[Bug pending/27986] New: unnecessary jump to middle of loop on entry

["dean at arctic dot org" <gcc-bugzilla at gcc dot gnu dot org>]

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