-sizeof(Helper1) ;

Just curious, but why Helper1? I would have expected:

struct Helper { char c; T v ; } ;
static const size_t value = sizeof( Helper ) - sizeof( T ) ;

Think about what the expression above does when T = char &.

I attached another version which has two alignof methods - one for
dealing with the reference as a type and the other to deal with the
references and the underlying type as the same.

>
Word addressed machines will typically require all struct's to
be aligned at word boundaries; only character types are exempt
from the requirement. But since the above seems so obvious, I
presume that there is some reason I haven't seen for not using
it.

You can probably do a static assert for this - i.e. safe since it fails
to compile on platforms that don't support it.

template <typename T>
struct noreference
{
typedef T type;
};

template <typename T>
struct noreference<T&>
{
typedef T type;
};

template <typename T>
struct alignof_static
{
struct Helper1 { T v; };
struct Helper2 { char c; Helper1 v; };

static const unsigned value = sizeof(Helper2)-sizeof(Helper1) ;
};

template <typename T>
struct alignof_noref_s tatic
{
static const unsigned value = alignof_static< typename noreference<T>: :type >::value;
};

template <typename T>
unsigned alignof()
{
return alignof_static< T>::value;
}

template <typename T>
unsigned alignof_noref()
{
return alignof_noref_s tatic<T>::value ;
}

#ifdef __GNUC__
struct X
{
int a;
} __attribute__ ((aligned (8)));
#else
typedef int X;
#endif
#include <iostream>
int main()
{
std::cout << alignof<char>() << "\n";
std::cout << alignof<short>( ) << "\n";
std::cout << alignof<int &>() << "\n";
std::cout << alignof<char &>() << " noref=" << alignof_noref<c har &>() << "\n";
std::cout << alignof<X>() << "\n";
}

[Gianni Mariani]

Gianni Mariani wrote:

James Kanze wrote:

....

I forgot to mention - I decided to switch news servers after my last
response didn't show for a few hours. So apologies again for double posts.

[James Kanze]

Gianni Mariani wrote:

James Kanze wrote:

On May 17, 11:34 am, Gianni Mariani <gi3nos...@mari ani.wswrote:

On May 17, 11:02 am, "Chris Thomasson" <cris...@comcas t.netwrote:
[...]
Now I need to figure out how to get the correct alignment
of any C++ type a user can come up with...

one way:
template <typename T>
struct alignof_static
{
struct Helper1 { T v; };
struct Helper2 { char c; Helper1 v; };
static const unsigned value = sizeof(Helper2)-sizeof(Helper1) ;

Just curious, but why Helper1? I would have expected:

struct Helper { char c; T v ; } ;
static const size_t value = sizeof( Helper ) - sizeof( T ) ;

Think about what the expression above does when T = char &.

Bingo. That's what I hadn't thought of.

I'm not sure that it's all that useful to ask the alignment of
something that the standard says might not occupy memory:-). On
the other hand, for most people, the case is probably more
likely to occur in practice that is the case of a word addressed
machine (unless you're a mainframes programmer, and your company
buys from Unisys). A good library should probably handle both
(i.e. word addressed machines, and the alignment of references)
correctly.

(The simplest solution would probably be to use both techniques,
forcing an error if the results weren't equal. That would
probably handle 99% of the cases, and would cause a compile time
error for the cases it didn't handle. Otherwise, I think that
it should be possible to use some metaprogramming tricks to use
one technique for references, and the other otherwise.)

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Chris Thomasson]

Thank you all for your time and energy!

Humm... This thread might be of interest to this group. I say this simply
because current C++ has no concept of threads... I can allow you to create a
single-threaded allocator in C++, and transform it into a full-blown
scaleable mulit-threaded one:

http://groups.google.com/group/comp....87f9cb2cb62d84
No kidding!
So, all of your single threaded C++ allocators can be transformed into
multi-threaded versions, simply by applying a simple lock-free algorithm I
invented.
Any thoughts? Could the technology/method I invented be of use to you or
your company?