Re: [Jack-Devel] Alignment (again)

On 08/04/2011 04:33 AM, Sampo Savolainen wrote:
> I would be very surprised if malloc() returned pointers that are not
> 4-byte aligned.

Looks like either my memory failed more... or they've changed eglibc.  I 
just ran a test on Ubuntu 11.04 (which uses eglibc) and it always 
returns 8-byte aligned pointers.

However... I'm pretty sure I've seen 1-byte alignment in the past.  :-/

 > Are you running vectorized code? x86 SIMD (SSE) vector
> instructions require 16-byte aligned data pointers. This is a more
> likely source for the segfaults.

True... but in this case the problem was observed while running tests on 
the jack_ringbuffer with floats and -O3.  I found a related bug report 
for GCC:

     http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49413

But now that I think of it... the problem with jack_ringbuffer is 
actually pretty fundamental.

Suppose you allocate a ringbuffer of size N (where N is a power of 2). 
The storage space is (N-1).  So even if you start with an aligned 
pointer... when you wrap around it will no longer be aligned. :-/

-gabriel

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