[Bug target/114100] [avr] Inefficient indirect addressing on Reduced Tiny

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114100

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Georg-Johann Lay <g...@gcc.gnu.org>:

https://gcc.gnu.org/g:cda3836161834c5f21f264885891fe4d0ce90da1

commit r14-9244-gcda3836161834c5f21f264885891fe4d0ce90da1
Author: Georg-Johann Lay <a...@gjlay.de>
Date:   Thu Feb 29 17:19:27 2024 +0100

    AVR: target/114100 - Better indirect accesses for reduced Tiny

    The Reduced Tiny core does not support indirect addressing with offset,
    which basically means that every indirect memory access with a size
    of more than one byte is effectively POST_INC or PRE_DEC.  The lack of
    that addressing mode is currently handled by pretending to support it,
    and then let the insn printers add and subtract again offsets as needed.
    For example, the following C code

       int vars[10];

       void inc_var2 (void) {
          ++vars[2];
       }

    is compiled to:

       ldi r30,lo8(vars)     ;  14   [c=4 l=2]  *movhi/4
       ldi r31,hi8(vars)
       subi r30,lo8(-(4))    ;  15   [c=8 l=6]  *movhi/2
       sbci r31,hi8(-(4))
       ld r20,Z+
       ld r21,Z
       subi r30,lo8((4+1))
       sbci r31,hi8((4+1))
       subi r20,-1             ;  16   [c=4 l=2]  *addhi3_clobber/1
       sbci r21,-1
       subi r30,lo8(-(4+1))    ;  17   [c=4 l=4]  *movhi/3
       sbci r31,hi8(-(4+1))
       st Z,r21
       st -Z,r20

    where the code could be -- and with this patch actually is -- like

       ldi r30,lo8(vars+4)    ;  28   [c=4 l=2]  *movhi/4
       ldi r31,hi8(vars+4)
       ld r20,Z+              ;  17   [c=8 l=2]  *movhi/2
       ld r21,Z+
       subi r20,-1            ;  19   [c=4 l=2]  *addhi3_clobber/1
       sbci r21,-1
       st -Z,r21              ;  30   [c=4 l=2]  *movhi/3
       st -Z,r20

    This is achieved in two steps:

    - A post-reload split into "real" instructions during .split2.
    - A new avr-specific mini pass .avr-fuse-add that runs before
      RTL peephole and that tries to combine the generated pointer
      additions into memory accesses to form POST_INC or PRE_DEC.

    gcc/
            PR target/114100
            * doc/invoke.texi (AVR Options) <-mfuse-add>: Document.
            * config/avr/avr.opt (-mfuse-add=): New target option.
            * common/config/avr/avr-common.cc (avr_option_optimization_table)
            [OPT_LEVELS_1_PLUS]: Set -mfuse-add=1.
            [OPT_LEVELS_2_PLUS]: Set -mfuse-add=2.
            * config/avr/avr-passes.def (avr_pass_fuse_add): Insert new pass.
            * config/avr/avr-protos.h (avr_split_tiny_move)
            (make_avr_pass_fuse_add): New protos.
            * config/avr/avr.md [AVR_TINY]: New post-reload splitter uses
            avr_split_tiny_move to split indirect memory accesses.
            (gen_move_clobbercc): New define_expand helper.
            * config/avr/avr.cc (avr_pass_data_fuse_add): New pass data.
            (avr_pass_fuse_add): New class from rtl_opt_pass.
            (make_avr_pass_fuse_add, avr_split_tiny_move): New functions.
            (reg_seen_between_p, emit_move_ccc, emit_move_ccc_after): New
functions.
            (avr_legitimate_address_p) [AVR_TINY]: Don't restrict offsets
            of PLUS addressing for AVR_TINY.
            (avr_regno_mode_code_ok_for_base_p) [AVR_TINY]: Ignore -mstrict-X.
            (avr_out_plus_1) [AVR_TINY]: Tweak ++Y and --Y.
            (avr_mode_code_base_reg_class) [AVR_TINY]: Always return
POINTER_REGS.