Options of fixing biggest alignment in PR target/38736

[Michael Meissner]

On Thu, Jan 08, 2009 at 03:35:29PM -0800, Ian Lance Taylor wrote:
> "H.J. Lu" <hjl.tools@gmail.com> writes:
> 
> > For 4.4, we can apply my patch:
> >
> > http://gcc.gnu.org/ml/gcc-patches/2009-01/msg00367.html
> >
> > and update document with
> >
> > As in the preceding examples, you can explicitly specify the alignment
> > (in bytes) that you wish the compiler to use for a given variable or
> > structure field.  Alternatively, you can leave out the alignment factor
> > and just ask the compiler to align a variable or field to the maximum
> > useful alignment for the target ABI you are compiling for.  For
> > example, you could write:
> >
> > @smallexample
> > short array[3] __attribute__ ((aligned));
> > @end smallexample
> >
> > Whenever you leave out the alignment factor in an @code{aligned} attribute
> > specification, the compiler automatically sets the alignment for the declared
> > variable or field to the largest alignment for the target ABI you are
> > compiling for.  Doing this can often make copy operations more efficient,
> > because the compiler can use whatever instructions copy the biggest
> > chunks of memory when performing copies to or from the variables or
> > fields that you have aligned this way.
> 
> Shouldn't the docs say something about memory allocation?  As in,
> malloc should return memory aligned to that boundary?
> 
> For that matter, don't we have a problem on x86 GNU/Linux, where
> malloc returns an 8-byte alignment but attribute((aligned)) is a 16
> byte alignment?

In that case, malloc should be changed to return items that are 16-byte aligned
if any type needs 16-byte alignment for any possible ISA.

-- 
Michael Meissner, IBM
4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA
meissner@linux.vnet.ibm.com