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Re: Options of fixing biggest alignment in PR target/38736
On Thu, Jan 8, 2009 at 10:49 AM, Ian Lance Taylor <iant@google.com> wrote:
> "H.J. Lu" <hjl.tools@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.