"Daniel Berlin" <[EMAIL PROTECTED]> wrote on 17/06/2007 18:18:19:
... > > The whole purpose of SLP was to enable straight line code > vectorization outside of loops. I wouldn't say that's the whole purpose of SLP. I think the purpose and beauty of SLP is that it's a simple algorithm that makes vectorization (including vectorization of loops) easier by removing the need to prove all kinds of properties about the loop as a whole, as well as the need to transform loops to make them vectorizable. The fact that this scheme also works out of loops is a neat property because it makes loop-vectorization a special case of SLP. However, as far as I know (also from talking with the SLP authors) pretty much all the opportunities they had found at the time were in loops. Also, a lot of the SLP based work that followed it focused on loops, and analysis to determine by how much to unroll loops in order to accommodate SLP. So, in reality, as always, there are no free meals - you often really have to compensate for the simple loop-ignorant SLP analysis by doing a lot of loop-level analysis and transformations before hand. While it cannot replace the classic SLP algorithm out-of-loops (e.g. for completely unrolled loops), doing SLP in loops actually makes a lot of sense, IMHO. It lets us leverage already existing infrastructure (the slp patch recently committed to autovect-branch is really not big), and opens up a lot of opportunities we couldn't vectorize before (partially unrolled loops, partially vectorizable loops, accesses to consecutive struct fields, and in the future also permutations), while taking advantage of the loop-context when it helps you do things more efficiently. In any case, we'll have to have a much better cost model before we start packing random sequences of stmts out of loops. > Simply because you can't find cases in SPEC2000 doesn't mean it's not useful. I don't know where you're taking this from. SPEC2000 is really not so interesting vectorization wise, inside or outside loops. dorit
