On 15 February 2014 22:49, Tim Prince <n...@aol.com> wrote: > In my experience, the (somewhat complicated) gcc --param options work > sufficiently well for specification of unrolling.
There is precedent for --param in LLVM, we could go this way, too. Though, I can't see how it'd be applied to a specific function, loop or lexical block. > In the same vein, I haven't seen any cases where gcc 4.9 is excessively > aggressive in > vectorization, so that a #pragma novector plus scalar unroll is needed, as > it is with Intel compilers. > (...) > If your idea is to obtain selective effective > auto-vectorization in source code which is sufficiently broken that -O2 > -ftree-vectorize can't be considered or -fno-strict-aliasing has to be set, > I'm not about to second such a motion. Our main idea with this is to help people report missed vectorization on their code, and a way to help them achieve performance while LLVM doesn't catch up. Another case for this (and other pragmas controlling the optimization level on a per-function basis) is to help debugging of specific functions while leaving others untouched. I'd not condone the usage of such pragmas on a persistent manner, nor for any code that goes in production, or to work around broken code at higher optimization levels. cheers, --renato
