On Mon, 10 Jul 2023, Jan Hubicka wrote: > Hi, > over weekend I found that vectorizer is missing scale_loop_profile for > epilogues. It already adjusts loop_info to set max iteraitons, so > adding it was easy. However now predicts the first loop to iterate at > most once (which is too much, I suppose it forgets to divide by epilogue > unrolling factor) and second never. > > > > The -O2 cost model doesn't want to do epilogues: > > > > /* If using the "very cheap" model. reject cases in which we'd keep > > a copy of the scalar code (even if we might be able to vectorize it). > > */ > > if (loop_cost_model (loop) == VECT_COST_MODEL_VERY_CHEAP > > && (LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo) > > || LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo) > > || LOOP_VINFO_PEELING_FOR_NITER (loop_vinfo))) > > { > > if (dump_enabled_p ()) > > dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location, > > "some scalar iterations would need to be > > peeled\n"); > > return 0; > > } > > > > it's because of the code size increase. > > I know, however -O2 is not -Os and here the tradeoffs of > performance/code size seems a lot better than other code expanding > things we do at -O2 (such as the unrolling 3 times). > I think we set the very cheap cost model very conservatively in order to > get -ftree-vectorize enabled with -O2 and there is some room for finding > right balance. > > I get: > > jan@localhost:~> cat t.c > int a[99]; > __attribute((noipa, weak)) > void > test() > { > for (int i = 0 ; i < 99; i++) > a[i]++; > } > void > main() > { > for (int j = 0; j < 10000000; j++) > test(); > } > jan@localhost:~> gcc -O2 t.c -fno-unroll-loops ; time ./a.out > > real 0m0.529s > user 0m0.528s > sys 0m0.000s > > jan@localhost:~> gcc -O2 t.c ; time ./a.out > > real 0m0.427s > user 0m0.426s > sys 0m0.000s > jan@localhost:~> gcc -O3 t.c ; time ./a.out > > real 0m0.136s > user 0m0.135s > sys 0m0.000s > jan@localhost:~> clang -O2 t.c ; time ./a.out > <warnings> > > real 0m0.116s > user 0m0.116s > sys 0m0.000s > > Code size (of function test): > gcc -O2 -fno-unroll-loops 17 bytes > gcc -O2 29 bytes > gcc -O3 50 bytes > clang -O2 510 bytes > > So unroling 70% code size growth for 23% speedup. > Vectorizing is 294% code size growth for 388% speedup > Clang does 3000% codde size growth for 456% speedup > > > > That's clearly much larger code. On x86 we're also fighting with > > large instruction encodings here, in particular EVEX for AVX512 is > > "bad" here. We hardly get more than two instructions decoded per > > cycle due to their size. > > Agreed, I found it surprising clang does that much of complette unrolling > at -O2. However vectorizing and not unrolling here seems like it may be > a better default for -O2 than what we do currently...
I was also playing with AVX512 fully masked loops here which avoids the epilogue but due to the instruction encoding size that doesn't usually win. I agree that size isn't everything at least for -O2. Richard.