Optimization question

This is a discussion on Optimization question within the C++ Programming forums, part of the General Programming Boards category; Alright thanks! That is good news....

  1. #16
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    Alright thanks! That is good news.

  2. #17
    C++まいる!Cをこわせ! Elysia's Avatar
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    Code:
    #include <iostream>
    
    template<int Num1, int Num2> struct divide
    {
    	static const int value = Num1 / Num2;
    };
    
    int main()
    {
    	std::cout << divide<10, 5>::value << std::endl;
    }
    No computation of division at runtime
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  3. #18
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    Quote Originally Posted by Elysia View Post
    Code:
    #include <iostream>
    *
    template<int Num1, int Num2> struct divide
    {
    	static const int value = Num1 / Num2;
    };
    
    int main()
    {
    	std::cout << divide<10, 5>::value << std::endl;
    }
    No computation of division at runtime
    The only compiles that are too retarded to optimize out "10 / 5" are unlikely to be able to handle these advanced template constructs, I think. So I think it would be really bad to use a messy construct like this.
    In fact, I find it more likely a compiler is unable to optimize something like divide<10, 5>*3 than it 10 / 5 * 3...

  4. #19
    Registered User whiteflags's Avatar
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    No one would write that, he's showing off

  5. #20
    C++まいる!Cをこわせ! Elysia's Avatar
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    The point was that it will be optimized even in Debug mode. In Release, I'm sure the compiler will optimize it.
    And any compiler that is able to instantiate a template like this will have no problems optimizing divide<10, 5> * 3.
    And some might write like this, if not for exactly division, but for other things such as binary -> decimal or whatever.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #21
    Registered User whiteflags's Avatar
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    Quote Originally Posted by Elysia View Post
    And some might write like this, if not for exactly division, but for other things such as binary -> decimal or whatever.
    Apples and oranges...

    Specific to your example, you can't do a/b for any non-constant expressions a or b, something you should have really mentioned maybe. So yeah you're showing off, no one would write that.

  7. #22
    C++まいる!Cをこわせ! Elysia's Avatar
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    The compiler can't optimize non-constant expressions to the results anyway, even without the template, so I figure it is kind of the same
    No constant expressions = no optimization (unless it's SSE or the like).
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  8. #23
    The larch
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    How do you know the compiler doesn't already evaluate constant expressions in debug build? E.g with gcc:

    Code:
    int a = 14 / 2;
    Code:
    .stabn 68,0,5,LM3-_main
    LM3:
    	movl	$7, -4(%ebp)
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  9. #24
    C++まいる!Cをこわせ! Elysia's Avatar
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    Well, as I said, with optimizations on (ie Release build), then it 99% likely will.
    But in Debug mode (ie no optimizations), it most likely will not (VC++ will not).
    And if they aren't constant expressions, then neither will work very well (unless we talk about SSE & Co).
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #25
    Registered User whiteflags's Avatar
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    Then why optimize arithmetic in debug builds at all?

  11. #26
    C++まいる!Cをこわせ! Elysia's Avatar
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    Because:
    Quote Originally Posted by iMalc View Post
    Option B is definitely better though because it almost certainly increases the speed of your debug builds, which can be a godsend.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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