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

[Jeroen van Bemmel]

Hi Ian,

I was confused by the Intel documentation, which states that pcmpistri 
provides its result in ECX. I stepped through a debugger, and verified 
that the upper 32 bits in RCX indeed get set to 0.

I also found that while I cannot distinguish between 32-bit and 64-bit 
forms of registers in constraints, GCC will choose the optimal flavor 
for me.

Thanks,
Jeroen

On 06/13/12 18:37, Ian Lance Taylor wrote:
> Jeroen van Bemmel<jbemmel@zonnet.nl>  writes:
>
>> 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? )
> Normally setting the low 32 bits of an x86 register will zero out the
> upper 32 bits.  Is that not true for pcmpistri?
>
> Otherwise, it sounds like you want the addressing mode (%rax,%ecx).
> Does x86 really have that addressing mode?
>
> Why not just zero extend %ecx to %rcx?
>
>> 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"
> That instruction will indeed set %ecx to zero.  Think about it.
>
> Ian
>