compiler-optimization

问题 Backgrond I am very experienced in C and pretty new to Android and Java, but this is rather environmental issues that programming. I have developed an administrative application in ANSI-C to be ported to any OS, just adding a UI in OS-dependent code. Well it uses quite some memory, especially for huge user files. I have a working Win32 program, trying to make an Android app using Android Studio with NDK. Android studio bundle NDK installation works fine I have installed and made a Win7

问题 How can I tell the MSVC compiler to use the 64bit/32bit division operation to compute the result of the following function for the x86-64 target: #include <stdint.h> uint32_t ScaledDiv(uint32_t a, uint32_t b) { if (a > b) return ((uint64_t)b<<32) / a; //Yes, this must be casted because the result of b<<32 is undefined else return uint32_t(-1); } I would like the code, when the if statement is true, to compile to use the 64bit/32bit division operation, e.g. something like this: ; Assume

问题 I'm working on a small mathematical optimization framework in C++11, and I wonder what's the best way for the user to provide domain-specific logic. I could force her to define classes with hook methods that can be called by the framework, but I'd like to keep it lean and take advantage of the new C++11 facilities whenever I can. So I'm thinking about accepting std::function objects, possibly instantiated from lambda expressions, as parameters, and call them when needed. The only think I'm

问题 I was intrigued by clang's ability to convert many == comparisons of small integers to to one big SIMD instruction, but then I noticed something strange. Clang generated "worse" code(in my amateur evaluation) when I had 7 comparisons compared to the code when I had 8 comparisons. bool f1(short x){ return (x==-1) | (x == 150) | (x==5) | (x==64) | (x==15) | (x==223) | (x==42) | (x==47); } bool f2(short x){ return (x==-1) | (x == 150) | (x==5) | (x==64) | (x==15) | (x==223) | (x==42); } My

问题 This question already has answers here : Can compiler generate std::move for a last use of lvalue automatically? (3 answers) Do compilers automatically use move semantics when a movable object is used for the last time? (1 answer) Closed 2 years ago . In C++11 you can use std::vector::push_back in combination with std::move to avoid copies while inserting elements into a vector. Is there a section in the standard that forbids compilers to use std::move automatically with local variables that

问题 I'm writing some debug code at work, and I'm wondering if what I'm doing might hurt performance or not. Let's get to the code: foreach (var item in aCollection) Debug.WriteLine(item.Name); I know that the Debug class uses the Conditional attribute to avoid compilation in release mode (or whenever DEBUG is undefined), but would this end up as a futile/empty iteration when compiled in release mode or would it be optimized by the compiler? 回答1: The C# compiler will not optimize away the

问题 I'm writing some debug code at work, and I'm wondering if what I'm doing might hurt performance or not. Let's get to the code: foreach (var item in aCollection) Debug.WriteLine(item.Name); I know that the Debug class uses the Conditional attribute to avoid compilation in release mode (or whenever DEBUG is undefined), but would this end up as a futile/empty iteration when compiled in release mode or would it be optimized by the compiler? 回答1: The C# compiler will not optimize away the

问题 I'm looking around OpenMP, partially because my program need to make additions of very large vectors (millions of elements). However i see a quite large difference if i use std::vector or raw array. Which i cannot explain. I insist that the difference is only on the loop, not the initialisation of course. The difference in time I refer to, is only timing the addition, especially not to take into account any initialization difference between vectors, arrays, etc. I'm really talking only about

问题 This is the code that I tested: #include <iostream> #include <chrono> using namespace std; #define CHRONO_NOW chrono::high_resolution_clock::now() #define CHRONO_DURATION(first,last) chrono::duration_cast<chrono::duration<double>>(last-first).count() int fib(int n) { if (n<2) return n; return fib(n-1) + fib(n-2); } int main() { auto t0 = CHRONO_NOW; cout << fib(45) << endl; cout << CHRONO_DURATION(t0, CHRONO_NOW) << endl; return 0; } Of course, there are much faster ways of calculating

问题 I found a question somewhat interesting, and went on an attempt to answer it. The author wants to compile -one- source file (which relies on template libraries) with AVX optimizations, and the rest of the project without those. So, to see what would happen, I've created a test project like this: main.cpp #include <iostream> #include <string> #include "fn_normal.h" #include "fn_avx.h" int main(int argc, char* argv[]) { int number = 10; // this will come from input, but let's keep it simple for