Re: Native support for vector shift

[Michael Meissner <meissner at linux dot vnet dot ibm dot com>]

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
meissner@linux.vnet.ibm.com