Re: Finding the correct alignment for all architectures
* Christian Seiler <christian@iwakd.de>, 2016-10-13, 00:20:
On 10/12/2016 11:51 PM, Thomas Weber wrote:
I am maintaining lcms2. In #749975, I received a patch to ensure correct
alignment for doubles von MIPS. I have forwarded the patch upstream[1], but
in the latest release, upstream has chosen a different way. It is now
possible to configure the alignment via a preprocessor variable
CMS_PTR_ALIGNMENT[2]:
[...]
I would like to drop the Debian-specific patch. But what value for
CMS_PTR_ALIGNMENT would be good/sufficient on all arches?
Use _Alignof(type), that will always be correct. :-)
For example:
#define POINTER_ALIGNMENT _Alignof(void *)
#define DOUBLE_ALIGNMENT _Alignof(double)
If you are not delighted with leading underscores, you can #include
<stdalign.h> and then do s/_Alignof/alignof/.
Technically, this was introduced in C11/C++11, so if you need to support
really old compilers, this may be problematic, but gcc/clang have supported
that for a while. (A quick test tells me that gcc and clang from Jessie
already support it.)
gcc in wheezy supports it too.
Alternatively, you may want to use the (gcc-specific) __BIGGEST_ALIGNMENT__
macro, but that kinda wasteful. For example, on i386 it's 16, even though
alignof(t) is <= 4 for most t you can think of.
--
Jakub Wilk
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