https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119702
--- Comment #17 from Avinash Jayakar <avinashd at linux dot ibm.com> --- Created attachment 62077 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=62077&action=edit Proposed patch for using add when vectorizing << 1 I looked at the slp vectorization pass that converts scalar gimple code to vectorized gimple. Analysis happens in vect_slp_analyze_bb_1 before actually scheduling in vect_schedule_slp. There are multiple patterns written here to optimize simple operations such as *2 to <<1 in vect_recog_mult_pattern. I have added a pattern just for detecting left shift by one and replacing it by add in vect_recog_lshift_by_one_pattern. Either this can be done, or I can move this logic in a shift pattern (vect_recog_widen_shift_pattern or vect_recog_vector_vector_shift_pattern). This does fix the original issue, where <<1 generates 2 instructions. With this patch it just generates 1 add instruction when code is vectorized. But other cases like *2 and a = a+a, is not handled right now. @Segher, I had a few questions on this - Do you suggest moving ahead in this direction? Since here I am manipulating the GIMPLE, it will affect different architectures as well, would this be ok? (In reply to Segher Boessenkool from comment #15) > Just have it recognised by a define_insn that generates an addition insn > when generating assembler code. You know... the same as always :-) > - Thank you for this suggestion, I did give this a try but ran into a few issues. Is there a way in define_insn to detect that one of the operand in rtl is dead? Because we need to be sure that const_1 is not used anywhere further before replacing the the 2 rtl insns (splat and shift), with just 1 (plus). I checked the define_peephole2, it provides a way to check if an operand is dead. Would using the peephole pass for this make sense?
