[cfe-dev] Bug of intrin functions

[Craig Topper via cfe-dev]

Can you file a bug on this? I suspect it was broken in r290539 when the
clobber list was removed because it was made an error to clobber an input
or output.

~Craig

On Wed, Nov 1, 2017 at 11:05 PM, Liu Hao via cfe-dev <cfe-dev at lists.llvm.org
> wrote:

> On 2017/11/1 22:08, LIANG Xingliang via cfe-dev wrote:
>
>> I just find a serious bug. Windows prebuilt binary.
>> ------------------------------------------------------------
>> -------------------------------------------
>> #include <stdio.h>
>> #include <intrin.h>
>>
>> //this function is just copyed from `intrin.h': __movsb(), __x86_64__
>> static __inline__ void __attribute__((__always_inline__, __nodebug__))
>> __movsb_buggy(unsigned char *__dst, unsigned char const *__src, size_t
>> __n) {
>> //for(size_t i=0; i<__n; i++) __dst[i] = __src[i]; //OK
>> __asm__("rep movsb" : : "D"(__dst), "S"(__src), "c"(__n)); //FAIL
>> }
>>
>> int main(int argc, const char ** argv)
>> {
>> unsigned char dst[4]={'a', 'b', 'c', '\0'};
>> unsigned char src[4]={'1', '2', '3', '\0'};
>> __movsb_buggy(dst, src, 1);
>> puts((char*)dst); //expect "1bc", but get "bc"
>> puts((char*)src); //expect "123", but get "23"
>> return 0;
>> }
>> ------------------------------------------------------------
>> ------------------------------------
>>
>> This implementation looks wrong to me.
>
> First, `movsb` alters RDI, RSI and RCX, so all three parameters must be
> in-out ones. Second, the memory area offset from RDI is written to and the
> one offset from RSI is read from, but this __asm__ statement fails to say
> that.
>
> AFAICT the correct implementation should be:
> ```
> static __inline__ void __attribute__((__always_inline__, __nodebug__))
> __movsb(unsigned char *__dst, unsigned char const *__src, size_t __n)
> {
>   __asm__ ("rep movsb"
>     : "+D"(__dst), "+S"(__src), "+c"(__n), "=m"(*(char (*)[])__dst)
>     : "m"(*(char (*)[])__src)
>   );
> }
> ```
>
>
> --
> Best regards,
> LH_Mouse
>
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