------- Comment #10 from xinliangli at gmail dot com 2008-02-19 06:00 -------
(In reply to comment #0)
> 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)
>
// David Li
Note that assignment of s0 = s in the loop is mostly dead except for the last
occurence. So it should be optimized into:
do {
s += *d;
++d;
} while (s < g);
v[0] = (s-*(d-1));
--
xinliangli at gmail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |xinliangli at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27986
