RE: AC_CHECK_ALIGNOF maximum ??

[Bob Friesenhahn]

On Thu, 20 Jun 2013, 'Chris Hall' wrote:
> > > Of course gcc gives __BIGGEST_ALIGNMENT__ for this.  And I guess
> > > other compilers do something similar...
> > >
> > > ...but I cannot help feeling that autoconf should cover this, but
> > > if it does, I have failed to find where :-(
>
> > How would Autoconf portably test for this
>
> I have no idea.  Correct me if I am wrong: but I thought Autoconf was
> there to save humble journeymen like me from having to know that sort
> of thing ?

Autoconf figures things out by running the compiler and linker in a 
portable way.  It does not usually want to run the compiled test 
program because that does not work for cross-compilation.  There needs 
to be a way to discover the desired alignment by compiling code.

> > and for what purpose would you use this information for?
>
> Specifically... I want to allocate lumps of memory with some red tape
> in front to be followed by the "body" of the allocation.  That body
> needs to be aligned, as if by malloc, to the maximum alignment.
>
> For completeness, given the sizeof() the body I wish to align to
> sizeof() % maximum alignment.
>
> With C11 I could use the alignof() spell to do something more precise.
> And I guess I could Autoconf my way to discovering if __alignof__() or
> equivalent whizzy-ness is available.
>
> You know and I know that 16 is probably an excellent guess that will
> be true for the foreseeable... but that's hardly satisfactory !

I am not so sure.  Are you talking about the alignment of a large type 
like 'double', 'long double', a 128-bit integer on PowerPC, the 
alignment required by SSE2 instructions, or the alignment required by 
some AVX instructions (see 
http://software.intel.com/en-us/forums/topic/299644)?

> > Compile-time estimation of biggest alignment seems risky to me if it
> > is used for any serious purpose.  If all of the software in the
> > running application was not compiled with the same flags, then the
> > compiled-in value may be wrong.
>
> AFAICS any disagreement about any alignment is a recipe for tears
> (before, after and during bedtime).  Why should the maximum alignment
> be any different ?

Depending on what you mean by 'maximum alignment', the answer could be 
different depending on compiler options and the target instruction 
set.

Autoconf usually creates a configuration header file which is used by 
all of the code in the project.  If some parts of an application are 
built with different flags, they might still work in the application 
but object code may require different maximum alignment.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/