[vlc-devel] Re: CVS Commit (gbazin)

[Gildas Bazin]

On Tuesday 10 December 2002 10:30, Sam Hocevar wrote:
> On Tue, Dec 10, 2002, cvs at videolan.org wrote:
> 
> > * include/configuration.h: rolled back the last changes made to this 
file.
> > There is no sense in manually initializing all the fields of 
module_config_t
> > as it will be done by the compiler anyway. Furthermore mingw32 gcc seems 
to
> > have problems optimizing code that manually initializes structures to 0.
> 
>    Uh? Why would any optimizations be needed at all here? Could you
> elaborate on the real problem?
> 

Ok, I'll give you the code gcc outputs for the 2 different cases.

1 - "module_config_t tmp = { CONFIG_HINT_CATEGORY, NULL, NULL, '\0', text, 
longtext };" :

LC3:
        .long   2              /* CONFIG_HINT_CATEGORY */
        .long   0
        .long   0
        .byte   0
        .space 3
        .long   LC2          /* text */
        .long   0              /* longtext */
        .space 28

This exactly what you would have expected.

2 - "module_config_t tmp = { CONFIG_HINT_CATEGORY, NULL, NULL, '\0', text, 
longtext, 0, 0 };" :

.globl _vlc_entry__0_5_0_cvs
        .def    _vlc_entry__0_5_0_cvs;  .scl    2;      .type   32;     
.endef
_vlc_entry__0_5_0_cvs:
        blablabla
        ...
        movl    $13, %ecx
        movl    %edx, %edi
        movl    $2, 32(%esp)                  /* CONFIG_HINT_CATEGORY */
        movl    $LC2, 48(%esp)              /* text */
        rep
        movsl

Strange no ? And the problem disappears with "module_config_t tmp = { 
CONFIG_HINT_CATEGORY, NULL, NULL, '\0', text, longtext, 0, 1 };"

I honestly don't understand why gcc is making this optimization (well it 
seems to save some space in the final object because it puts things in the 
stack, but still...) but this seems to be a pretty old behaviour as this is 
what gcc-2.95 and gcc-3.2 do.

Regards,

--
Gildas
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