Unjustified warning?

[Marcel Keller <m.keller@xxxxxxxxxxxxx>]

Hi,

Compiling the attached code with GCC 7.2 (Ubuntu 17.10) gives the 
following warning:

$ g++ -mavx -O3 foo.cpp -c
In function ‘void foo(size_t)’:
cc1plus: warning: ‘void* __builtin_memset(void*, int, long unsigned 
int)’: writing between 32 and 18446744073709551584 bytes into a region 
of size 16 overflows the destination [-Wstringop-overflow=]

However, the length in the memset call would be between 1 and 31, and 
the range 1 to 16 should be fine, as far as I can tell. Should that be 
considered a bug?

Best regards,
Marcel

#include <immintrin.h>
#include <string.h>

long long bar[2];

void avx_memzero(void* dest, size_t length)
{
	__m256i* d = (__m256i*)dest;
	__m256i s = _mm256_setzero_si256();
	while (length >= 32)
	{
		_mm256_storeu_si256(d++, s);
		length -= 32;
	}
	if (length)
		memset((void*)d, 0, length);
}

void foo(size_t t)
{
	avx_memzero(bar, t);
}