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'