Visual Studio 2019 C6385 / C6386 (buffer overrun warning) on __m256 array - c++

Question

I'm allocating an array as follows:
__m256 *v256f_valid_mask = (__m256*)malloc(sizeof(__m256) * p_ranks);
The compiler is showing warning C6385 / C6386 (depending on exact context) on all lines where I access this array, except for at [0], indicating that 64 bytes may be read. The definition clearly states it's an array of 32-byte values.
Using _aligned_malloc() doesn't help.
Sample code to reproduce the warning:
void func(const size_t p_ranks)
{
__m256 v256f_x = _mm256_set1_ps(1.0f);
__m256* v256f_valid_mask = (__m256*)malloc(sizeof(__m256) * p_ranks);
for (size_t rank = 1; rank < p_ranks; rank++)
{
v256f_valid_mask[rank] = _mm256_cmp_ps(v256f_x, _mm256_setzero_ps(), _CMP_GT_OQ); // <<
}
}
Exact warning:
I fixed the C6011 warning with a null check.
Is there an error in my code or is this a false positive?

Answer 1

It is a false positive but the code analyser doesn't know it (probably because it doesn't 'trust' the malloc() call)! Using 'new' instead clears the warning (at least, in my VS2019 solution) …
void func(const size_t p_ranks)
{
__m256 v256f_x = _mm256_set1_ps(1.0f);
// __m256* v256f_valid_mask = (__m256*)malloc(sizeof(__m256) * p_ranks);
#if defined(__cplusplus)
__m256* v256f_valid_mask = new __m256[p_ranks];
#else
#define MAXRANKS 100 // Would probably be defined elsewhere!
__m256 v256f_valid_mask[MAXRANKS];
#endif
for (size_t rank = 1; rank < p_ranks; rank++)
{
v256f_valid_mask[rank] = _mm256_cmp_ps(v256f_x, _mm256_setzero_ps(), _CMP_GT_OQ); // <<
}
}
Please try and see!