Convert (vectorize) code with per32-bit element conditional to SSE2 SSE3 - optimization

Question

I want to vectorize code for Core2. I think, I can use intrinsic functions from gcc or icc, and the SSE, SSE2, SSE3, SSSE3 instructions are allowed.
My code works on arrays of 8 uint32_t elements and it is like this (only hotspot is here):
const uint32_t p[8] = {2147483743, 2147483713, 2147483693, 2147483659,
2147483647, 2147483629, 2147483587, 2147483579};
void vector_mod_add(uint32_t *a /* a[8] */, uint32_t *b /* b[8] */) {
int n;
for(n=0;n<8;n++)
a[n]+=b[n];
for(n=0;n<8;n++)
if(a[n]>=p[n])
a[n]-=p[n];
}
Addition is rather easy, but I don't know how it is possible to do an conditional subtraction.
Also, I have no experience in manual vectorizing with SSE2, so, please, tell me how should I define all types here.

Answer 1

You can write it as a[n] -= p[n] & ~(a[n] < p[n]). Note that the < here is not the C one, it's the SSE one (pcmpltd) that returns -1 in each true element and 0 in each false element (to allow the AND operation), and &~ is pandn. Here is an attempt at the code:
__m128i a, p;
a = _mm_sub_epi32(a, _mm_andnot_si128(_mm_cmplt_epi32(a, p), p));
Note that this uses signed operations, and so your numbers will need to stay below 2^31 - 1 for it to work correctly. If you need to go beyond that, change _mm_cmplt_epi32(a, p) to _mm_cmplt_epi32(_mm_xor_si128(a, signs), _mm_xor_si128(p, signs)), where signs is a vector of 32-bit words whose elements are all 0x80000000. Here is a version that seems like it will handle wider ranges more efficiently:
__m128i a, p;
a = _mm_sub_epi32(a, p);
a = _mm_add_epi32(a, _mm_and_si128(_mm_srai_epi32(a, 31), p));