[FFmpeg-cvslog] r21432 - trunk/libavcodec/h264_cavlc.c

[Michael Niedermayer]

On Sun, Jan 24, 2010 at 11:59:25PM +0000, M?ns Rullg?rd wrote:
> michael <subversion at mplayerhq.hu> writes:
> 
> > Author: michael
> > Date: Sun Jan 24 19:42:22 2010
> > New Revision: 21432
> >
> > Log:
> > a[b-1] -> (a-1)[b].
> > Helps gcc not to add seperate -1 instructions.
> 
> Do you have some asm to illustrate?

from h264_cavlc.c with this patch
and thats with gcc-4.4 (Debian 4.4.3-1) 4.4.3

before:

        movl    %eax, %edx
        movl    %eax, %ecx
        leal    -1(%rbp), %r10d          <-----------
        shrl    $3, %edx
        andl    $7, %ecx
        mov     %edx, %edx
        movl    (%r11,%rdx), %edx
        bswap   %edx
        sall    %cl, %edx
        movslq  %r10d,%rcx               <-----------
        leaq    (%rcx,%rcx,2), %r12      <-----------
        shrl $-9, %edx
        mov     %edx, %edx
        leaq    0(,%rdx,4), %rcx
        addq    total_zeros_vlc+8(,%r12,8), %rcx

after:
        movl    %eax, %edx
        movl    %eax, %ecx
        shrl    $3, %edx
        andl    $7, %ecx
        mov     %edx, %edx
        movl    (%r11,%rdx), %edx
        bswap   %edx
        sall    %cl, %edx
        movslq  %ebp,%rcx             <-------------
        leaq    -3(%rcx,%rcx,2), %r10 <-------------
        shrl $-9, %edx
        mov     %edx, %edx
        leaq    0(,%rdx,4), %rcx
        addq    total_zeros_vlc+8(,%r10,8), %rcx

you can also see 2 of the 
mov     %edx, %edx
in there that i spoke off

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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