On Wed, 5 Sep 2012, Gabriel Dos Reis wrote:
On Wed, Sep 5, 2012 at 5:09 PM, Iyer, Balaji V <balaji.v.i...@intel.com> wrote:
Let's say we have two for loops like this:
int my_func (int x, int y);
For (ii = 0; ii < 10000; ii++)
X[ii] = my_func (Y[ii], Z[ii]);
I assume X, Y and Z are __restrict pointers (or something the compiler can
detect doesn't alias).
2. Considering this example, won't you get the same behaviour
if my_func was declared with "pure" attribute? If not, why?
AFAIU, my_func is defined in a separate library and because of the
attribute on the definition, it will actually export overloads:
int myfunc(int,int);
v2si myfunc(v2si,v2si);
v4si myfunc(v4si,v4si);
etc (where does it stop? seems problematic if the library is compiled for
sse4 and I then compile and link an avx program)
(hopefully with implementations more clever than breaking the vectors into
pieces and calling the basic myfunc on each)
The attribute on the declaration then lets gcc's vectorizer know it can
call those overloads.
With suitable pure/const attribute you could unroll the loop a bit and
reorder the calls to myfunc, but without myfunc's body, you couldn't do as
much.
Note that this is my guess from reading the example and completely
ignoring the patch, it could be miles from the truth, and it needs better
explanation (the doc patch is coming later in the series IIRC).
--
Marc Glisse
