On 16 February 2014 17:23, Tobias Burnus <bur...@net-b.de> wrote: > As '#pragma omp simd' doesn't generate any threads and doesn't call the > OpenMP run-time library (libgomp), I would claim that it only controls the > tree vectorizer. (Hence, -fopenmp-simd was added as it permits this control > without enabling thread parallelization or dependence on libgomp or > libpthread.)
Right, this is a bit confusing, but should suffice for out purposes, which are very similar to GCC's. > Compiler vendors (and users) have different ideas whether the SIMD pragmas > should give the compiler only a hint or completely override the compiler's > heuristics. In case of the Intel compiler, the user rules; in case of GCC, > it only influences the heuristics unless one passes explicitly > -fsimd-cost-model=unlimited (cf. also -Wopenmp-simd). We prefer to be on the safe side, too. We're adding a warning callback mechanism to warn about possible dangerous situations (debug messages already do that), possibly with the same idea as -Wopenmp-simd. But the intent is to not vectorize if we're sure it'll break things. Only on doubt we'll trust the pragmas/flags. The flag -fsimd-cost-model=unlimited might be a bit too heavy on other loops, and is the kind of think that I'd rather have as a pragma or not at all. > As a user, I found Intel's pragmas interesting, but at the end regarded > OpenMP's SIMD directives/pragmas as sufficient. That was the kind of user experience that I was looking for, thanks! > According to http://gcc.gnu.org/projects/tree-ssa/vectorization.html, > basic-block vectorization (SLP) support exists since 2009. Would it be desirable to use some pragmas to control lexical blocks, too? I'm not sure omp/cilk pragmas apply to lexical blocks... cheers, --renato
