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From: "pali at kernel dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/109317] New: -Os generates bigger code than -O2 on
 32-bit ARM
Date: Tue, 28 Mar 2023 16:27:11 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 12.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: pali at kernel dot org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109317

            Bug ID: 109317
           Summary: -Os generates bigger code than -O2 on 32-bit ARM
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pali at kernel dot org
  Target Milestone: ---
            Target: arm-linux-gnueabi

Simple loops like the one in the example below are better optimized for siz=
e on
32-bit ARM with -O2 option than -Os option.


$ cat test-arm.c
char *test1(char *ptr) {
  while (*ptr !=3D '\0' || *(ptr+1) !=3D '\0') ptr++;
  ptr++;
  return ptr;
}



$ arm-linux-gnueabi-gcc -O2 -c test-arm.c && arm-linux-gnueabi-objdump -d
test-arm.o

test-arm.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <test1>:
   0:   e5d02000        ldrb    r2, [r0]
   4:   e2800001        add     r0, r0, #1
   8:   e3520000        cmp     r2, #0
   c:   1afffffb        bne     0 <test1>
  10:   e5d02000        ldrb    r2, [r0]
  14:   e3520000        cmp     r2, #0
  18:   1afffff8        bne     0 <test1>
  1c:   e12fff1e        bx      lr



$ arm-linux-gnueabi-gcc -Os -c test-arm.c && arm-linux-gnueabi-objdump -d
test-arm.o

test-arm.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <test1>:
   0:   e1a03000        mov     r3, r0
   4:   e5d32000        ldrb    r2, [r3]
   8:   e2800001        add     r0, r0, #1
   c:   e3520000        cmp     r2, #0
  10:   1afffffa        bne     0 <test1>
  14:   e5d02000        ldrb    r2, [r0]
  18:   e3520000        cmp     r2, #0
  1c:   1afffff7        bne     0 <test1>
  20:   e12fff1e        bx      lr



$ arm-linux-gnueabi-gcc -v
Using built-in specs.
COLLECT_GCC=3Darm-linux-gnueabi-gcc
COLLECT_LTO_WRAPPER=3D/usr/lib/gcc-cross/arm-linux-gnueabi/12/lto-wrapper
Target: arm-linux-gnueabi
Configured with: ../src/configure -v --with-pkgversion=3D'Debian 12.2.0-14'
--with-bugurl=3Dfile:///usr/share/doc/gcc-12/README.Bugs
--enable-languages=3Dc,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=3D/usr
--with-gcc-major-version-only --program-suffix=3D-12 --enable-shared
--enable-linker-build-id --libexecdir=3D/usr/lib --without-included-gettext
--enable-threads=3Dposix --libdir=3D/usr/lib --enable-nls --with-sysroot=3D/
--enable-clocale=3Dgnu --enable-libstdcxx-debug --enable-libstdcxx-time=3Dy=
es
--with-default-libstdcxx-abi=3Dnew --enable-gnu-unique-object --disable-lib=
itm
--disable-libquadmath --disable-libquadmath-support --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=3Drelea=
se
--without-target-system-zlib --enable-multiarch --disable-sjlj-exceptions
--with-arch=3Darmv5te --with-float=3Dsoft --disable-werror
--enable-checking=3Drelease --build=3Dx86_64-linux-gnu --host=3Dx86_64-linu=
x-gnu
--target=3Darm-linux-gnueabi --program-prefix=3Darm-linux-gnueabi-
--includedir=3D/usr/arm-linux-gnueabi/include
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (Debian 12.2.0-14)=