memcheck seems to not understand that 0 multiplied with whatever is still 0 when using simd instructions. 

valgrind version: 3.8.1

example program to provoke the error:

#include "emmintrin.h"
#include <stdio.h>
#include <valgrind/memcheck.h>
int main(void)
{
    __m128i a;
    __m128i r = _mm_madd_epi16(a,_mm_setzero_si128());
    
    if (VALGRIND_CHECK_MEM_IS_DEFINED(&r,16))
        printf("\n\nVALGRIND BUG! Memory is defined and 0 (undefined*0 == 0)\n\n");
    return _mm_cvtsi128_si32(r);
}   

gcc -msse -o test test.c && valgrind ./test
Observe that VALGRIND_CHECK_MEM_IS_DEFINED will report an error,  as well as the return value is deemed uninitialised.

expected result: no printf and no valgrind/memcheck errors.

A typical use-case is a filter function with padded data and 0 coefficients for edge handling.

Reproducible: Always