On Thu, Jan 8, 2009 at 10:49 AM, Ian Lance Taylor <i...@google.com> wrote: > "H.J. Lu" <hjl.to...@gmail.com> writes: > >> It it hard to guess what the alignment should be for C++ new operator. >> Whatever you choose today may not be appropriate tomorrow >> or for others. I think g++ should issue a warning when new operator >> is used on a type whose alignment greater than MALLOC_ABI_ALIGNMENT: > > The real problem is not a single value, but an array of some type. > Auto-vectorization optimizations may apply, and they may require a > larger alignment, and the compiler may believe that the memory > allocator provides a larger alignment than it actually does. The > compiler can not know the minimal alignment returned by the memory > allocator, because the user can change the memory allocator. > > We need to write __attribute__ ((aligned (XXX))) accordingly. > > I see that MALLOC_ABI_ALIGNMENT exists, but the value is overly > conservative by default, and no target defines it, and the only thing > which actually uses is the Ada frontend. >
Compiler can only use the minimum alignment on memory returned by memory allocator, which is MALLOC_ABI_ALIGNMENT. I am not sure how __attribute__ ((aligned (max)) can solve this. For the memory allocator writers, they should provide an interface with a parameter to specify alignment requirement. User can use __attribute__ ((aligned (XXX))) to tell compiler memory alignment. -- H.J.
