[Bug target/64215] New: -Os misses an opportunity to merge two ret instructions

["mpeg.blue at free dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64215

            Bug ID: 64215
           Summary: -Os misses an opportunity to merge two ret
                    instructions
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpeg.blue at free dot fr

The following code:

int find_mismatch(const char *s, const char *t)
{
  int n;
  for (n = 0; s[n] == t[n]; ++n) if (s[n] == 0) return -1;
  return n;
}

optimized for space, by version 4.8.2-19ubuntu1,
produces the following amd64 assembly code:

0000000000000000 <find_mismatch>:
   0:    31 c0                    xor    %eax,%eax
   2:    8a 14 07                 mov    (%rdi,%rax,1),%dl
   5:    3a 14 06                 cmp    (%rsi,%rax,1),%dl
   8:    75 0b                    jne    15 <find_mismatch+0x15>
   a:    48 ff c0                 inc    %rax
   d:    84 d2                    test   %dl,%dl
   f:    75 f1                    jne    2 <find_mismatch+0x2>
  11:    83 c8 ff                 or     $0xffffffff,%eax
  14:    c3                       retq   
  15:    c3                       retq   

The last two instructions could be merged, saving a grand total of 1 byte.


$ gcc -v -Os -c parse.c
Using built-in specs.
COLLECT_GCC=gcc
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1'
--with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) 
COLLECT_GCC_OPTIONS='-v' '-Os' '-c' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.8/cc1 -quiet -v -imultiarch x86_64-linux-gnu
parse.c -quiet -dumpbase parse.c -mtune=generic -march=x86-64 -auxbase parse
-Os -version -fstack-protector -Wformat -Wformat-security -o /tmp/ccZIGWnY.s
GNU C (Ubuntu 4.8.2-19ubuntu1) version 4.8.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.8.2, GMP version 5.1.3, MPFR version 3.1.2-p3,
MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=97 --param ggc-min-heapsize=127216
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/4.8/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C (Ubuntu 4.8.2-19ubuntu1) version 4.8.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.8.2, GMP version 5.1.3, MPFR version 3.1.2-p3,
MPC version 1.0.1
GGC heuristics: --param ggc-min-expand=97 --param ggc-min-heapsize=127216
Compiler executable checksum: dc75e0628c9356affcec059d0c81cc01
COLLECT_GCC_OPTIONS='-v' '-Os' '-c' '-mtune=generic' '-march=x86-64'
 as -v --64 -o parse.o /tmp/ccZIGWnY.s
GNU assembler version 2.24 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.24
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.8/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-Os' '-c' '-mtune=generic' '-march=x86-64'