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'
