RFA (libstdc++): PATCH to implement C++17 over-aligned new

[Marc Glisse]

On Fri, 16 Sep 2016, Jonathan Wakely wrote:

> On 16/09/16 09:04 +0200, Rainer Orth wrote:
>> Hi Jason,
>> 
>>> OK, one more:
>> 
>> this works just fine on both sparc-sun-solaris2.12 and
>> i386-pc-solaris2.12.
>> 
>> Once Jonathan's patch to heed aligned_alloc's requirement on size being
>> a multiple of alignment is in, all is fine on Solaris.
>
> I've got a slightly different fix now.
>
> We only need to make the size a multiple of alignment for
> aligned_alloc, however for posix_memalign we need to ensure the
> alignment is a multiple of sizeof(void*).
>
> I'm testing this now (but only on x86_64 GNU/Linux where it wasn't
> failing anyway).

+  // The value of alignment shall be a power of two multiple of sizeof(void *).
+  if (al < sizeof(void*))
+    al = sizeof(void*);

The code doesn't exactly match the comment. I can't find the precondition 
in the standard that says operator new can only be called on a power of 
2... (maybe we can add it if it is really missing?)

> Would using __builtin_expect (sz == 0, false) make sense?  Surely it's
> rare to try to allocate zero bytes.

https://gcc.gnu.org/ml/libstdc++/2014-03/msg00001.html

gcc already guesses that a test like sz == 0 is usually false (not with as 
large a probability as if you use __builtin_expect, but enough that the 
generated code is unlikely to differ). But adding __builtin_expect cannot 
hurt...

Is the division (by a non-constant denominator) really necessary? Since 
align has to be a power of 2, x % align should be the same as x & (align - 
1), for instance.

I guess people interested in performance will do for aligned new the same 
as for the old new: provide an inline version that skips all the overhead 
to forward directly to malloc/aligned_alloc (and avoid questionable calls 
in their code).

-- 
Marc Glisse