g++: __attribute__ ((aligned)) weirdness

g++: __attribute__ ((aligned)) weirdness

[Andrew Haley]

The doc says

     The `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

but this doesn't seem to be true.  For example, on x86 g++ you have a non-POD
class and a subclass:

-----------------------------------------------------------------------------
class Boof
{
  int arse ()
  {
    return 25;
  }

  void *prickle;
};

class foo : Boof
{
  public: long long i;
};
-----------------------------------------------------------------------------

then try:

-----------------------------------------------------------------------------
  foo f;
  fprintf(stderr, "offset: %d\n", (char *)&f.i - (char *)&f);
-----------------------------------------------------------------------------

offset: 4

Fair enough, this long long is probably 4-aligned.

Now do this:

-----------------------------------------------------------------------------
class bar : Boof
{
  public: long long __attribute__((aligned(4))) i;
};

  bar b;
  fprintf(stderr, "offset: %d\n", (char *)&b.i - (char *)&b);

-----------------------------------------------------------------------------

offset: 8

So you have a field at offset 4.  You ask for it to be 4-aligned, and
it jumps to offset 8.

What is going on?
 
Andrew.

Re: __attribute__ ((aligned)) weirdness

[Piotr Wyderski]

> So you have a field at offset 4.  You ask for it to be 4-aligned, and
> it jumps to offset 8.
> 
> What is going on?

The alignment of x means that the offset of a variable is divisible
by x with no remainder. Each y=x*2^n (where n is natural) is
a good alignment too, because f mod y = 0 => f mod x = 0;
8-aligned field is also 4-aligned. So where's the problem?

    Best regards
    Piotr Wyderski



Serwis www.logo.hoga.pl - sciagaj bajery na telefony
Nokia, Siemens, Alcatel, Ericsson, Motorola,Samsung
------------------------------------------------------------
Promocja!!! rabat 40 % na zakup mks_vir 2003 dla klientów Connect , którzy
posiadaja kupony rabatowe.

Re: __attribute__ ((aligned)) weirdness

[Andrew Haley]

Piotr Wyderski writes:
 > > So you have a field at offset 4.  You ask for it to be 4-aligned, and
 > > it jumps to offset 8.
 > > 
 > > What is going on?
 > 
 > The alignment of x means that the offset of a variable is divisible
 > by x with no remainder. Each y=x*2^n (where n is natural) is
 > a good alignment too, because f mod y = 0 => f mod x = 0;
 > 8-aligned field is also 4-aligned. So where's the problem?

I need to know why this happens.

Andrew.

Re: __attribute__ ((aligned)) weirdness

[Andreas Tobler]

Andrew Haley wrote:
> Piotr Wyderski writes:
>  > > So you have a field at offset 4.  You ask for it to be 4-aligned, and
>  > > it jumps to offset 8.
>  > > 
>  > > What is going on?
>  > 
>  > The alignment of x means that the offset of a variable is divisible
>  > by x with no remainder. Each y=x*2^n (where n is natural) is
>  > a good alignment too, because f mod y = 0 => f mod x = 0;
>  > 8-aligned field is also 4-aligned. So where's the problem?
> 
> I need to know why this happens.

Could this be related? No explanation, sorry.

http://gcc.gnu.org/ml/gcc/2003-02/msg00353.html

Andreas

PPC support in GCC ?

[A.R. Ashok Kumar]

Hi,

  I have gcc 2.95.3 source. As per the source, there is no support for
PowerPC in GCC. But GCC website(http://gcc.gnu.org/install/specific.html) 
says that the following configurations are supported in GCC,

   powerpc-*-*
   powerpc-*-darwin*
   powerpc-*-elf, powerpc-*-sysv4
   powerpc-*-linux-gnu*
   powerpc-*-netbsd*
   powerpc-*-eabiaix
   powerpc-*-eabisim
   powerpc-*-eabi
   powerpcle-*-elf, powerpcle-*-sysv4
   powerpcle-*-eabisim
   
I couldn't find any target specific config directory for PowerPC under
"gcc/config". Can any one help me?

Thanks,
-AshokA-

Re: PPC support in GCC ?

[Falk Hueffner]

"A.R. Ashok Kumar" <ashokar@sankhya.com> writes:

> I couldn't find any target specific config directory for PowerPC
> under "gcc/config".

It's integrated into rs6000.

-- 
	Falk

Re: PPC support in GCC ?

[A.R. Ashok Kumar]

Hi,

> > I couldn't find any target specific config directory for PowerPC
> > under "gcc/config".
> 
> It's integrated into rs6000.

  Thanks for the info.

  Does GCC has support for MPC555? If so, can you please mention which
  configuration is to be used for "--target" option for building
  GCC for MPC555 on HP-UX.

  (I am new to PowerPC architecture).

-AshokA-