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POextremely slow program from using AVX instructions
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  1. POextremely slow program from using AVX instructions
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    copied!<p>I'm trying to write a geometric mean sqrt(a * b) using AVX intrinsics, but it runs slower than molasses!</p> <pre class="lang-c prettyprint-override"><code>int main() { int count = 0; for (int i = 0; i &lt; 100000000; ++i) { __m128i v8n_a = _mm_set1_epi16((++count) % 16), v8n_b = _mm_set1_epi16((++count) % 16); __m128i v8n_0 = _mm_set1_epi16(0); __m256i temp1, temp2; __m256 v8f_a = _mm256_cvtepi32_ps(temp1 = _mm256_insertf128_si256(_mm256_castsi128_si256(_mm_unpacklo_epi16(v8n_a, v8n_0)), _mm_unpackhi_epi16(v8n_a, v8n_0), 1)), v8f_b = _mm256_cvtepi32_ps(temp2 = _mm256_insertf128_si256(_mm256_castsi128_si256(_mm_unpacklo_epi16(v8n_b, v8n_0)), _mm_unpackhi_epi16(v8n_b, v8n_0), 1)); __m256i v8n_meanInt32 = _mm256_cvtps_epi32(_mm256_sqrt_ps(_mm256_mul_ps(v8f_a, v8f_b))); __m128i v4n_meanLo = _mm256_castsi256_si128(v8n_meanInt32), v4n_meanHi = _mm256_extractf128_si256(v8n_meanInt32, 1); g_data[i % 8] = v4n_meanLo; g_data[(i + 1) % 8] = v4n_meanHi; } return 0; } </code></pre> <p>The key to this mystery is that I'm using Intel ICC 11 and it's only slow when compiling with icc -O3 sqrt.cpp. If I compile with icc -O3 -xavx sqrt.cpp, then it runs 10x faster.</p> <p>But it's not obvious if there's emulation happening because I used performance counters and the number of instructions executed for both versions is roughly 4G:</p> <pre><code> Performance counter stats for 'a.out': 16867.119538 task-clock # 0.999 CPUs utilized 37 context-switches # 0.000 M/sec 8 CPU-migrations # 0.000 M/sec 281 page-faults # 0.000 M/sec 35,463,758,996 cycles # 2.103 GHz 23,690,669,417 stalled-cycles-frontend # 66.80% frontend cycles idle 20,846,452,415 stalled-cycles-backend # 58.78% backend cycles idle 4,023,012,964 instructions # 0.11 insns per cycle # 5.89 stalled cycles per insn 304,385,109 branches # 18.046 M/sec 42,636 branch-misses # 0.01% of all branches 16.891160582 seconds time elapsed </code></pre> <p>-----------------------------------with -xavx----------------------------------------</p> <pre><code> Performance counter stats for 'a.out': 1288.423505 task-clock # 0.996 CPUs utilized 3 context-switches # 0.000 M/sec 2 CPU-migrations # 0.000 M/sec 279 page-faults # 0.000 M/sec 2,708,906,702 cycles # 2.102 GHz 1,608,134,568 stalled-cycles-frontend # 59.36% frontend cycles idle 798,177,722 stalled-cycles-backend # 29.46% backend cycles idle 3,803,270,546 instructions # 1.40 insns per cycle # 0.42 stalled cycles per insn 300,601,809 branches # 233.310 M/sec 15,167 branch-misses # 0.01% of all branches 1.293986790 seconds time elapsed </code></pre> <p>Is there some kind of processor internal emulation going on? I know for denormal numbers, adds end up being 64 times slower than normal.</p>
 

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