Inline assembler: register clobber

Hi, first of all: sorry for my bad english.

I have a arm1136 based SOC, and I'm playing with the inline assembler.
Please, look at the following code:

void function() {
  asm volatile("mov r8, r0\n"
               : : : "r8");
}

Due to ARM calling convention r8 must be saved before the asm statment and after it must be restored, right?
As expected, if I "objdump" this, r8 is saved/restored:

00000000 <function>:
   0: e92d4100 push {r8, lr}
   4: e1a08000 mov r8, r0
   8: e8bd8100 pop {r8, pc}

But if I compile this code in thumb mode (-mthumb), r8 doesn't get saved/restored:

00000000 <function>:
   0: 4680 mov r8, r0
   2: 4770 bx lr

I can't undestand this, r8 should be saved/restored in thumb mode too, where am I wrong?

This is the output of "arm-none-eabi-gcc -v":
Using built-in specs.
COLLECT_GCC=arm-none-eabi-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-none-eabi/4.8/lto-wrapper
Target: arm-none-eabi
Configured with: ../gcc-4.8.3/configure --build=x86_64-linux-gnu --prefix=/usr --includedir='/usr/lib/include' --mandir='/usr/lib/share/man' --infodir='/usr/lib/share/info' --sysconfdir=/etc --localstatedir=/var --disable-silent-rules --libexecdir='/usr/lib/lib/gcc-arm-none-eabi' --disable-maintainer-mode --disable-dependency-tracking --enable-languages=c,c++ --prefix=/usr/lib --infodir=/usr/share/doc/gcc-arm-none-eabi/info --mandir=/usr/share/man --htmldir=/usr/share/doc/gcc-arm-none-eabi/html --pdfdir=/usr/share/doc/gcc-arm-none-eabi/pdf --bindir=/usr/bin --libexecdir=/usr/lib --libdir=/usr/lib --with-system-zlib --enable-multilib --disable-decimal-float --disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath --disable-libssp --disable-libstdcxx-pch --disable-libstdc++-v3 --disable-nls --disable-shared --disable-threads --disable-tls --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=arm-none-eabi --with-gnu-as --with-gnu-ld --with-headers=no --without-newlib --with-pkgversion=4.8.3-9+11 --without-included-gettext --with-multilib-list=armv6-m,armv7-m,armv7e-m,armv7-r INHIBIT_LIBC_CFLAGS=-DUSE_TM_CLONE_REGISTRY=0 AR_FOR_TARGET=arm-none-eabi-ar AS_FOR_TARGET=arm-none-eabi-as LD_FOR_TARGET=arm-none-eabi-ld NM_FOR_TARGET=arm-none-eabi-nm OBJDUMP_FOR_TARGET=arm-none-eabi-objdump RANLIB_FOR_TARGET=arm-none-eabi-ranlib READELF_FOR_TARGET=arm-none-eabi-readelf STRIP_FOR_TARGET=arm-none-eabi-strip
Thread model: single
gcc version 4.8.3 20140820 (release) (4.8.3-9+11)

I'm in debian testing (jessie)

Thanks in advance for your help.