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POmingw-w64: slow sprintf in <cstdio>
 

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  1. POmingw-w64: slow sprintf in <cstdio>
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    <p>Is that <code>&lt;cstdio&gt;</code> header in C++ contains just the same functions as <code>&lt;stdio.h&gt;</code> but put in <code>std</code> namespace?</p> <p>I experienced strange efficiency problems in my program compiled with mingw-w64, which is more than ten times slower then on linux. After some test I found that the problem is in <code>sprintf</code>.</p> <p>Then I did the following test:</p> <pre><code>#include &lt;stdio.h&gt; // #include &lt;cstdio&gt; // using std::sprintf; int main () { int i; for (i = 0; i &lt; 500000; i++){ char x[100]; sprintf(x, "x%dx%dx", i, i&lt;&lt;2); } } </code></pre> <p>When compiled with <code>&lt;stdio.h&gt;</code> it is 15 times faster then using <code>&lt;cstdio&gt;</code>. Here is the timing:</p> <pre><code>$ time ./stdio real 0m0.557s user 0m0.046s sys 0m0.046s $ time ./cstdio real 0m7.465s user 0m0.031s sys 0m0.077s $ g++ --version g++.exe (rubenvb-4.8-stdthread) 4.8.1 20130324 (prerelease) Copyright (C) 2013 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. </code></pre> <p><strong>UPDATE 1</strong>: I further timed with different mingw-w64 build (rubenvb, drangon, and mingw-build), and find that all 32bit version using <code>&lt;cstdio&gt;</code> timed 4.x seconds and 64bit versions 7.x~8.x seconds. And all versions using <code>&lt;stdio.h&gt;</code> timed around 0.4~0.6 second.</p> <p><strong>UPDATE 2</strong>: I disassembled the main function in gdb and find only one line differs: the <code>&lt;stdio.h&gt;</code> version calls <code>callq 0x4077c0 &lt;sprintf&gt;</code> but the <code>&lt;cstdio&gt;</code> version calls <code>callq 0x407990 &lt;_Z7sprintfPcPKcz&gt;</code>.</p> <p><code>sprintf</code> contains:</p> <pre><code>0x00000000004077c0 &lt;+0&gt;: jmpq *0x7c6e(%rip) # 0x40f434 &lt;__imp_sprintf&gt; 0x00000000004077c6 &lt;+6&gt;: nop 0x00000000004077c7 &lt;+7&gt;: nop </code></pre> <p>Following <code>__imp_sprintf</code> I reached the <code>sprinf</code> inside <code>msvcrt.dll</code>.</p> <p><code>_Z7sprintfPcPKcz</code> contains some mingw codes:</p> <pre><code>0x0000000000407990 &lt;+0&gt;: push %rbp 0x0000000000407991 &lt;+1&gt;: push %rbx 0x0000000000407992 &lt;+2&gt;: sub $0x38,%rsp 0x0000000000407996 &lt;+6&gt;: lea 0x80(%rsp),%rbp 0x000000000040799e &lt;+14&gt;: mov %rcx,-0x30(%rbp) 0x00000000004079a2 &lt;+18&gt;: mov %r8,-0x20(%rbp) 0x00000000004079a6 &lt;+22&gt;: mov %r9,-0x18(%rbp) 0x00000000004079aa &lt;+26&gt;: mov %rdx,-0x28(%rbp) 0x00000000004079ae &lt;+30&gt;: lea -0x20(%rbp),%rax 0x00000000004079b2 &lt;+34&gt;: mov %rax,-0x58(%rbp) 0x00000000004079b6 &lt;+38&gt;: mov -0x58(%rbp),%rdx 0x00000000004079ba &lt;+42&gt;: mov -0x28(%rbp),%rax 0x00000000004079be &lt;+46&gt;: mov %rdx,%r8 0x00000000004079c1 &lt;+49&gt;: mov %rax,%rdx 0x00000000004079c4 &lt;+52&gt;: mov -0x30(%rbp),%rcx 0x00000000004079c8 &lt;+56&gt;: callq 0x402c40 &lt;__mingw_vsprintf&gt; 0x00000000004079cd &lt;+61&gt;: mov %eax,%ebx 0x00000000004079cf &lt;+63&gt;: mov %ebx,%eax 0x00000000004079d1 &lt;+65&gt;: add $0x38,%rsp 0x00000000004079d5 &lt;+69&gt;: pop %rbx 0x00000000004079d6 &lt;+70&gt;: pop %rbp </code></pre> <p>Why does <code>cstdio</code> use a different (and much slower) function?</p>
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    1. USJiaming Lu
    1. USJiaming Lu
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    1. COYes, `printf()` is slow. Sorry. (And I'm not surprised either that it performs better on Linux than on Windows.)
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    2. COMy question is: why it is slow in `cstdio` but fast in `stdio.h`.
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      1. POmingw-w64: slow sprintf in <cstdio>
      1. USJiaming Lu
    3. COCould you try running the tests a few times? `sprint` and `std::sprintf` are the [exact same function](http://gcc.gnu.org/onlinedocs/gcc-4.6.2/libstdc++/api/a00832_source.html).
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      1. USJesse Good
 

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