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From: "mike.robins at talktalk dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/99937] New: Optimization needed for ARM with single cycle
 multiplier
Date: Tue, 06 Apr 2021 14:02:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99937

            Bug ID: 99937
           Summary: Optimization needed for ARM with single cycle
                    multiplier
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mike.robins at talktalk dot net
  Target Milestone: ---

Created attachment 50512
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50512&action=3Dedit
Source file(s)

I am cross-compiling using "arm-none-eabi-gcc -mcpu=3Dcortex-m0plus -Wall -=
Wextra
-fno-strict-aliasing -fwrapv -O3 -S foobar.c" for a target architecture that
performs a multiply in a single cycle, using gcc version 10.2.0 on a PC run=
ning
Fedora Linux.

Is there an option to persuade the compiler to use the multiply instruction
automatically instead of shifts and adds when multiplying by a constant?

In the example code attached, gcc uses the trick of multiplying by a big nu=
mber
instead of dividing by a small one (12 in this case). For my target, the co=
de
from "-O3" is both longer and slower then that for "-Os".=