[PATCH] PR string/19907: Incorrect memcpy tests

[Florian Weimer]

* H. J. Lu:

> On Sun, May 8, 2016 at 10:14 AM, Florian Weimer <fw@deneb.enyo.de> wrote:
>> * H. J. Lu:
>>
>>> +  /* Must clear the destination buffer set by the previous run.  */
>>> +  for (i = 0; i < len; i++)
>>> +    dst[i] = 0;
>>
>> Doesn't this need some sort of compiler barrier so that GCC will not
>> eliminate the dead stores if it recognizes a memset-style loop?
>
> The code looks like:
>
>  /* Must clear the destination buffer set by the previous run.  */
>   for (i = 0; i < len; i++)
>     dst[i] = 0;
>
>   if (CALL (impl, dst, src, len) != MEMCPY_RESULT (dst, len))
>     {
>
> Compiler doesn't know what CALL (impl, dst, src, len) does
> and won't optimize it out.

I looked at the underlying mechanism and there's an indirection
through the “impls” section.  You are right that it's going to take a
while until the compiler will see through that.  In any case, it's a
pre-existing problem, not related to your change.