constantfolding

Having had a look at previous questions 1 , 2 , I was wondering if I can force the compiler to perform a constant folding for the following code which prints prime numbers. #include <iostream> using namespace std; inline bool is_prime(int n) { if(n<2) return false; for(int i=2;i*i<=n;i++) if(n%i==0) return false; return true; } int main() { for(int i=0;i<20;i++) if(is_prime(i)) cout<<i<<endl; return 0; } And I build it via: g++ -O3 -S main.cpp -o main.asm As the result are a few: 2,3,5,7,11,13,17,19 I would like to force compiler look at the code similar to for(int x:{2,3,5,7,11,13,17,19})

It seems that gcc has some limitation on complex constant folding. Here is an example: static inline unsigned int DJBHash(const char *str) { int i; unsigned int hash = 5381; for(i = 0; i < strlen(str); i++) { hash = ((hash << 5) + hash) + str[i]; } return hash; } int f(void) { return DJBHash("01234567890123456"); } When running with -O3 optimization level (gcc 4.8), it unfolds the loop in DJBHash nicely and calculates the hash value for that string during compile time. However, when making the string one character longer return DJBHash("012345678901234567"); it does not fold it any more and

Possible Duplicate: is there any concept called “Constant Folding” in java? Hi I have come across line Java compiler uses something known as Constant Folding.What is this? and how Does it affect? Constant folding is where the compiler finds expressions that contain compile-time constants and replaces them with the result effectively removing redundant runtime calculations. // code static final int a = 2; int b = 30 * a; // folding would create int b = 60; Constant folding is the process of simplifying constant expressions at compile time. Terms in constant expressions are typically simple

Here's my code: int f(double x) { return isnan(x); } If I #include <cmath> I get this assembly: xorl %eax, %eax ucomisd %xmm0, %xmm0 setp %al This is reasonably clever: ucomisd sets the parity flag if the comparison of x with itself is unordered, meaning x is NAN. Then setp copies the parity flag into the result (only a single byte, hence the initial clear of %eax ). But if I #include <math.h> I get this assembly: jmp __isnan Now the code is not inline, and the __isnan function is certainly no faster the the ucomisd instruction, so we have incurred a jump for no benefit. I get the same thing

I found out that Java supports constant folding of primitive types , but what about String s? Example If I create the following source code out.write("" + "<markup>" + "<nested>" + "Easier to read if it is split into multiple lines" + "</nested>" + "</markup>" + ""); What goes into the compiled code? Combined Version? out.write("<markup><nested>Easier to read if it is split into multiple lines</nested></markup>"); Or the less efficient run-time concatenation version? out.write(new StringBuilder("").append("<markup>").append("<nested>").append("Easier to read if it is split into multiple lines"