[PATCH] Support addsub/subadd as non-isomorphic operations for SLP vectorizer.

[Cong Hou]

On Fri, Nov 15, 2013 at 10:18 AM, Richard Earnshaw <rearnsha@arm.com> wrote:
> On 15/11/13 02:06, Cong Hou wrote:
>> Hi
>>
>> This patch adds the support to two non-isomorphic operations addsub
>> and subadd for SLP vectorizer. More non-isomorphic operations can be
>> added later, but the limitation is that operations on even/odd
>> elements should still be isomorphic. Once such an operation is
>> detected, the code of the operation used in vectorized code is stored
>> and later will be used during statement transformation. Two new GIMPLE
>> opeartions VEC_ADDSUB_EXPR and VEC_SUBADD_EXPR are defined. And also
>> new optabs for them. They are also documented.
>>
>
> Not withstanding what Richi has already said on this subject, you
> certainly don't need both VEC_ADDSUB_EXPR and VEC_SUBADD_EXPR.  The
> latter can always be formed by vec-negating the second operand and
> passing it to VEC_ADDSUB_EXPR.
>

Right. But I also considered targets without the support to addsub
instructions. Then we could still selectively negate odd/even elements
using masks then use PLUS_EXPR (at most 2 instructions). If I
implement VEC_ADDSUB_EXPR by negating the second operand then using
VEC_ADDSUB_EXPR, I end up with one more instruction.


thanks,
Cong



> R.
>
>