Unfortunately there's no way to specify alignment attribute of pointers in
GCC - the syntax was allowed in the past but not really supported
correctly, and then entirely disallowed (by this patch
http://gcc.gnu.org/ml/gcc-patches/2005-04/msg02284.html). This issue was
discussed in details in these threads:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20794
http://gcc.gnu.org/ml/gcc/2005-03/msg00483.html
(and recently came up again also in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27827#c56).
The problem is that
"We don't yet implement either attributes on array parameters applying to
the array not the pointer, or attributes inside the [] of the array
parameter applying to the pointer. (This is documented in "Attribute
Syntax".)" (from the above thread).
dorit
> Hello,
>
> I have been playing with gcc's new (to me) auto vectorization
> optimizations. I have a particular loop for which I have made external
> provisions to ensure that the data is 16-byte aligned. I have tried
> everything I can think of to give gcc the hint that it is operating on
> aligned data, but still the vectorizer warns that it is operating on
> unaligned data and generates the less efficient MOVLPS/MOVUPS instead
> of MOVAPS.
>
> The code is like this:
>
> #define SSE __attribute__((aligned (16)))
>
> typedef float matrix_t[100][1024];
>
> matrix_t aa SSE, bb SSE, cc SSE;
>
> void calc(float *a, float *b, float *c) {
> int i, n = 1024;
>
> for (i=0; i<n; ++i) {
> a[i] = b[i] / (b[i] + c[i]);
> }
>
> }
>
> int main(void) {
> int i, n = 100;
> for (i=0; i<n; ++i) {
> calc(a[i], b[i], c[i]);
> }
> }
>
> gcc rejects if I specify alignment attributes on the formal parameters
> (obviously it was dumb to even try that), and there does not seem to
> be a way to get the alignment hint to apply to the object referenced
> by the pointer instead of the pointer itself.
>
> In my application it is important that the function doing the
> computations remains abstracted away from the data definitions, as
> there is over 1G of data dynamically arranged and the actual alignment
> provisions are made with mmap().
>
> Does anyone have a suggestion?
>
> Regards,
> Michael James
