Inline assembly - how to get gcc to clear the full rcx register in x86-64 mode

[Jeroen van Bemmel <jbemmel@xxxxxxxxx>]

Hi,

I have ported an SSE4 strcmp function from 
http://www.strchr.com/strcmp_and_strlen_using_sse_4.2
to GCC inline assembly:

long __res;
__asm__ __volatile__(
        "sub        $16, %4                    \n"
        "1:\n"
        "add        $16, %4                     \n"
        "movdqu        (%4), %%xmm0 \n"    // Could use any XMM, using 
register constraint "x"
        // ".byte 0x48                           \n"    // REX prefix 
with REX.w=1, to get result in RCX
        "pcmpistri    $0x18, (%4,%0), %%xmm0  \n"    // 
EQUAL_EACH(0x08) + NEGATIVE_POLARITY(0x10)
        "ja 1b                                \n"
        "jc 2f                                 \n"
        "xor %0, %0                     \n"
        "jmp 3f                              \n"    // XXX Extra jump 
could be avoided in pure asm
        "2:\n"
        "add %4, %0                     \n"
        "movzxb (%0,%1), %0      \n"
        "movzxb (%4,%1), %4      \n"
        "sub %4, %0                     \n"
        "3:\n"
    : "=a"(__res), "=c"(cs) : "0"(cs-ct), "1"(0L), "r"(ct) : "xmm0" );

    return (int) __res;

The problem with this code is that "pcmpistri" returns its result in ECX 
(i.e. the lower 32 bits of RCX), while the "movzxb" instructions use the 
full RCX register.
One solution is to insert a REX prefix with REX.w bit set ( any gas 
directive for this? )

However, I'd prefer to have gcc clear RCX at the beginning of the 
function. The above code loads the "c" register with 0, but the 
resulting asm code is
"xorl    ecx, ecx"

Is this a bug in GCC? Or how do I get it to clear the full RCX, without 
doing it 'manually' in the asm block?

Thanks,
Jeroen