Re: PATCH: Malloc fails if MALLOC_ALIGNMENT > 2 * SIZE_SZ

[Roland McGrath <roland at hack dot frob dot com>]

> And as a reminder for those who didn't follow the previous thread, in the 
> existing state arbitrary code mallocing datastructures containing long 
> double may be *quietly* broken (AltiVec loads and stores used at runtime, 
> expecting 16-byte alignment, on data that is only 8-byte aligned - these 
> don't trap, the documented semantics are to quietly ignore the low bits of 
> the address).  As I understand, the Emacs breakage is it least immediately 
> obviously visible rather than quiet.

That is probably so (though I don't think there's any real guarantee that
we can expect an immediate crash on malloc_set_state).  But, in the time
scales of ABI changes, -mlong-double-128 was a recent addition and things
using that long double type on powerpc32 are neither very common at all,
nor did they ever work reliably in the past.  So both issues affect few
programs in practice.  The one is a bad situation that has existed for a
short time for using a new feature, while the other would be a breakage of
an existing ABI that always worked before.


Thanks,
Roland