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POWhy is std::sin() and std::cos() slower than sin() and cos()?
 

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  1. POWhy is std::sin() and std::cos() slower than sin() and cos()?
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    <p>Test code:</p> <pre><code>#include &lt;cmath&gt; #include &lt;cstdio&gt; const int N = 4096; const float PI = 3.1415926535897932384626; float cosine[N][N]; float sine[N][N]; int main() { printf("a\n"); for (int i = 0; i &lt; N; i++) { for (int j = 0; j &lt; N; j++) { cosine[i][j] = cos(i*j*2*PI/N); sine[i][j] = sin(-i*j*2*PI/N); } } printf("b\n"); } </code></pre> <p>Here is the time:</p> <pre><code>$ g++ main.cc -o main $ time ./main a b real 0m1.406s user 0m1.370s sys 0m0.030s </code></pre> <p>After adding <code>using namespace std;</code>, the time is:</p> <pre><code>$ g++ main.cc -o main $ time ./main a b real 0m8.743s user 0m8.680s sys 0m0.030s </code></pre> <p>Compiler:</p> <pre><code>$ g++ --version g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2 </code></pre> <p>Assembly:</p> <pre><code>Dump of assembler code for function sin@plt: 0x0000000000400500 &lt;+0&gt;: jmpq *0x200b12(%rip) # 0x601018 &lt;_GLOBAL_OFFSET_TABLE_+48&gt; 0x0000000000400506 &lt;+6&gt;: pushq $0x3 0x000000000040050b &lt;+11&gt;: jmpq 0x4004c0 End of assembler dump. Dump of assembler code for function std::sin(float): 0x0000000000400702 &lt;+0&gt;: push %rbp 0x0000000000400703 &lt;+1&gt;: mov %rsp,%rbp 0x0000000000400706 &lt;+4&gt;: sub $0x10,%rsp 0x000000000040070a &lt;+8&gt;: movss %xmm0,-0x4(%rbp) 0x000000000040070f &lt;+13&gt;: movss -0x4(%rbp),%xmm0 0x0000000000400714 &lt;+18&gt;: callq 0x400500 &lt;sinf@plt&gt; 0x0000000000400719 &lt;+23&gt;: leaveq 0x000000000040071a &lt;+24&gt;: retq End of assembler dump. Dump of assembler code for function sinf@plt: 0x0000000000400500 &lt;+0&gt;: jmpq *0x200b12(%rip) # 0x601018 &lt;_GLOBAL_OFFSET_TABLE_+48&gt; 0x0000000000400506 &lt;+6&gt;: pushq $0x3 0x000000000040050b &lt;+11&gt;: jmpq 0x4004c0 End of assembler dump. </code></pre>
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      1. POWhy is std::sin() and std::cos() slower than sin() and cos()?
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    1. CO@Nawaz: It might. It's an implementation detail whether `<cmath>` provides `double sin(double)` and `double cos(double)` in the global namespace. Ditto for `<cstdio>` and `printf`.
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      1. POWhy is std::sin() and std::cos() slower than sin() and cos()?
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    2. CO@Nawaz It does compile. It is my real coding.
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      1. POWhy is std::sin() and std::cos() slower than sin() and cos()?
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    3. COThe easiest way to answer issues like this is by comparing the assembly output of the compiler.
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      1. POWhy is std::sin() and std::cos() slower than sin() and cos()?
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