[FFmpeg-devel] Question about -fPIC usage for some files

[Michael Niedermayer]

On Fri, Feb 08, 2008 at 04:48:43PM -0500, Alexander Strange wrote:
> On Feb 8, 2008 4:26 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > On Fri, Feb 08, 2008 at 01:03:56PM -0800, Trent Piepho wrote:
> > > On Fri, 8 Feb 2008, Thorsten Jordan wrote:
> > > > > Why does it fail with pic for you?
> > > > the same problem that was discussed several times on this list, gcc
> > > > fails to generate the code because it runs out of registers (ebx is used
> > > > with -fPIC):
> > >
> > > Since version 3 something, gcc can use other registers besides ebx, and
> > > might not use ebx at all if the function doesn't do anything that requires
> > > access to the pic pointer.  If a function accesses no globals, does not
> > > take the address of a fuction, or call a function in another shared
> > > library, it shouldn't need to load the pic register.
> > >
> > > The real problem isn't ebx, it's accessing globals.  In non-PIC code, a
> > > memory reference to a global takes zero registers.  In PIC code, it takes
> > > one register.  In some cases multiple global references can share the same
> > > register(s), so gcc doesn't always need one per global.  But this could
> > > still easily add a half dozen extra registers to an asm block.
> >
> > [...]
> >
> > Why does it fail?
> > This is not a compiler being presented with a situation too complex to solve
> > This is a compiler failing to fit a grain of sand in a bus.
> 
> apply_welch_window_sse2 has six constraints:
>         :"+&r"(i), "+&r"(j)\
>         :"r"(w_data+n2), "r"(w_data+len-2-n2),\
>          "r"(data+n2), "r"(data+len-2-n2)\
> 
> It only fails if that's inlined; I think the VLA use is breaking it:
>     double tmp[len + lag + 2];
> 
> If you comment out everything below line 78 it still fails.

Report all this to the gcc devels not here. Noone here can help.
The error reported is in line 87 and the asm i quoted. If removial of
the asm causes the previous one to fail thats mighty suspicious.

And 6 contraints need 6 registers, 6 < 8 ! gcc even has 2 spare ones
for the stack pointer and the unneeded ebx PIC thingy.

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Its not that you shouldnt use gotos but rather that you should write
readable code and code with gotos often but not always is less readable
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