In his book The C++ Standard Library (Second Edition) Nicolai Josuttis states that lambdas can be better optimized by the compiler than plain functions. In addition, C++ compilers optimize lambdas better than they do ordinary functions. (Page 213) Why is that? I thought when it comes to inlining there shouldn’t be any difference any more. … Read more
I’m compiling my C++ app using GCC 4.3. Instead of manually selecting the optimization flags I’m using -march=native, which in theory should add all optimization flags applicable to the hardware I’m compiling on. But how can I check which flags is it actually using? 5 Answers 5
#include <stdio.h> volatile int i; int main() { int c; for (i = 0; i < 3; i++) { c = i &&& i; printf(“%d\n”, c); } return 0; } The output of the above program compiled using gcc is 0 1 1 With the -Wall or -Waddress option, gcc issues a warning: warning: the … Read more
This is the message received from running a script to check if Tensorflow is working: I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcublas.so.8.0 locally I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcudnn.so.5 locally I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcufft.so.8.0 locally I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcuda.so.1 locally I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcurand.so.8.0 locally … Read more
As far as I know, reference/pointer aliasing can hinder the compiler’s ability to generate optimized code, since they must ensure the generated binary behaves correctly in the case where the two references/pointers indeed alias. For instance, in the following C code, void adds(int *a, int *b) { *a += *b; *a += *b; } when … Read more
This question already has answers here: Closed 11 years ago. Possible Duplicate: Why does volatile exist? I have never used it but I wonder why people use it? What does it exactly do? I searched the forum, I found it only C# or Java topics. 2 Answers 2
I first noticed in 2009 that GCC (at least on my projects and on my machines) have the tendency to generate noticeably faster code if I optimize for size (-Os) instead of speed (-O2 or -O3), and I have been wondering ever since why. I have managed to create (rather silly) code that shows this … Read more
I was implementing an algorithm in Swift Beta and noticed that the performance was very poor. After digging deeper I realized that one of the bottlenecks was something as simple as sorting arrays. The relevant part is here: let n = 1000000 var x = [Int](repeating: 0, count: n) for i in 0..<n { x[i] … Read more
I was looking for the fastest way to popcount large arrays of data. I encountered a very weird effect: Changing the loop variable from unsigned to uint64_t made the performance drop by 50% on my PC. The Benchmark #include <iostream> #include <chrono> #include <x86intrin.h> int main(int argc, char* argv[]) { using namespace std; if (argc … Read more
I am doing some numerical optimization on a scientific application. One thing I noticed is that GCC will optimize the call pow(a,2) by compiling it into a*a, but the call pow(a,6) is not optimized and will actually call the library function pow, which greatly slows down the performance. (In contrast, Intel C++ Compiler, executable icc, … Read more
