"H.J. Lu" <hjl.to...@gmail.com> writes: > 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.
But the compiler does not actually use MALLOC_ABI_ALIGNMENT, MALLOC_ABI_ALIGNMENT is not correctly defined for most target, (i.e., actually using the current definition would be an optimization regression in some cases), and even if the compiler did use MALLOC_ABI_ALIGNMENT there is no guarantee that it would be correct. > 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. Think harder about new[] and auto-vectorization. What you describe seems to me to be a theoretical argument rather than a practical one. Ian
