Why is `float` so much faster than `int` in this program?
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Thread: Why is `float` so much faster than `int` in this program?

  1. #1
    Join Date
    May 2017
    Posts
    1

    Why is `float` so much faster than `int` in this program?

    This simple program copy data from the array `xy` to `arr` and prints the time it takes
    Code:
        #include <stdio.h>
        #include <time.h>
        
        class Point
        {
        public:
        	float x, y;
        
        	Point() :x(0), y(0) {}
        	Point(float x, float y) : x(x), y(y) {}
        };
        
        int main()
        {
        	const size_t n = 99999999;
        	Point *arr = new Point[n];
        
        	float *xy = new float[2 * n];
        	for (size_t i = 0; i < 2 * n; i++)
        		xy[i] = (float)i;
        
        	clock_t start = clock();
        
        	for (size_t i = 0, j = 0; i < n; i++)
        	{
        		float x = xy[j++];
        		float y = xy[j++];
        		arr[i] = Point(x, y);
        	}
        
        	clock_t end = clock();
        
        	printf("time: %d\n", (end - start));
        
        	delete[] arr;
        	delete[] xy;
        
        	return 0;
        }

    It takes about 250 ms. If I change everything to `int`:
    Code:
        #include <stdio.h>
        #include <time.h>
        
        class Point
        {
        public:
        	int x, y;
        
        	Point() :x(0), y(0) {}
        	Point(int x, int y) : x(x), y(y) {}
        };
        
        int main()
        {
        	const size_t n = 99999999;
        	Point *arr = new Point[n];
        
        	int *xy = new int[2 * n];
        	for (size_t i = 0; i < 2 * n; i++)
        		xy[i] = (int)i;
        
        	clock_t start = clock();
        
        	for (size_t i = 0, j = 0; i < n; i++)
        	{
        		int x = xy[j++];
        		int y = xy[j++];
        		arr[i] = Point(x, y);
        	}
        
        	clock_t end = clock();
        
        	printf("time: %d\n", (end - start));
        
        	delete[] arr;
        	delete[] xy;
        
        	return 0;
        }
    it takes almost 3 times the time, around 700 ms. Why is there is a difference? I expected it to be the same because they are both 32 bits and there are no arithmetic operations involved.
    I am using Visual Studio 2015, Configuration: Release x64. Processor: intel i7-4510U.

  2. #2
    Join Date
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    Armenia, Yerevan
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    700

    Re: Why is `float` so much faster than `int` in this program?

    I run the following code on a 32 bit processor, the timings are identical.

    Code:
    #include <stdio.h>
    #include <time.h>
    
    class PointInt
    {
    public:
    	int x, y;
    
    	PointInt() :x(0), y(0) {}
    	PointInt(float x, float y) : x(x), y(y) {}
    };
    
    class PointFloat
    {
    public:
    	float x, y;
    
    	PointFloat() :x(0), y(0) {}
    	PointFloat(int x, int y) : x(x), y(y) {}
    };
    
    int main()
    {
    	const size_t n = 10000000;
    	PointInt *arr1 = new PointInt[n];
    	PointFloat *arr2 = new PointFloat[n];
    
    	int *xy1 = new int[2 * n];
    	float *xy2 = new float[2 * n];
    
    	for (size_t i = 0; i < 2 * n; i++)
    		xy1[i] = (int)i, xy2[i] = (float)i;
    
    	clock_t start1 = clock();
    	for (size_t i = 0, j = 0; i < n; i++)
    	{
    		int x = xy1[j++];
    		int y = xy1[j++];
    		arr1[i] = PointInt(x, y);
    	}
    	clock_t end1 = clock();
    	printf("time: %d\n", (end1 - start1));
    
    	clock_t start2 = clock();
    
    	for (size_t i = 0, j = 0; i < n; i++)
    	{
    		float x = xy2[j++];
    		float y = xy2[j++];
    		arr2[i] = PointFloat(x, y);
    	}
    
    	clock_t end2 = clock();
    	printf("time: %d\n", (end2 - start2));
    
    	delete[] arr1;
    	delete[] xy1;
    	delete[] arr2;
    	delete[] xy2;
    
    	return 0;
    }
    Probably, this is caused due to more improved floating point command set of the 64 architecture.
    Last edited by AvDav; May 28th, 2017 at 02:22 AM. Reason: corrected

  3. #3
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
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    Re: Why is `float` so much faster than `int` in this program?

    For the code in post #2 for my Windows 7 64 bit using VS2017 in release mode with full optimisations enabled

    For 32-bit .exe
    Code:
    int time: 21
    float time: 20
    For 64 bit .exe
    Code:
    int time: 11
    float time: 58
    Why is `float` so much faster than `int`
    What optimisations are enabled for the project properties?
    Last edited by 2kaud; May 27th, 2017 at 07:29 AM.
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++17 Compiler: Microsoft VS2017 (15.5.0)

  4. #4
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
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    Re: Why is `float` so much faster than `int` in this program?

    Eh, no. In the code in post #2, PointInt is dealing with float and PointFloat is dealing with int!

    Consider
    Code:
    #include <iostream>
    #include <ctime>
    #include <typeinfo>
    using namespace std;
    
    using fp = float;
    //using fp = double;
    
    using in = _int32;
    //using in = _int64;
    
    const size_t n = 10000000;
    const size_t n2 = n * 2;
    
    template<typename T>
    class Point
    {
    public:
    	T x{}, y{};
    
    	Point() = default;
    	Point(T x1, T y1) : x(x1), y(y1) {}
    };
    
    template<typename T>
    size_t timing(const T* xy)
    {
    	const auto arrbeg = new Point<T>[n];
    	auto arr = arrbeg;
    
    	clock_t start1 = clock();
    	for (size_t i = 0; i < n; ++i)
    	{
    		auto x = *xy++;
    		auto y = *xy++;
    		*arr++ = Point<T>(x, y);
    	}
    	clock_t end1 = clock();
    
    	delete[] arrbeg;
    
    	return end1 - start1;
    }
    
    int main()
    {
    	auto xyi = new in[n2];
    	auto xyf = new fp[n2];
    
    	for (size_t i = 0; i < n2; ++i)
    		xyi[i] = (in)i, xyf[i] = (fp)i;
    
    	cout << typeid(in).name() << " time " << timing(xyi) << endl;
    	cout << typeid(fp).name() << " time " << timing(xyf) << endl;
    
    	delete[] xyi;
    	delete[] xyf;
    
    	return 0;
    }
    For 64 bit .exe
    Code:
    int time: 56
    float time: 12
    For 32 bit .exe
    Code:
    int time: 11
    float time: 22
    Changing type float to type double gives
    For 64 bit .exe
    Code:
    int time: 57
    double time: 23
    For 32 bit .exe
    Code:
    int time: 11
    double time: 24
    Last edited by 2kaud; May 27th, 2017 at 10:09 AM.
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++17 Compiler: Microsoft VS2017 (15.5.0)

  5. #5
    Join Date
    Jun 2003
    Location
    Armenia, Yerevan
    Posts
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    Re: Why is `float` so much faster than `int` in this program?

    Accept my apologies, didn't want to mislead anyone.

  6. #6
    Join Date
    Jun 2003
    Location
    Armenia, Yerevan
    Posts
    700

    Re: Why is `float` so much faster than `int` in this program?

    I ran the code shown by 2kaud in his original posting no. 4, the timings are nearly equal on a 32-bit machine.
    OMG, I need to update my CPU as the speed is relatively slow.
    Popular opinion is the greatest lie in the world.

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