Why is _mm_set_epi16 sometimes faster than _mm_load_si128? - c++

Question

I've understood it's best to avoid _mm_set_epi*, and instead rely on _mm_load_si128 (or even _mm_loadu_si128 with a small performance hit if the data is not aligned). However, the impact this has on performance seems inconsistent to me. The following is a good example.
Consider the two following functions that utilize SSE intrinsics:
static uint32_t clmul_load(uint16_t x, uint16_t y)
{
const __m128i c = _mm_clmulepi64_si128(
_mm_load_si128((__m128i const*)(&x)),
_mm_load_si128((__m128i const*)(&y)), 0);
return _mm_extract_epi32(c, 0);
}
static uint32_t clmul_set(uint16_t x, uint16_t y)
{
const __m128i c = _mm_clmulepi64_si128(
_mm_set_epi16(0, 0, 0, 0, 0, 0, 0, x),
_mm_set_epi16(0, 0, 0, 0, 0, 0, 0, y), 0);
return _mm_extract_epi32(c, 0);
}
The following function benchmarks the performance of the two:
template <typename F>
void benchmark(int t, F f)
{
std::mt19937 rng(static_cast<unsigned int>(std::time(0)));
std::uniform_int_distribution<uint32_t> uint_dist10(
0, std::numeric_limits<uint32_t>::max());
std::vector<uint32_t> vec(t);
auto start = std::chrono::high_resolution_clock::now();
for (int i = 0; i < t; ++i)
{
vec[i] = f(uint_dist10(rng), uint_dist10(rng));
}
auto duration = std::chrono::duration_cast<
std::chrono::milliseconds>(
std::chrono::high_resolution_clock::now() -
start);
std::cout << (duration.count() / 1000.0) << " seconds.\n";
}
Finally, the following main program does some testing:
int main()
{
const int N = 10000000;
benchmark(N, clmul_load);
benchmark(N, clmul_set);
}
On an i7 Haswell with MSVC 2013, a typical output is
0.208 seconds. // _mm_load_si128
0.129 seconds. // _mm_set_epi16
Using GCC with parameters -O3 -std=c++11 -march=native (with slightly older hardware), a typical output is
0.312 seconds. // _mm_load_si128
0.262 seconds. // _mm_set_epi16
What explains this? Are there actually cases where _mm_set_epi* is preferable over _mm_load_si128? There are other times where I've noticed _mm_load_si128 to perform better, but I can't really characterize those observations.

Answer 1

Your compiler is optimizing away the "gather" behavior of your _mm_set_epi16() call since it really isn't needed. From g++ 4.8 (-O3) and gdb:
(gdb) disas clmul_load
Dump of assembler code for function clmul_load(uint16_t, uint16_t):
0x0000000000400b80 <+0>: mov %di,-0xc(%rsp)
0x0000000000400b85 <+5>: mov %si,-0x10(%rsp)
0x0000000000400b8a <+10>: vmovdqu -0xc(%rsp),%xmm0
0x0000000000400b90 <+16>: vmovdqu -0x10(%rsp),%xmm1
0x0000000000400b96 <+22>: vpclmullqlqdq %xmm1,%xmm0,%xmm0
0x0000000000400b9c <+28>: vmovd %xmm0,%eax
0x0000000000400ba0 <+32>: retq
End of assembler dump.
(gdb) disas clmul_set
Dump of assembler code for function clmul_set(uint16_t, uint16_t):
0x0000000000400bb0 <+0>: vpxor %xmm0,%xmm0,%xmm0
0x0000000000400bb4 <+4>: vpxor %xmm1,%xmm1,%xmm1
0x0000000000400bb8 <+8>: vpinsrw $0x0,%edi,%xmm0,%xmm0
0x0000000000400bbd <+13>: vpinsrw $0x0,%esi,%xmm1,%xmm1
0x0000000000400bc2 <+18>: vpclmullqlqdq %xmm1,%xmm0,%xmm0
0x0000000000400bc8 <+24>: vmovd %xmm0,%eax
0x0000000000400bcc <+28>: retq
End of assembler dump.
The vpinsrw (insert word) is ever-so-slightly faster than the unaligned double-quadword move from clmul_load, likely due to the internal load/store unit being able to do the smaller reads simultaneously but not the 16B ones. If you were doing more arbitrary loads, this would go away, obviously.

Answer 2

The slowness of _mm_set_epi* comes from the need to scrape together various variables into a single vector. You'd have to examine the generated assembly to be certain, but my guess is that since most of the arguments to your _mm_set_epi16 calls are constants (and zeroes, at that), GCC is generating a fairly short and fast set of instructions for the intrinsic.