On Tue, 23 Jan 2024, Jakub Jelinek wrote: > On Tue, Jan 23, 2024 at 01:03:46PM +0100, Richard Biener wrote: > > On Tue, 23 Jan 2024, Jakub Jelinek wrote: > > > > > On Tue, Jan 23, 2024 at 12:56:52PM +0100, Richard Biener wrote: > > > > The following avoids using exact_log2 on the number of SIMD clone calls > > > > to be emitted when vectorizing calls since that can easily be not > > > > a power of two in which case it will return -1. For different simd > > > > clones the number of calls will differ by a multiply with a power of two > > > > only so using floor_log2 is good enough here. > > > > > > > > Bootstrap and regtest running on x86_64-unknown-linux-gnu. > > > > > > > > PR tree-optimization/113552 > > > > * tree-vect-stmts.cc (vectorizable_simd_clone_call): Use > > > > floor_log2 instead of exact_log2 on the number of calls. > > > > > > Is there any target which supports non-power-of-two simdlen? > > > If not, perhaps we should add !pow2p_hwi (num_calls) to the continue; > > > condition a few lines earlier? > > > > Is non-power-of-two simdlen a thing? Note there's nothing wrong > > with non-power-of-two num_calls, with VF == 4 and a group size > > of 3 you get 12 lanes and either 3 (simdlen == 4) or 6 (simdlen == 2) > > calls. > > > > Iff non-power-of-two simdlen is a thing then we could bias > > by + num_calls (no idea why we use *_log2 in the first place, but it > > was that way since the beginning). > > Ah, with SLP I can understand it doesn't have to be a power of two, > the original > + if (n->simdclone->simdlen > + < (unsigned) LOOP_VINFO_VECT_FACTOR (loop_vinfo)) > + this_badness += (exact_log2 (LOOP_VINFO_VECT_FACTOR (loop_vinfo)) > + - exact_log2 (n->simdclone->simdlen)) * 1024; > was written for loop vectorization only and I think correctly assumed > power of 2 loop vectorization factors as well as simdlens.
Note even without SLP we could vectorize a group size of 3 with interleaving. But yes, VF for interleaving is a power of two and so is simdlen so the above should have worked. > I admit I don't remember why log2 rather than the count has been used, > the first version of the patch is > https://gcc.gnu.org/pipermail/gcc-patches/2013-November/374728.html > But if we keep using log2, perhaps better ceil_log2 because num_calls of > 3 is certainly more expensive than 2. I think only the relative accounting to the other this_badness increments matter since as said, at least with power-of-two simdlen we'd get different {ceil,floor}_log2 values for different simdlen. It's never going to be 3 vs 2 but 3 * 2^n vs. 3 * 2^m so floor or ceil doesn't matter. In fact we could have just using some inverse of exact_log2 (n->simdclone->simdlen). That is, it's only simdlen that's varying in this addend to this_badness. For the exact_log2 case the behavior is unchanged and we now only get a sensible number for the others now. Maybe log2 was for the fear of overflow or over-accounting compared to the other increments. When overflow is not an issue we could also use floor_log2 (num_calls * 4096) to be more "precise". I don't know why we have all these different weights and why they are the way they are - but it's a lot of apples and oranges we put together in a magic number to compare ;) I prefer to do a minimal change now, but both floor and ceil work for me, so if you prefer I can change (but as said, it would only matter with the mixing with the other cost factors, and in unknown ways since the desire is not spelled out). Richard.
