Strange behaviour when comparing in SSE - c++

Question

I don't get the error in my code. I try to compare a buffer of unsigned char values to a constant. Then I want to store 1 or 0 depending on the comparison. Here is my code (in a structure):
void operator()(const uint8* src, int32 swidth, int32 sheight, uint8* dst, uint8 value) {
uint8 t[16];
__m128i v_one = _mm_set1_epi8((uint8)1);
__m128i v_value = _mm_set1_epi8(value);
printf("value = %d\n", value);
SHOW(t, v_one);
SHOW(t, v_value);
std::cout << "****" << std::endl;
for (int32 i = 0; i < sheight; ++i) {
const uint8* sdata = src + i * swidth;
uint8* ddata = dst + i * swidth;
int32 j = 0;
for ( ; j <= swidth - 16; j += 16) {
__m128i s = _mm_load_si128((const __m128i*)(sdata + j));
__m128i mask = _mm_cmpgt_epi8(s, v_value);
SHOW(t, s);
SHOW(t, mask);
std::cout << std::endl;
}
}
}
My first line are what I would expect:
value = 100
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
But then my comparison are wrong:
214 100 199 203 232 50 85 195 70 141 121 160 93 130 242 233
0 0 0 0 0 0 0 0 0 0 255 0 0 0 0 0
And I really don't get where the mistakes are.
The SHOW macro is:
#define SHOW(t, r) \
_mm_storeu_si128((__m128i*)t, r); \
printf("%3d", (int32)t[0]); \
for (int32 k = 1; k < 16; ++k) \
printf(" %3d", (int32)t[k]); \
printf("\n")

Answer 1

You are comparing the elements in your s array with your value array.
All the values in the value array are 100.
You have a mix of values in your s array.
However, _mm_cmpgt_epi8 works on signed values and as these are bytes it considers values from -128 to +127.
So the only possible values that are > 100 are values in the range 101 to 127.
As you've only got 1 value in that range (121) thats the only one which has its mask set.
To see this, change uint8 t[16]; to int8 t[16]; and you should get a more expected result.