On Tue, Feb 24, 2009 at 06:15:37AM -0800, Bingfeng Mei wrote:
> Hello,
> For the targets that support vectors, we can write the following code:
>
> typedef short V4H __attribute__ ((vector_size (8)));
>
> V4H tst(V4H a, V4H b){
> return a + b;
> }
>
> Other operators such as -, *, |, &, ^ etc are also supported. However,
> vector shift
> is not supported by frontend, including both scalar and vector second
> operands.
>
> V4H tst(V4H a, V4H b){
> return a << 3;
> }
>
> V4H tst(V4H a, V4H b){
> return a << b;
> }
>
> Currently, we have to use intrinsics to support such shift. Isn't syntax of
> vector
> shift intuitive enough to be supported natively? Someone may argue it breaks
> the
> C language. But vector is a GCC extension anyway. Support for vector
> add/sub/etc
> already break C syntax. Any thought? Sorry if this issue had been raised in
> past.
Note, internally there are two different types of vector shift. Some machines
support a vector shift by a scalar, some machines support a vector shift by a
vector. One future machine (x86_64 with -msse5) can support both types of
vector shifts.
The auto vectorizer now can deal with both types:
for (i = 0; i < n; i++)
a[i] = b[i] << c
will generate a vector shift by a scalar on machines with that support, and
splat the scalar into a vector for the second set of machines.
If the machine only has vector shift by a scalar, the auto vectorizer will not
generate a vector shift for:
for (i = 0; i < n; i++)
a[i] = b[i] << c[i]
Internally, the compiler uses the standard shift names for vector shift by a
scalar (i.e. ashl<type>, ashr<type>, lshl<type>), and a v prefix for the vector
by vector shifts (i.e. vashl<type>, vashr<type>, vlshl<type>).
The rotate patterns are also similar.
--
Michael Meissner, IBM
4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA
meiss...@linux.vnet.ibm.com
