compiler-optimization

问题 The C11 standard appears to imply that iteration statements with constant controlling expressions should not be optimized out. I'm taking my advice from this answer, which specifically quotes section 6.8.5 from the draft standard: An iteration statement whose controlling expression is not a constant expression ... may be assumed by the implementation to terminate. In that answer it mentions that a loop like while(1) ; should not be subject to optimization. So...why does Clang/LLVM optimize

问题 The Problem We have a mid-sized program for a simulation task, that we need to optimize. We have already done our best optimizing the source to the limit of our programming skills, including profiling with Gprof and Valgrind. When finally finished, we want to run the program on several systems probably for some months. Therefore we are really interested in pushing the optimization to the limits. All systems will run Debian/Linux on relatively new hardware (Intel i5 or i7). The Question What

问题 The Problem We have a mid-sized program for a simulation task, that we need to optimize. We have already done our best optimizing the source to the limit of our programming skills, including profiling with Gprof and Valgrind. When finally finished, we want to run the program on several systems probably for some months. Therefore we are really interested in pushing the optimization to the limits. All systems will run Debian/Linux on relatively new hardware (Intel i5 or i7). The Question What

问题 This question already has answers here : Events and multithreading once again (2 answers) C# Events and Thread Safety (15 answers) Is C#'s null-conditional delegate invocation thread safe? (3 answers) Is C# 6 ?. (Elvis op) thread safe? If so, how? (2 answers) c# ?. conditional operator (3 answers) Closed 13 days ago . The documentation says that PropertyChanged?.Invoke(…) is thread safe, because it is equivalent to this: var handler = this.PropertyChanged; if (handler != null) { handler(…); }

问题 This question already has answers here : MS C# compiler and non-optimized code (3 answers) Closed 4 years ago . The IL code (generated with https://dotnetfiddle.net) of this piece of code: public class Program { public static void Main() { int i = 10; if (i < 4) Console.WriteLine("Hello World"); } } contains ldstr "Hello World". Shouldn't the compiler know that Console.WriteLine never gets executed? The IL code of this: public class Program { public static void Main() { if (10 < 4) Console

问题 Consider the following code: #include <stdio.h> void foo() { printf("Hello world\n"); } void bar() { printf("Hello world"); } The assembly produced by both these two functions is: .LC0: .string "Hello world" foo(): mov edi, OFFSET FLAT:.LC0 jmp puts bar(): mov edi, OFFSET FLAT:.LC0 xor eax, eax jmp printf Now I know the difference between puts and printf, but I find this quite interesting that gcc is able to introspect the const char* and figure out whether to call printf or puts. Another

问题 How does noexcept in C++ change the assembly? I tried a bit with small functions, in godbolt, but the assembly did not change. float pi() //noexcept // no difference { return 3.14; } int main(){ float b{0}; b = pi(); return 0; } I am looking for a minimal working example, where I can see a change in the assembly due to noexcept . 回答1: Pretty simple examples can be constructed that involve destructors directly rather than introspection on noexcept status: void a(int); void b() noexcept; void c

问题 How does noexcept in C++ change the assembly? I tried a bit with small functions, in godbolt, but the assembly did not change. float pi() //noexcept // no difference { return 3.14; } int main(){ float b{0}; b = pi(); return 0; } I am looking for a minimal working example, where I can see a change in the assembly due to noexcept . 回答1: Pretty simple examples can be constructed that involve destructors directly rather than introspection on noexcept status: void a(int); void b() noexcept; void c

问题 Is it OK to say that 'volatile' keyword makes no difference if the compiler optimization is turned off i.e (gcc -o0 ....)? I had made some sample 'C' program and seeing the difference between volatile and non-volatile in the generated assembly code only when the compiler optimization is turned on i.e ((gcc -o1 ....). 回答1: No, there is no basis for making such a statement. volatile has specific semantics that are spelled out in the standard. You are asserting that gcc -O0 always generates code

问题 Is it OK to say that 'volatile' keyword makes no difference if the compiler optimization is turned off i.e (gcc -o0 ....)? I had made some sample 'C' program and seeing the difference between volatile and non-volatile in the generated assembly code only when the compiler optimization is turned on i.e ((gcc -o1 ....). 回答1: No, there is no basis for making such a statement. volatile has specific semantics that are spelled out in the standard. You are asserting that gcc -O0 always generates code