[x264-devel] [bug] asm operand has impossible constraints (x86)

[Sérgio Basto]

Hi,
thank you for share your acknowledgment .

On Qua, 2015-08-12 at 02:20 +0000, Loren Merritt wrote:
> On Mon, 10 Aug 2015, Johannes Dewender wrote:
> 
> > when compiling the 32 bit package of libx264 (on a 64 bit machine) I
> > get (current git master):
> >
> > yasm -I. -I. -DARCH_X86_64=0 -I./common/x86/ -f elf32 -Worphan-labels
> > -DSTACK_ALIGNMENT=32 -DPIC -DHIGH_BIT_DEPTH=0 -DBIT_DEPTH=8 -o
> > common/x86/pixel-32.o common/x86/pixel-32.asm
> 
> Btw, this isn't the command that produced the error. You're probably
> using multithreaded make, which means that compilation messages aren't
> necessarily displayed in order.
> 
> > In file included from ./common/common.h:1014:0,
> >                  from encoder/me.c:28:
> > encoder/me.c: In function 'x264_me_search_ref':
> > ./common/x86/util.h:130:5: error: 'asm' operand has impossible constrain
> > ts
> >      asm(
> >       ^
> > ./common/x86/util.h:193:5: error: 'asm' operand has impossible constrain
> > ts
> >      asm(
> >       ^
> > <builtin>: recipe for target 'encoder/me.o' failed
> >
> > The problem appears in building the Arch Linux 32 bit package
> > "lib32-libx264":
> > https://aur.archlinux.org/packages/lib32-libx264/
> >
> > Built on a 64 bit machine with:
> > ./configure --prefix=/usr --libdir=/usr/lib32 --host=i686-linux-gnu \
> >         --enable-shared \
> >         --enable-pic
> > make
> >
> > The 64 bit build runs fine on the same machine, but that isn't using
> > files out of common/x86/, I guess.
> 
> The 64bit build does use the same code, it's just that it has more
> registers available, so the same constraint is easier to solve.

So , you are clearly saying that we should enable asm in _all_ arches ? 
 
> > Arch Linux always ships the newest (stable) versions of all software
> > so the problem is probably related to a new version of important build
> > tools.
> > However, I couldn't pinpoint the exact update generating the problem.
> >
> > The complete build log is in
> > https://gist.github.com/JonnyJD/6aa13527e8b98055a1d0
> >
> > This is the list of packages and versions updated (breaking the build):
> > https://gist.github.com/JonnyJD/22b65e6101d0e971ee4d
> > (It worked before the Arch Linux update)
> >
> > I can't tell if that is a bug in the build tools or in x264 and I
> > don't have pratical experience in assembler.
> 
> It's a gcc bug.
> More precisely: Allocating registers to inline-asm blocks is an NP Hard
> optimization problem. N is sufficiently small that "NP Hard" doesn't imply
> "intractable", but it still requires a certain amount of intelligence on
> the compiler's part. That's true of ordinary C code too. The difference is
> that in C code, a suboptimal compilation is just a bit slower than it
> could have been, whereas in inline-asm it can result in the compiler
> incorrectly throwing an "impossible constraints" error.
> 
> I have verified that the constraints are not in fact impossible. But if
> you want to make it easier on the compiler, you could disable pic.

Thanks,
-- 
Sérgio M. B.