a c++ program returns different results in two IDE

问题

I write the following c++ program in CodeBlocks, and the result was 9183. again I write it in Eclipse and after run, it returned 9220. Both use MinGW. The correct result is 9183. What's wrong with this code? Thanks. source code:

#include <iostream>
#include <set>
#include <cmath>

int main()
{
   using namespace std;
   set<double> set_1;
   for(int a = 2; a <= 100; a++)
   {
       for(int b = 2; b <= 100; b++)
       {
           set_1.insert(pow(double(a), b));
       }
   }
    cout << set_1.size();

return 0;
}

回答1:

You are probably seeing precision errors due to CodeBlocks compiling in 32-bit mode and Eclipse compiling in 64-bit mode:

$ g++ -m32 test.cpp
$ ./a.out
9183
$ g++ -m64 test.cpp
$ ./a.out
9220

回答2:

If I cast both arguments to double I get what you would expect:

pow(static_cast<double>(a), static_cast<double>(b))

回答3:

The difference appears to be due to whether the floating point operations are using 53-bit precision or 64-bit precision. If you add the following two lines in front of the loop (assuming Intel architecture), it will use 53-bit precision and give the 9220 result when compiled as a 32-bit application:

uint16_t precision = 0x27f;
asm("fldcw %0" : : "m" (*&precision));

It is bits 8 and 9 of the FPU that control this precision. The above sets those two bits to 10. Setting them to 11 results in 64-bit precision. And, just for completeness, if you set the bits to 00 (value 0x7f), the size is printed as 9230.

回答4:

Actually you're not really supposed to rely on == (or technically, x <= y && y <= x) for doubles anyway. So this code produces implementation-dependent results (not strictly speaking UB, per comments, but what I meant :) )

来源：https://stackoverflow.com/questions/13635546/a-c-program-returns-different-results-in-two-ide

标签

c++

eclipse

gcc

codeblocks