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From: "dean at arctic dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug pending/27986] New: unnecessary jump to middle of loop on entry Date: Sat, 10 Jun 2006 22:32:00 -0000 [thread overview] Message-ID: <bug-27986-5748@http.gcc.gnu.org/bugzilla/> (raw) in the following code gcc choses the registers in such a way that it causes itself an extra copy for every loop iteration and has to jump past the copy to start the loop... it's probably easiest to describe just by looking at the code: % cat jmp-over.c void foo(int *v, int *d, int g) { int s = v[1]; int s0; do { s0 = s; s += *d; ++d; } while (s < g); v[0] = s0; } % gcc -g -O3 -Wall -c -o jmp-over.o jmp-over.c % objdump -dr jmp-over.o jmp-over.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 <foo>: 0: 8b 4f 04 mov 0x4(%rdi),%ecx 3: eb 02 jmp 7 <foo+0x7> 5: 89 c1 mov %eax,%ecx 7: 89 c8 mov %ecx,%eax 9: 03 06 add (%rsi),%eax b: 48 83 c6 04 add $0x4,%rsi f: 39 d0 cmp %edx,%eax 11: 7c f2 jl 5 <foo+0x5> 13: 89 0f mov %ecx,(%rdi) 15: c3 retq the jump-over is unnecessary... mov 0x4(%rdi),%ecx 1: mov %ecx,%eax add (%rsi),%ecx add $0x4,%rsi cmp %edx,%ecx jl 1b mov %eax,(%rdi) retq -dean % gcc -v Using built-in specs. Target: x86_64-linux-gnu Configured with: ../gcc/configure --prefix=/home/odo/gcc --host=x86_64-linux-gnu --disable-multilib --enable-languages=c,c++ --disable-bootstrap Thread model: posix gcc version 4.2.0 20060603 (experimental) -- Summary: unnecessary jump to middle of loop on entry Product: gcc Version: 4.2.0 Status: UNCONFIRMED Severity: minor Priority: P3 Component: pending AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: dean at arctic dot org GCC build triplet: x86_64-linux-gnu GCC host triplet: x86_64-linux-gnu GCC target triplet: x86_64-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27986
next reply other threads:[~2006-06-10 22:23 UTC|newest] Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top 2006-06-10 22:32 dean at arctic dot org [this message] 2006-06-11 8:35 ` [Bug middle-end/27986] jump to middle of loop on entry with using old version of an variable pinskia at gcc dot gnu dot org 2006-09-03 7:19 ` [Bug middle-end/27986] [4.0/4.1/4.2 Regression] " pinskia at gcc dot gnu dot org 2006-09-07 1:14 ` mmitchel at gcc dot gnu dot org 2006-09-30 9:25 ` steven at gcc dot gnu dot org 2006-10-02 13:56 ` amacleod at redhat dot com 2006-10-02 14:01 ` amacleod at redhat dot com 2006-10-02 21:46 ` steven at gcc dot gnu dot org 2006-10-02 22:14 ` amacleod at redhat dot com 2007-02-03 17:36 ` [Bug middle-end/27986] [4.0/4.1/4.2/4.3 " gdr at gcc dot gnu dot org 2007-02-03 20:42 ` pinskia at gcc dot gnu dot org 2007-02-14 9:12 ` mmitchel at gcc dot gnu dot org 2007-03-10 1:41 ` mmitchel at gcc dot gnu dot org 2008-02-19 6:01 ` xinliangli at gmail dot com 2008-02-19 17:43 ` [Bug middle-end/27986] [4.0/4.1/4.2/4.3/4.4 " dean at arctic dot org 2008-07-04 21:23 ` [Bug middle-end/27986] [4.2/4.3/4.4 " jsm28 at gcc dot gnu dot org 2009-02-12 13:59 ` [Bug middle-end/27986] [4.2 " ubizjak at gmail dot com 2009-03-30 16:58 ` jsm28 at gcc dot gnu dot org
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