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From: "willy at w dot ods dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug optimization/11873] New: inefficient use of registers induces size and time overhead Date: Sun, 10 Aug 2003 08:39:00 -0000 [thread overview] Message-ID: <20030810083857.11873.willy@w.ods.org> (raw) PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11873 Summary: inefficient use of registers induces size and time overhead Product: gcc Version: 3.3.1 Status: UNCONFIRMED Severity: enhancement Priority: P2 Component: optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: willy at w dot ods dot org CC: gcc-bugs at gcc dot gnu dot org GCC build triplet: i586-linux-gnu GCC host triplet: i586-linux-gnu GCC target triplet: i586-linux-gnu I wrote a simple test function which uses an unsigned long long arg and returns its integer part + 1 if not 0, otherwise 0. GCC 3.3.1 passes some intermediate values through several registers while it's unneeded. Here comes all my analysis. I hope this can help improving the optimizer. The following code : int test(unsigned long long x) { if (x) { return (int)x + 1; } else { return (int)x; } } processed this way : # 1 "bool.c" # 1 "<built-in>" # 1 "<command line>" # 1 "bool.c" int test(unsigned long long x) { if (x) { return (int)x + 1; } else { return (int)x; } } produces the following code when compiled with gcc 2.95.3 : (gcc -c -O2 -fomit-frame-pointer bool.c) 00000000 <test>: 0: 8b 54 24 04 mov 0x4(%esp,1),%edx 4: 8b 4c 24 08 mov 0x8(%esp,1),%ecx 8: 89 d0 mov %edx,%eax a: 09 c8 or %ecx,%eax c: 75 03 jne 11 <test+0x11> e: 89 d0 mov %edx,%eax 10: c3 ret 11: 8d 42 01 lea 0x1(%edx),%eax 14: c3 ret 15: 8d 76 00 lea 0x0(%esi),%esi 18: and this one with gcc-3.3.1 : (gcc-3.3.1 -c -O2 -fomit-frame-pointer bool.c) 00000000 <test>: 0: 53 push %ebx 1: 8b 4c 24 0c mov 0xc(%esp,1),%ecx 5: 8b 54 24 08 mov 0x8(%esp,1),%edx 9: 89 cb mov %ecx,%ebx b: 89 d0 mov %edx,%eax d: 09 d3 or %edx,%ebx f: 74 03 je 14 <test+0x14> 11: 8d 42 01 lea 0x1(%edx),%eax 14: 5b pop %ebx 15: c3 ret 16: 8d 76 00 lea 0x0(%esi),%esi 19: 8d bc 27 00 00 00 00 lea 0x0(%edi,1),%edi 20: the EBX register is used and clobbered for nothing here. The same code could be written this way, which is fully equivalent and saves some cycles and bytes : 00000000 <test>: 0: 8b 4c 24 0c mov 0xc(%esp,1),%ecx 4: 8b 54 24 08 mov 0x8(%esp,1),%edx 8: 89 d0 mov %edx,%eax a: 09 d1 or %edx,%ecx c: 74 03 je 11 <test+0x11> e: 8d 42 01 lea 0x1(%edx),%eax 11: c3 ret 12: Now we can also save EDX and some more bytes : 00000000 <test>: 0: 8b 4c 24 0c mov 0xc(%esp,1),%ecx 4: 8b 54 24 08 mov 0x8(%esp,1),%eax a: 09 c1 or %eax,%ecx c: 74 01 je f <test+0xf> e: 41 inc %eax f: c3 ret 10: Here are the compilers versions : wtap:~/dev$ gcc -v Reading specs from /usr/lib/gcc-lib/i586-pc-linux-gnu/2.95.3/specs gcc version 2.95.3 20010315 (release) wtap:~/dev$ gcc-3.3.1 -v Reading specs from /usr/lib/gcc-lib/i586-pc-linux-gnu/3.3.1/specs Configured with: ./configure --prefix=/usr --with-cpu=i386 --host=i586-pc-linux-gnu --enable-languages=c,c++ --disable-nls --disable-locale --enable-shared --enable-target-optspace --enable-version-specific-runtime-libs --program-suffix=-3.3.1 --enable-threads Thread model: posix gcc version 3.3.1
next reply other threads:[~2003-08-10 8:39 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2003-08-10 8:39 willy at w dot ods dot org [this message] 2003-08-10 13:52 ` [Bug optimization/11873] " pinskia at gcc dot gnu dot org 2003-08-23 1:15 ` dhazeghi at yahoo dot com 2004-06-03 4:27 ` [Bug rtl-optimization/11873] " pinskia at gcc dot gnu dot org 2004-06-03 4:29 ` pinskia at gcc dot gnu dot org 2004-11-08 23:44 ` pinskia at gcc dot gnu dot org 2004-11-13 6:01 ` pinskia at gcc dot gnu dot org
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