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POHow to tell compiler to NOT optimize certain code away?
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copied!<p>Is there a way to tell the compiler (g++ in my case) to <strong>not</strong> optimize certain code away, even if that code is not reachable? I just want those symbols in the object file.</p> <p>Example: Here is a simple function, and I do want this function to be compiled, even if it's never called.</p> <pre><code>void foo(){ Foo<int> v; } </code></pre> <p>If there is no official compiler directive, is there a trick to make the compiler think that it's an important function? Or at least make it think that it can't safely be ignored? I tried something like this:</p> <pre><code>extern bool bar; void foo(){ if(bar){ Foo<int> v; } } </code></pre> <p>but that didn't seem to do it.</p> <p>(If you really want to know why I on earth would want that -- it has to do with <a href="https://stackoverflow.com/questions/555330/templates-use-forward-declarations-to-reduce-compile-time">this</a> question, where, instead of explicit template instantiation with <code>template class Foo<int></code> I simply want to be able to write <code>Foo<int> v</code>, since in many cases that's easier since it implicitly instantiates all functions needed, and it does work fine in debug mode without optimizations ...)</p> <p><strong>UPDATE:</strong></p> <p>Here is what I want to do (as a compilable mini example):</p> <p><strong>foo.h</strong> (such files are given to me and not changeable)</p> <pre><code>template<class T> struct Foo { T val_; Foo(T val) : val_(val) { // heavy code, long compile times } }; </code></pre> <p><strong>foo-instantiation.cpp</strong></p> <pre><code>#include "foo.h" void neverCalled() { Foo<int> f(1); } // The standard way to instantiate it is this: // template class Foo<int>; // but in reality it is often hard to find out // exactly what types I have to declare. // Usage like Foo<int> f(1); will instantiate all // dependent types if necessary. </code></pre> <p><strong>foo-decl.h</strong> (an interface that I extracted from foo.h)</p> <pre><code>template<class T> struct Foo { T val_; Foo(T val); // no heavy code, can include anywhere and compile fast }; </code></pre> <p><strong>main.cpp</strong></p> <pre><code>#include <iostream> #include "foo-decl.h" int main(int argc, char** argv){ Foo<int> foo(1); return 0; } </code></pre> <p><strong>Compilation (no optimization)</strong></p> <pre><code>g++ -c main.cpp g++ -c foo-instantiation.cpp g++ main.o foo-instantiation.oo </code></pre> <p><strong>Compilation (optimization)</strong></p> <pre><code>g++ -O2 -c main.cpp g++ -O2 -c foo-instantiation.cpp g++ main.o foo-instantiation.oo main.o(.text+0x13): In function `main': : undefined reference to `Foo<int>::Foo(int)' collect2: ld returned 1 exit status </code></pre> <ul> <li>I tried precompiled headers instead, but the template instantiation method makes for much faster compilation.</li> <li>Compiling <code>foo-instantiation.cpp</code> without optimization is not so ideal because then the library code (<code>foo.h</code> and others) will run slower.</li> </ul>

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