On Fri, Nov 15, 2013 at 10:18 AM, Richard Earnshaw <rearn...@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. > >
