[Bug middle-end/122871] [13/14/15/16/17 Regression] de-optimized synthesis of long long shift and add

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122871

--- Comment #17 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:373e2c30b1710e271664cf95fcfb66fea7dd15ed

commit r17-1373-g373e2c30b1710e271664cf95fcfb66fea7dd15ed
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Fri Jun 5 14:53:27 2026 +0100

    PR target/56102: Improve rtx_costs from -mthumb on ARM.

    This patch provides improved (more accurate) RTX costs for -mthumb on ARM.
    My recent patch for double word multiplication, PR 122871, revealed that
    the current costs for THUMB code on ARM are... let's say a little dubious.

    To demonstrate the code generation improvements provided by better
    thumb1_rtx_costs consider the function below (from PR middle-end/122871).

    long long foo (long long a)
    {
      long long c = a << 33;
      c += a;
      return c;
    }

    With the ARM backend's current costs, this produces 11 instructions with
    -O2 -mthumb.

    Before: movs    r3, r0
            movs    r2, #0
            adds    r2, r2, r0
            adcs    r3, r3, r1
            adds    r2, r2, r2
            adcs    r3, r3, r3
            subs    r2, r2, r0
            sbcs    r3, r3, r1
            movs    r0, r2
            movs    r1, r3
            bx      lr

    With sane RTX costs, GCC now generate the much more reasonable 5 insns:

    After:  movs    r2, #0
            lsls    r3, r0, #1
            adds    r0, r0, r2
            adcs    r1, r1, r3
            bx      lr

    2026-06-05  Roger Sayle  <roger@nextmovesoftware.com>
                Richard Earnshaw  <richard.earnshaw@arm.com>

    gcc/ChangeLog
            PR target/56102
            PR middle-end/122871
            * config/arm/arm.cc (thumb1_rtx_costs): Provide reasonable costs
            for PLUS, MINUS, COMPARE, AND, XOR, IOR, NEG, NOT, ASHIFT,
            ASHIFTRT and ROTATERT for SImode, DImode, HImode and QImode.
            (thumb1_size_rtx_costs): Likewise.

            (comp_not_to_clear_mask_str_un): Silence host compiler warning.