https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121340
--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-15 branch has been updated by Georg-Johann Lay <g...@gcc.gnu.org>: https://gcc.gnu.org/g:fc03165fb8829a78224882d053e7850132bfb9c9 commit r15-10180-gfc03165fb8829a78224882d053e7850132bfb9c9 Author: Georg-Johann Lay <a...@gjlay.de> Date: Wed Jul 30 12:34:56 2025 +0200 AVR: rtl-optimization/121340 - New mini-pass to undo superfluous moves from insn combine. Insn combine may come up with superfluous reg-reg moves, where the combine people say that these are no problem since reg-alloc is supposed to optimize them. The issue is that the lower-subreg pass sitting between combine and reg-alloc may split such moves, coming up with a zoo of subregs which are only handled poorly by the register allocator. This patch adds a new avr mini-pass that handles such cases. As an example, take int f_ffssi (long x) { return __builtin_ffsl (x); } where the two functions have the same interface, i.e. there are no extra moves required for the argument or for the return value. However, $ avr-gcc -S -Os -dp -mno-fuse-move ... f_ffssi: mov r20,r22 ; 29 [c=4 l=1] movqi_insn/0 mov r21,r23 ; 30 [c=4 l=1] movqi_insn/0 mov r22,r24 ; 31 [c=4 l=1] movqi_insn/0 mov r23,r25 ; 32 [c=4 l=1] movqi_insn/0 mov r25,r23 ; 33 [c=4 l=4] *movsi/0 mov r24,r22 mov r23,r21 mov r22,r20 rcall __ffssi2 ; 34 [c=16 l=1] *ffssihi2.libgcc ret ; 37 [c=0 l=1] return where all the moves add up to a no-op. The -mno-fuse-move option stops any attempts by the avr backend to clean up that mess. PR rtl-optimization/121340 gcc/ * config/avr/avr.opt (-mfuse-move2): New option. * config/avr/avr-passes.def (avr_pass_2moves): Insert after combine. * config/avr/avr-passes.cc (make_avr_pass_2moves): New function. (pass_data avr_pass_data_2moves): New static variable. (avr_pass_2moves): New rtl_opt_pass. * config/avr/avr-protos.h (make_avr_pass_2moves): New proto. * common/config/avr/avr-common.cc (default_options avr_option_optimization_table) <-mfuse-move2>: Set for -O1 and higher. * doc/invoke.texi (AVR Options) <-mfuse-move2>: Document. * config/avr/avr.opt.urls (-mfuse-move2): Add url. (cherry picked from commit 7d75e87f3051fa29d48e923016f04a8042a8ec53)
